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Clinical Microbiology Reviews, April 2004, p. 465-493, Vol. 17, No. 2
0893-8512/04/$08.00+0     DOI: 10.1128/CMR.17.2.465-493.2004

Foot-and-Mouth Disease

Marvin J. Grubman* and Barry Baxt

Plum Island Animal Disease Center, USDA, Agricultural Research Service, North Atlantic Area, Greenport, New York 11944

SUMMARY
THE DISEASE AND ITS AGENT
DESCRIPTION OF THE AGENT
    Genome Organization
    Virus Structure
    Infectious Cycle
        Early interactions: adsorption, penetration, and uncoating.
        Viral translation.
        Viral transcription and genome replication.
        Encapsidation and maturation.
    Antigenic Variation
PATHOGENESIS
    Virulence Factors
    Host Response
    Carrier State
DISEASE OUTBREAKS AND CONTROL MEASURES
    Outbreaks
    Reemergence of FMD in Developed Countries
    The 2001 United Kingdom Outbreak and Its Aftermath
    Disease Control
        Current vaccines.
        Alternative vaccine strategies. (i) Proteins and peptides.
        (ii) Live attenuated vaccines.
        (iii) Empty viral capsids.
        Antiviral approach.
        Diagnostics.
        (i) Rapid detection of virus.
        (ii) Assays to differentiate infected from vaccinated animals.
CONCLUDING REMARKS
REFERENCES

   SUMMARY
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Foot-and-mouth disease (FMD) is a highly contagious disease of cloven-hoofed animals. The disease was initially described in the 16th century and was the first animal pathogen identified as a virus. Recent FMD outbreaks in developed countries and their significant economic impact have increased the concern of governments worldwide. This review describes the reemergence of FMD in developed countries that had been disease free for many years and the effect that this has had on disease control strategies. The etiologic agent, FMD virus (FMDV), a member of the Picornaviridae family, is examined in detail at the genetic, structural, and biochemical levels and in terms of its antigenic diversity. The virus replication cycle, including virus-receptor interactions as well as unique aspects of virus translation and shutoff of host macromolecular synthesis, is discussed. This information has been the basis for the development of improved protocols to rapidly identify disease outbreaks, to differentiate vaccinated from infected animals, and to begin to identify and test novel vaccine candidates. Furthermore, this knowledge, coupled with the ability to manipulate FMDV genomes at the molecular level, has provided the framework for examination of disease pathogenesis and the development of a more complete understanding of the virus and host factors involved.


   THE DISEASE AND ITS AGENT
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The recent outbreaks of foot-and-mouth disease (FMD) in a number of FMD-free countries, in particular Taiwan in 1997 and the United Kingdom in 2001, have significantly increased public awareness of this highly infectious disease of cloven-hoofed livestock. Furthermore, worldwide concern following the terrorist attacks in the United States has raised the possibility that terrorist organizations or rogue states might target the $100 billion/year U.S. livestock industry by employing the etiologic agent of FMD. These events have directed the efforts of the scientific community to reexamine our knowledge of FMD, the viral agent that causes the disease, and current methods of disease control. In this review, we summarize the history of this disease; present, in detail, our current knowledge of the molecular biology, pathogenesis, and virulence factors; and discuss recent disease outbreaks as well as new disease control strategies.

The first written description of FMD probably occurred in 1514, when Fracastorius described a similar disease of cattle in Italy (159). Almost 400 years later, in 1897, Loeffler and Frosch (271) demonstrated that a filterable agent caused FMD. This was the first demonstration that a disease of animals was caused by a filterable agent and ushered in the era of virology. Subsequently it was shown that the agent, FMD virus (FMDV), consists of a single-stranded, plus-sense RNA genome of approximately 8,500 bases surrounded by four structural proteins to form an icosahedral capsid (401). FMDV is the type species of the Aphthovirus genus of the Picornaviridae family. The only other member of this genus is equine rhinitis A virus (240). Seven serotypes (A, O, C, Asia 1, and South African Territories 1, 2, and 3) have been identified serologically, and multiple subtypes occur within each serotype (21).

Outbreaks have occurred in every livestock-containing region of the world with the exception of New Zealand, and the disease is currently enzootic in all continents except Australia and North America (Fig. 1). The disease affects domestic cloven-hoofed animals, including cattle, swine, sheep, and goats, as well as more than 70 species of wild animals, including deer (155), and is characterized by fever, lameness, and vesicular lesions on the tongue, feet, snout, and teats (see "Pathogenesis" below). In sheep and goats the disease is generally mild and can be difficult to distinguish from other common conditions (138, 168). In addition, other vesicular diseases, such as swine vesicular disease (SVD), vesicular stomatitis, and vesicular exanthema of swine, cause signs so similar to those of FMD that differential clinical diagnosis alone can be difficult (21). Although FMD does not result in high mortality in adult animals, the disease has debilitating effects, including weight loss, decrease in milk production, and loss of draught power, resulting in a loss in productivity for a considerable time. Mortality, however, can be high in young animals, where the virus can affect the heart. In addition, cattle, sheep, and goats can become carriers, and cattle can harbor virus for up to 2 to 3 years (70) (see "Carrier state" below).



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  		FIG. 1. Countries in which FMD was reported to the OIE between 1990 and 2002. The data and maps were compiled by Nick Knowles and can be found at www.iah.bbsrc.ac.uk/virus/picornaviridae/aphthovirus.

 
FMD is one of the most highly contagious diseases of animals or humans, and FMDV rapidly replicates and spreads within the infected animal, among in-contact susceptible animals, and by aerosol. Disease signs can appear within 2 to 3 days after exposure and can last for 7 to 10 days. FMD is on the A list of infectious diseases of animals of the Office International des Épizooties (OIE) and has been recognized as the most important constraint to international trade in animals and animal products (266). Countries that are free of the disease have introduced a number of measures to retain this status because of the detrimental economic consequences resulting from its presence. The Smoot-Hawley Tariff Act of 1930, which was passed after the last outbreak of FMD in the United States in 1929, contained restrictions on importation of susceptible livestock, fresh meat, and animal products from countries where FMD was present (21). To protect disease-free countries, the OIE has developed control policy recommendations for affected countries to reacquire FMD-free status and therefore participate in international trade. These recommendations favor the more rapid return to FMD-free status if an outbreak is controlled by slaughter and vaccination is not employed. Thus, disease-free countries such as the United States, the United Kingdom, and other Western European countries have adopted control strategies that include inhibition of animal movement and slaughter of infected and in-contact susceptible animals but generally do not include vaccination. However, recent events, specifically the 2001 outbreak in the United Kingdom, have had a profound effect on this control strategy, and new recommendations have evolved and are evolving.


   DESCRIPTION OF THE AGENT
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Genome Organization

The virion is a 140S particle consisting of a single-stranded RNA genome and 60 copies each of four structural proteins (VP1 [1D], VP2 [1B], VP3 [1C], and VP4 [1A]). The FMDV genome has a basic organization similar to those of other members of the Picornaviridae, and the nomenclature for the viral proteins was established by Rueckert and Wimmer (402). In this review, we refer to the viral structural proteins by their more common designations, VP1 to -4, and to viral nonstructural (NS) proteins mainly by their designations as described by Rueckert and Wimmer (402). Within the virion, there are small amounts of a cleavage precursor of VP2 and VP4, called VP0 (1AB) (401), and one copy of a 23- to 24-amino-acid genome-linked protein, 3B (VPg [we use this designation for the protein]), covalently bound to the 5' terminus of the RNA (186, 420). The organization of the viral genome is shown in Fig. 2. The RNA is translated as a single long open reading frame (ORF) into a polyprotein, followed by a series of posttranslational proteolytic cleavages to generate both the intermediate and mature structural and NS viral proteins (191, 387, 402).



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  		FIG. 2. Schematic map of the FMDV genome. The ORF is shown in the boxed area, with the viral proteins named according to the nomenclature of Rueckert and Wimmer (402). Also shown are the functional elements of the genome as described in the text and the partial protein cleavage products. The sites of the primary cleavages and the proteases responsible are indicated. PKs, pseudoknot structures. (Adapted from reference 295.)

 
Based on the initial cleavage products, the genome ORF is divided into four regions (Fig. 2). The 5' end, the L region, which encodes the N-terminal component of the polyprotein, contains two in-frame AUG initiation codons that result in the generation of two L proteins, Lab and Lb (387, 419). While both forms of L are synthesized during in vitro translation of viral RNA (48, 418) and in infected cells (102), it has been shown, by using site-directed mutagenesis, that deletion of the second AUG from an FMDV infectious clone abolished viral replication upon transfection of the transcribed RNA into cells, while deletion of the first AUG had no effect on viral replication (84). In addition, Piccone et al. (356) generated synthetic viral genomes lacking the L gene and showed that only the genome that initiated polyprotein synthesis at the second AUG codon produced live virus. These results strongly suggest that Lb may be the biologically functional protein in vivo. The L protein, a papain-like protease (Lpro) (251, 356, 386, 435), is autocatalytically cleaved from the polyprotein at its C terminus (440). A crystallographic structural analysis of Lpro suggested that the self-cleavage reaction might occur intermolecularly, in trans, based on the position of the protein's C terminus within the active site of an adjacent Lpro molecule (194). However, other structural features within the same crystals suggested that an intramolecular cis self-cleavage reaction is also possible (194). In addition, Lpro has unique cation concentration and pH range requirements, due to differences within the molecule's active site, which distinguish it from other papain-like enzymes (193). The Lpro also plays a role in inhibition of host protein synthesis and has been identified as a viral virulence factor (see "Viral translation" and "Virulence factors" below).

Directly downstream of the L region is the P1 region of the genome (Fig. 2), encoding the four viral structural proteins VP4, VP2, VP3, and VP1. Following the P1 region is the P2 region (Fig. 2), encoding three viral NS proteins, 2A, 2B, and 2C, and the P3 region, encoding NS proteins 3A, three copies of VPg, 3Cpro, and 3Dpol. Historically the 2A region was considered part of the P2 region; however, genetic and biochemical evidence has shown that the FMDV 2A peptide is cleaved as a P1-2A precursor (463) (see "Viral translation" below). 3Cpro was identified as a viral protease (252) and is involved in processing the viral polyprotein, while 3Dpol is the viral RNA-dependent RNA polymerase (110, 275, 335, 368-370, 389). The roles that each of these proteins play in viral replication are discussed in Infectious Cycle below.

The FMDV genome also contains untranslated RNA found upstream (5' untranslated region [5' UTR]) and downstream (3' UTR) of the ORF (Fig. 2). The 5' UTR of FMDV contains about 1,300 bases (157, 191, 387) and can be divided into five functional elements which play roles in virus translation and RNA replication (Fig. 2). The most 5' segment, the S fragment, encompasses about 360 bases and folds into a long stem-loop (Fig. 2) (77, 203, 336). The function of the S fragment is not known, but analogies with other picornaviral genomes suggest that it may play a role in maintaining genome stability in infected cells (33) and may also be involved in the binding of proteins involved in genome replication (12, 13, 33, 212, 481). There have been some suggestions that the S fragment may affect viral pathogenicity, but currently there is no direct supporting evidence for this (85, 150).

Following the S fragment, there is a poly(C) tract comprising over 90% C residues with a small number of U and A residues (Fig. 2). This segment is over 100 bases in length; however, the length of the poly(C) tract can be extremely variable (108), and, in one case, a tissue culture-adapted virus has been shown to have a poly(C) length of over 400 bases (150). Although an early study suggested that the length of the poly(C) tract was associated with virulence (203), other studies have been unable to correlate poly(C) length with this property of the virus (108). It has been shown that the poly(C) lengths of natural viral isolates are increased by repeated passage in cell culture (150), as is the length of this segment in genetically engineered viruses (381). It is possible to replicate virus with essentially no poly(C), and while this virus is virulent in suckling mice, it has a much higher particle/PFU ratio than viruses containing longer poly(C) tracts (381). The virulence of this virus in susceptible animals has not been determined. While the exact role that the poly(C) tract plays in FMDV replication is unknown, recent studies with poliovirus have shown an association of the host factor, poly(rC) binding protein (PCBP), with the 5' end of the poliovirus genome (165), which could regulate the switch from translation to genome replication (see "Viral transcription and genome replication" below) (33, 212, 470). Just 3' of the poly(C) tract there is a series of RNA pseudoknot structures of unknown function (Fig. 2) (149, 381).

Downstream of the pseudoknots there is a short hairpin loop structure, the cis-acting replicative element (cre) (Fig. 2) (294). The cre, which has been identified in the genomes of human rhinoviruses (169, 310, 311), poliovirus (176, 382), and cardioviruses (270), has a stem-loop with a conserved AAACA sequence in the loop region. In contrast to the case for other picornaviruses, where the cre is located within different regions of the ORF, the cre of FMDV is located within the 5' UTR (294). The cre is essential for RNA genome replication, and its function is discussed in "Viral transcription and genome replication" below.

The region between the cre and ORF contains a series of highly conserved stem-loop structures which together constitute the internal ribosome entry site (IRES) (Fig. 2). All picornavirus mRNAs lack the 7-methyl-G cap structure present at the 5' ends of most eukaryotic mRNAs. In addition, the 5' UTR of picornaviral genomes is quite long, and it was demonstrated for poliovirus and encephalomyocarditis virus (EMCV) that ribosomes enter the genome internally at the IRES (231, 351). An IRES element was subsequently identified within the FMDV genome (51, 261). The IRES elements of picornaviruses contain a high degree of secondary and tertiary structure. They have been divided into three groups, based on conserved structure as opposed to primary sequence (359, 360, 385). The IRES element for aphthoviruses, which is similar in structure to the cardiovirus IRES (group 2 IRES), is about 450 nucleotides in length (51, 261) and has been modeled into a five-domain structure (359). IRES elements contain a pyrimidine-rich region at their 3' ends immediately preceding the AUG translation initiation codon, and FMDV contains pyrimidine-rich regions directly upstream of each of the alternative AUG initiation codons (51, 261, 361).

The 3' UTR, which follows the ORF termination codon, contains a short stretch of RNA which folds into a specific stem-loop structure (362) followed by a poly(A) tract of variable length carried on the genome (Fig. 2) (141). The 3' UTR also appears to be important for genome replication (312, 364, 392). This is supported by studies showing that the 3' UTR can bind a number of picornaviral proteins that are involved in RNA replication (114, 115, 202). Gutierrez and coworkers (195) demonstrated that hybridization of antisense RNA to the 3' UTR of FMDV did not effect in vitro translation of viral RNA but did inhibit RNA replication in infected cells. In contrast, more recent studies have demonstrated that deletion of the FMDV 3' UTR reduced the efficiency of in vitro translation (274) and blocked the ability to recover viable virus from transfected cells (413). Replacing the FMDV 3' UTR with that of the enterovirus, SVD virus, resulted in a nonviable genome (413), suggesting that the 3' UTR is specific for each picornavirus. The poly(A) tract probably functions in FMDV translation (274) and may also play a role in picornavirus RNA replication (33, 212).

Virus Structure

By electron microscopy, the FMD virion appears to be a round particle with a smooth surface and a diameter of about 25 nm (21). The fine structure of the viral capsid has been determined for a number of serotypes by using X-ray crystallographic techniques (2, 117, 227, 263, 264), and the structural features of type O1BFS are shown in Fig. 3. The structural proteins, VP1 to -3, fold into an eight-stranded wedge-shaped ß-barrel which fit together to form the majority of the capsid structure (Fig. 3a) (2). The VP4 protein is buried within the capsid and has a myristyl group covalently attached to its N terminus (50, 100). The strands of the ß-barrels of VP1 to -3 are connected by loops which form the outer surface of the virion (Fig. 3a) (227). FMDV is distinguished from other picornaviruses by the lack of a surface canyon, or pit, which has been shown to be the receptor binding site for the entero- and cardioviruses (49, 207, 215, 257, 280, 325, 398, 483). Another feature of the virion is the presence of a channel at the fivefold axis which permits the entry of small molecules, such as CsCl, into the capsid, resulting in FMDV having the highest buoyant density of the picornaviruses (Fig. 3c) (2, 227).



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  		FIG. 3. Structure of the mature type O1BFS FMD virion based on X-ray crystallographic data. The structures shown are based on the data of Acharya et al. (2) and Logan et al. (272). (a) A viral protomer highlighting the ß-barrel-and-loop organization of the viral proteins. (b) A pentamer positioned on the virion looking down the fivefold axis. (c) The organization of the entire virion, highlighting the G-H loop (yellow) and the RGD sequence (white). Note the pore located at the top of the fivefold axis (see text). (d) A protomer highlighting the positions of the G-H loop (purple) and RGD sequence (yellow). All structures are representative of the mature virion (cleaved VP0 [see text]), and the viral proteins are colored blue (VP1), green (VP2), and red (VP3). VP4 is buried within the particle and is visible only in panel a, where it is colored yellow. The graphic renderings were done by using RasMole.

 
Unlike those of other picornaviruses, the FMDV capsid is dissociated at pHs of below 6.5 into 12S pentameric subunits (76). The reason for this instability is thought to be a cluster of His residues at the interface between VP2 and VP3 which become protonated at low pH, weakening the capsid through electrostatic repulsion (116, 147). This low-pH-induced instability of FMDV leads to differences in the mechanism of its uncoating upon infection of cells compared to that for other picornaviruses (see "Early interactions: adsorption, penetration and uncoating" below) and also probably plays a role in the targeting of the virus to specific tissues and organs in susceptible hosts.

Infectious Cycle

FMDV, like other members of the Picornaviridae, has a relatively short infectious cycle in cultured cells. Depending on the multiplicity of infection, newly formed infectious virions begin to appear at between 4 and 6 h after infection. The virus is cytocidal, and infected cells exhibit morphological alterations, commonly called cytopathic effects, which include cell rounding and alteration and redistribution of internal cellular membranes. The virus also causes biochemical alterations, including inhibition of host translation and transcription (401).

Early interactions: adsorption, penetration, and uncoating. The interactions of FMDV with cells have been extensively studied for many years. It is generally accepted that FMDV receptors, as well as other picornavirus receptors, play a role in tissue and organ tropism which leads to disease pathogenesis (112, 151, 429, 476). FMDV binds rapidly to cells in culture at both 4 and 37°C by using a limited number of receptor sites, which has measured at between 103 and 104 per cell (39). These studies also suggested that while six of the seven serotypes bound to a single class of receptor site, some of the serotypes bound to a second class of receptors which were present at a high copy number (39, 431). Early studies showed that limited trypsin digestion of virus resulted in viral particles which were noninfectious due to the inability to bind to cells in culture (22, 30, 38, 90, 318). Analysis of trypsin-treated virus revealed a single cleavage of the VP1 protein at Arg144 (388), which was later shown to be located within a surface loop connecting the ßG and ßH strands of the protein (G-H loop) (2, 272) (Fig. 3c and d). These results suggested that this region of the VP1 protein interacted with the cell surface receptor.

In 1984 Pierschbacher and Ruoslahti (357), studying the binding of fibronectin to cells, reported that the tripeptide sequence Arg-Gly-Asp (RGD) was a cellular recognition site on the molecule and that this sequence was also found in the FMDV VP1 protein (358) (Fig. 3d). The fibronectin receptor was subsequently shown to be part of a large family of transmembrane glycoproteins called integrins (449). These molecules are type l membrane glycoproteins, consisting of two subunits ({alpha} and ß) which are noncovalently bound at the cell surface. They are involved in cell adhesion, cell migration, thrombosis, and lymphocyte interactions (220, 221, 223). In FMDV, while sequences surrounding the RGD sequence within the G-H loop are variable, the RGD sequence itself is highly conserved (353). The first indication that the RGD sequences might be involved in the virus-receptor interaction came from studies showing that small peptides containing RGD could inhibit the binding of virus to cells (40, 158). Direct genetic evidence for this interaction was obtained by mutating or deleting the RGD sequence in infectious cDNA clones, resulting in viral particles which were noninfectious, could not adsorb to susceptible cells, and could not cause disease in susceptible animals (268, 297, 309). Of the 24 known integrin receptors, only 8 use the RGD tripeptide as a recognition sequence, and 5 of those are part of the {alpha}V subgroup of integrin receptors (222, 403). The first identification of the integrin receptor for FMDV was made based on comparison of the receptor specificity of the virus with that of a human enterovirus, coxsackievirus A9 (CAV9). This virus contains an extension in its VP1 protein that includes an RGD sequence (92, 93), which was shown to be involved in virus binding to cells in culture (393). The receptor utilized by CAV9 was demonstrated to be the integrin {alpha}Vß3 (394), and competition binding studies revealed that CAV9 and antibodies to the {alpha}Vß3 integrin were able to inhibit the binding of FMDV to monkey kidney cells (56). These results were confirmed genetically by demonstrating that cells which did not express this integrin and were not susceptible to FMDV infection became permissive for viral infection upon transfection of cDNAs encoding either the human (334) or the bovine (144, 332, 333) {alpha}Vß3 integrin. Following these studies, it was further demonstrated that FMDV could also utilize two additional {alpha}V integrins, {alpha}Vß6 (229) and {alpha}Vß1 (228); however, the virus was unable to utilize the integrin {alpha}Vß5 as a receptor (144; for reviews, see references 44 and 227). To date, none of the other integrins which utilize the RGD recognition sequence have been analyzed for FMDV receptor activity.

A number of alternative receptors have been shown to mediate FMDV infection in vitro. Antibody-complexed virus can infect cells via Fc receptor-mediated adsorption (42, 293, 297). In addition, an artificial receptor which consists of a single-chain anti-FMDV monoclonal antibody fused to intercellular adhesion molecule 1 (ICAM-1) has been engineered, and this receptor was also able to mediate infection with RGD-deleted virus (380). In 1996, Jackson and coworkers (226) reported that a type O1 virus was able to utilize the glycosaminoglycan heparan sulfate (HS) as a coreceptor. We had previously shown that there appeared to be a second, unidentified receptor present at a high copy number (39, 431), and the report by Jackson et al., (226) was consistent with this finding. It contrasted, however, with our findings that a type A virus could not bind to CHO cells which lack the integrin receptor for FMDV but express HS (293). We reconciled these seemingly opposite findings by showing that tissue culture adaptation of a type O1 virus selects a variant which has a positively charged Arg at residue 56 of VP3 and can grow in CHO cells (409). Interestingly, this variant was relatively avirulent in cattle (409). In contrast, a second variant of this virus containing a His at residue 56 of VP3 could not grow in CHO cells and was relatively virulent in cattle (409). We expanded these studies to show that the virus with the Arg residue required only HS to replicate in CHO cells but that the variant with the His residue required the integrin to replicate in cell culture (334). A similar result was also noted for type C viruses, where multiple passages in tissue culture selected viruses with positive surface charges which can replicate in the absence of the known integrin receptors (26, 27, 290). Structural analysis of HS-complexed type O virus revealed a direct interaction between HS and residue 56 in VP3 (162, 227).

While the penetration and uncoating of FMDV have not been studied in great detail, there have been some observations which suggest possible mechanisms of how they might occur. We and others have shown that after adsorption to the cell surface, the 140S virion breaks down into 12S pentameric subunits, releasing the RNA (37-39, 89). This breakdown does not occur at the cell surface, since particles which are eluted from the cell after adsorption are fully infectious and still sediment at 140S (39). By using a series of lysosomotropic agents, which raise the pH of intracellular endosomes, it has been demonstrated that the virus probably breaks down upon entering an acidic endosome (37, 87, 88). Recently it has been shown that a genetically engineered FMDV, which is unable to perform the maturation cleavage of VP0 to VP2 and VP4 (see "Encapsidation and maturation" below) is noninfectious, can adsorb to cells in culture, and is acid sensitive (253). Thus, the breakdown of 140S virus to pentameric subunits by itself does not lead to productive infection, but there must be other events after the breakdown. These results indicate that the viral receptor is responsible only for docking the virus to the membrane of the susceptible cell and plays no role in viral uncoating, which is consistent with the ability of FMDV to utilize multiple receptors for infection in cell culture.

Viral translation. Following uncoating, the RNA is released into the cytoplasm by an as-yet-unknown mechanism and begins a round of viral translation. The genome-linked protein VPg is cleaved by a cellular enzyme prior to translation of the incoming RNA (10, 11); however, protein synthesis initiation complexes can be formed with mRNA containing VPg (174). Unlike most host mRNAs, actively translated viral mRNA does not contain a 7-methyl-G cap structure at its 5' end (187) and initiates protein synthesis internally at the IRES by a cap-independent mechanism (51, 231, 261, 351). Cap-dependent mRNA translation is inhibited in infected cells as the result of the cleavage of the protein synthesis initiation factor eIF4G by Lpro (124, 241). Intact eIF4G acts as a bridge connecting the mRNA cap to the 40S ribosomal subunit. This is accomplished by the binding of the cap binding protein, eIF4E, to the N-terminal domain of eIF4G, while the C-terminal domain binds eIF4A and the 40S ribosomal subunit via eIF3 (262). In contrast, initiation of FMDV RNA translation requires only the Lpro-generated C-terminal eIF4G cleavage product, which binds to the FMDV IRES and interacts with 40S ribosomal subunit-bound eIF4A and eIF3 (273, 414). In addition, eIF4B is bound to the FMDV IRES and has been identified in both 48S preinitiation complexes and 80S ribosomes (273, 314, 343, 405).

The FMDV IRES interacts with a number of cellular proteins, including initiation factors important for normal cellular mRNA translation. A host factor of 57 kDa, subsequently identified as the nuclear polypyrimidine tract binding protein (PTBP) (209, 237, 337, 338), was shown to interact with at least two regions of the IRES (281, 363). Deletion of these two sites inhibited both the binding of the protein and in vitro translation (282). More recently a second host factor, which is required for FMDV IRES-driven translation but not for translation of cardiovirus mRNA (363), has been identified. This 45-kDa protein, IRES-specific trans-acting factor (ITAF45), along with PTBP, is required for the formation of the 48S translation-initiation complex (291, 363). A third host factor, PCBP, which is required for translation of poliovirus RNA (65) has not to date been shown to be involved in FMDV translation. However, the presence of the poly(C) tract upstream of the IRES suggests that it may also play a role in FMDV translation, genome replication, or both. It has also been postulated that PCBP facilitates a circularization of the poliovirus genome to modulate the balance between translation and RNA replication (33, 212).

Interestingly, the 3' end of the genome may also be required for FMDV translation, since deletion of either the poly(A) tract or the 3' stem-loop and the poly(A) tract generated noninfectious FMDV RNAs which had a lowered translation efficiency in in vitro translation reactions (274). Furthermore, addition of either the poly(A) tract, the FMDV 3' stem-loop, or both to a bicistronic construct, driven by the FMDV IRES, stimulated IRES-directed translation in vitro (274).

Following initiation, translation results in the production of a single polypeptide which undergoes a series of cleavages leading to the production of both structural and NS proteins (Fig. 2). The primary cleavage reactions are performed by three different proteases. As discussed above, Lpro autocatalytically cleaves itself from polyprotein. 2A, (an 18-amino-acid peptide), autocatalytically removes itself from the P2 polyprotein, and remains associated with the P1 precursor (192, 408, 463). There has been a suggestion that this cleavage is not a proteolytic event but rather is a modification of the translational machinery by the 2A peptide which allows the release of P1-2A from the ribosome while permitting the synthesis of the downstream proteins to proceed (139, 140). This hypothesis, however, has not been confirmed by other laboratories. Nevertheless, the 2A peptide has been used in nonviral systems to cleave foreign genes from polyproteins (177, 185). All of the other cleavages of the polyprotein, as outlined in Fig. 2 (with the exception of the maturation cleavage [see below]), are performed by 3Cpro (20, 101, 463). This protein is related to the trypsin family of serine proteases (18, 45, 179), and Grubman and coworkers have mapped the active site of FMDV 3Cpro to Cys163, His46, and Asp84 (192). The protease cleaves at a number of different dipeptide sequences, including Gln-Gly, Glu-Gly, Gln-Leu, and Glu-Ser (348, 387).

Viral transcription and genome replication. Picornavirus RNA replication presents a number of unique challenges. The 5' end of the genome RNA is covalently linked to VPg, and the 3' end has a genetically coded poly(A) tail. Thus, the viral RNA-dependent RNA polymerase (3Dpol) must distinguish between viral RNAs and cellular mRNAs, which also contain 3'-terminal poly(A) tracts. In addition, since the mRNA and the genome RNA are the same molecule, with the exception of the genome-linked VPg, there must be a mechanism to distinguish RNAs which are bound for the ribosome and those which will be packaged into virion particles. While there have been very few studies on transcription and replication in the FMDV system, extensive studies on these activities have been performed with enteroviruses.

The first step in picornavirus RNA replication is the synthesis of a minus-strand RNA molecule. This system has not been studied in FMDV; however, the models of RNA replication developed for poliovirus are probably quite similar (350). It is thought that translation of the plus-strand RNA must cease before minus-strand synthesis begins (164). The mechanism of this shutdown of translation of the plus strand is unclear; however, it has been proposed that, in poliovirus-infected cells, when the polymerase precursor (3CD) accumulates in the cell, it binds to the 5' cloverleaf structure and modifies the affinity of PCBP for the IRES, an interaction which is essential for translation (see "Viral translation" above). Whether 3CD plays a similar role in FMDV RNA replication is not known; however, it has been shown that in FMDV-infected cells, this protein is rapidly cleaved to 3Cpro and 3Dpol (191).

There is still controversy about the initiation of picornavirus minus-strand RNA synthesis. One model proposes that initiation begins after circularization of the genome facilitated by the interactions of poly(A) binding protein (PABP) with the 3' poly(A) tail and the PCBP-3CD-5' cloverleaf structure (211). In the FMDV genome, the 5' interactions probably take place within the S fragment and may also involve the poly(C) tract (see above). Since initiation of minus-strand synthesis occurs in the cytoplasm in the presence of cellular mRNAs, which also contain poly(A) tails, picornaviruses must have developed mechanisms enabling the polymerase to recognize viral RNA. In picornavirus-infected cells, both plus- and minus-strand RNAs are linked to VPg (339), and the presence of a small protein-linked dinucleotide, VPgpUpU, in infected cells (111) suggested that VPg might be a primer for the RNA polymerase. The discovery of the cre provided a rational mechanism for both the uridylylation of VPg and the ability of the polymerase to discriminate viral RNA from cellular mRNAs. The cre is a stem-loop RNA structure, found within the 5' UTR in the FMDV genome (Fig. 2), with a conserved AAACA motif within the loop (see "Genome organization" above). This conserved motif is required for poliovirus and rhinovirus minus-strand synthesis (176, 311), and the first two A's serve as the template for the synthesis of VPgpU and VPgpUpU and for viral replication initiation in enteroviruses (169, 382). Mutations in this motif within the FMDV cre severely reduced viral replication in cell culture; however, the cre is positionally independent within picornavirus genomes (176, 294, 310, 487). It is not clear at this time whether free VPg is utilized in the initiation step or whether a cleavage precursor (either 3AB, 3ABC, or 3BCD) is needed. The second model, discussed below, postulates that minus-strand synthesis is primed on the poly(A) tail.

Following the initiation reaction, elongation of the minus strand begins, catalyzed by 3Dpol. For this to occur, the initiation complex must translocate to the 3' end of the plus-strand template. The mechanism by which this occurs is unknown, but one hypothesis suggests that binding of PABP to the poly(A) tract positions this region of the plus strand near the cre (350). The elongation of the nascent strands results in the formation of a double-stranded molecule, the replicative form (RF) (3, 477). Free minus strands are not detectable in vivo.

After formation of the RF, new plus-strand synthesis can begin. In poliovirus-infected cells, the ratio of plus to minus strands is about 50:1 (340), indicating that a single minus strand can be a template for the synthesis of numerous plus strands, resulting in the formation of a partially double-stranded RNA molecule, the replicative intermediate (3). The initiation of plus-strand synthesis from the RF has not been elucidated; however, two possible mechanisms to generate VPgpUp have been suggested. The first proposes either using existing uridylylated VPg, made in abundance during minus-strand synthesis, or uridylylating VPg at the 3' end of the minus strand (350). The second hypothesis, which also disputes the mechanism for the initiation of minus strands presented above, proposes that VPgpUpU is generated on the 3' poly(A) tail of the plus strand and utilized for minus-strand synthesis, while cre-generated VPgpUpU is utilized for plus-strand synthesis (33, 321, 330). Since the data which led to the latter hypothesis was generated totally with cell-free systems, it is still uncertain whether these mechanisms are utilized in infected cells. In addition, it has recently been suggested that FMDV cre function can be complemented in trans (456). While more studies are necessary to confirm this result, it should be noted that cre function could not be complemented in trans in either the human rhinovirus (176) or the poliovirus (176, 382) system.

For plus-strand synthesis to proceed, the RF must be unwound. The mechanism for this is also unclear. The picornavirus 2C protein both has ATPase activity (249, 354, 390) and contains helicase motifs (125, 128, 178, 180, 250), but helicase activity has not been demonstrated (355). It has been shown that 2C and a cellular protein (p38) bind to the minus-strand 3' stem-loop (25, 391, 392), and this may act to destabilize the RF molecule. The possibility of involvement of either a cellular helicase or a nuclear protein has also been suggested, since the RF is infectious when transfected into whole cells (317) but not when transfected into enucleated cells (322). The elongation of the plus strand by 3Dpol also occurs by an unknown mechanism. The complete replication of a picornaviral RNA in a cell-free system, including de novo protein synthesis, genome replication, and encapsidation to produce infectious virus, has been accomplished for poliovirus (32, 211, 316, 461) and EMCV (446).

RNA synthesis occurs within a membranous replication complex, which is derived from membranes of the endoplasmic reticulum and Golgi and contains viral NS proteins encoded by both the P2 (2B, 2BC, and 2C) and P3 (3A and its precursors, 3Cpro, and 3Dpol) regions (58-61, 66, 145, 170, 181, 232, 428, 442, 452, 460, 465). Structures similar to enterovirus replication complexes containing RNA and 3Dpol have been detected in FMDV-infected cells (371, 372). The 2B protein has been shown to enhance membrane permeability and block protein secretion (129, 232, 465, 466), but its role in RNA synthesis is not clear. Protein 2C has been found in membranous aggregates along the periphery of FMDV-infected cells (450) and has been directly implicated in FMDV RNA synthesis by using the picornavirus RNA synthesis inhibitor guanidine hydrochloride (81). FMDV mutants that are resistant to guanidine inhibition had an altered 2C isoelectric point (426), and changes in the viral 2C protein in resistant mutants were directly shown by using recombination and RNase T1 oligonucleotide mapping (427). Infection with picornaviruses results in a rapid inhibition of host cell transcription, which does not appear to be related to the inhibition of host cell translation (235). For FMDV-infected cells it has been shown that histone H3 is cleaved by 3Cpro, and this cleavage does not occur in cells infected by either poliovirus or EMCV (184, 451). In the case of enterovirus infections, transcription catalyzed by RNA polymerases I, II, and III is inhibited, and this inhibition requires the synthesis of 3Cpro, which appears to cleave cellular transcription factors required for the activity of these three enzymes (see reference 119 and references therein). Thus, FMDV may inhibit host cell transcription by a mechanism different from that of the entero- and cardioviruses.

Encapsidation and maturation. The final steps in the replication cycle are the encapsidation of the plus-strand viral RNA and maturation cleavage of VP0 to VP2 and VP4 to form the mature virion. The mechanisms of encapsidation and maturation are still unresolved and are probably the least studied of all of the steps in the replication cycle. Again, most of the studies have been done with the enteroviruses, and therefore analogies must be drawn with FMDV. In broad terms, the 3Cpro cleavage products of the P1 region are assembled into a protomer structure containing one copy of each of the proteins VP0, VP1, and VP3 (Fig. 3a). Five protomers can assemble into a pentamer (Fig. 3b), and 12 pentamers assemble into the final capsid structure (Fig. 3c). Following encapsidation of the RNA, the maturation cleavage reaction (VP0 to VP2 and VP4) takes place (see below). A number of intermediate particles have been identified in picornavirus-infected cells, including protomers, pentamers, a particle containing RNA with an uncleaved VP0 (provirion) (197, 265), and a particle with an uncleaved VP0 lacking RNA (empty capsid) (190, 485). Two unresolved issues in picornaviral maturation are what signals are necessary for encapsidation of the RNA and what are the roles of the empty capsid and provirion.

Picornaviruses encapsidate only plus-strand RNA, linked to VPg, to the exclusion of all other viral and cellular RNAs (339, 340, 475). In addition, only newly synthesized plus-strand RNAs are encapsidated, indicating that there is a link between active RNA replication and encapsidation (201, 341). Thus, there may be cis-acting packaging signals present within the plus-strand RNA to facilitate encapsidation. The putative packaging signal does not appear to reside within the P1 region of the genome, since defective-interfering particles have been demonstrated in poliovirus-infected cells (198, 236, 279), all of which have deletions within the P1 region. Interestingly, no defective-interfering particles have been detected in FMDV-infected cells (103, 367). In addition, the complete P1 regions of a number of picornaviruses, including FMDV (294), can be deleted and replaced with a reporter gene generating a replicon, which, in the case of poliovirus, can be encapsidated when capsid proteins are provided in trans by a coinfecting virus (15, 28, 308, 373). Further studies, using the poliovirus replicon system, have shown that if the structural proteins of a heterologous picornavirus are provided in trans, packaging of the replicon RNA does not occur (28, 374). These data provide additional evidence for a packaging element, specific for each individual picornavirus, located within the genome outside the P1 region. There has been a single unconfirmed report of heterologous trans encapsidation of the FMDV genome with bovine enterovirus structural proteins (462). Recently, a cis-acting encapsidation element was identified within the stem of a stem-loop beginning four bases from the 5' end of the genome of a newly discovered picornavirus, Aichi virus (425). This is the first report of such an element within the Picornaviridae.

While naturally occurring provirions have not been demonstrated in FMDV-infected cells, they have been shown to occur during the replication of other picornaviruses (63, 197, 214). There are currently two models of picornavirus assembly. One proposes that pentamers assemble into empty capsids, followed by insertion of the RNA, and the second proposes that pentamers directly interact with the RNA to form the provirion. In either case, it is known that myristylation of the N terminus of VP0 is necessary for capsid formation (16, 286, 323). Studies with FMDV have demonstrated that radioactive label can be chased from structural proteins into protomers, pentamers, empty capsids, and finally virions (485). In addition, poliovirus pentamers can self-assemble into empty capsids in vitro in the absence of viral RNA (287, 347, 395). More recently, however, it has been shown in a cell-free replication system that only poliovirus pentamer structures can interact with newly synthesized RNA to form virions (467), indicating that empty capsids may be either a storage particle for pentamers or a by-product of the assembly reaction.

The final step in virion assembly is the maturation cleavage of VP0 into VP4 and VP2, requiring the presence of viral RNA (19, 215, 230, 398). An aberrant cleavage of VP0 in FMDV empty capsids has been demonstrated, suggesting that viral RNA is required for the proper cleavage event to take place (116, 118). Cleavage is thought to be autocatalytic and results from a conserved His residue in VP2 which activates local water molecules, leading to a nucleophilic attack on the scissile bond and cleavage (34, 118, 213). Maturation cleavage is required for the generation of infectious virus (201, 253, 265). In FMDV, site-directed mutations within VP0 led to the formation of noninfectious provirions that exhibited receptor binding and acid sensitivity, similar to the case for infectious virus (253). Upon acid dissociation, however, the generated pentamers were more hydrophobic than those from mature virions, suggesting that VP0 cleavage may be necessary for release of the RNA into the cytoplasm (253).

Antigenic Variation

The presence of seven serotypes and multiple subtypes and variants has added to the difficulty of laboratory diagnosis and control of FMD. The rise of new variants is inevitably caused by continued circulation of the virus in the field and the quasispecies nature of the RNA genome (134, 206). RNA viruses in general, and FMDV in particular, have very high mutation rates, in the range of 10–3 to 10–5 per nucleotide site per genome replication, due to the lack of error correction mechanisms during RNA replication (135, 142). This high error rate leads to differences of FMDV replicated genomes from the original parental genome of 0.1 to 10 base positions (206), and the quasispecies concept was developed to explain the effects of errors in replication on the evolution of replicating RNA molecules (146). In its simplest terms, the concept envisions that within any population of virus, all genome sequences are not identical, and that selection occurs at the population level rather than at the individual level (134). Thus, there is not a "wild type" as such but rather an observed "average" phenotype which has adapted to and replicates "best" within any given environment. The environment can be in either tissue culture or a particular host species, and in either situation, immunologic pressure or physical conditions such as temperature or pH are influential. Any change in the environment can lead to the emergence of a new "average" phenotypic trait. The quasispecies nature of the FMDV genome was described over 20 years ago (132, 437), and while the concept is studied at the nucleotide level, the variability of FMDV populations is manifested when mutations lead to codon changes resulting in a change in the viral phenotype. Most of this variation occurs within the capsid-coding region of the genome (the P1 region [Fig. 2]) and leads to antigenic variation. While mutations also occur within the NS protein-coding regions of the genome, they are probably less tolerated, since proteins encoded by these regions are necessary for viral replication and changes are more likely to be lethal. In addition to variation caused by mutation, FMDV has been shown to undergo RNA recombination in tissue culture (239, 304, 474). Interestingly, these studies indicated that recombination was more likely to occur within the regions of the genome coding for the NS proteins; however, a more recent study has suggested that RNA recombination within the capsid-coding P1 region of the genome may contribute to genetic diversity in FMDVs isolated from the field (459).

Antigenic variation in the field increases with time and most probably results from immunologic pressure placed on the virus by either the infected or vaccinated host species (134, 205). In addition, antigenic variation in FMDV has also been observed in tissue culture in the absence of immunologic pressure (67, 126, 133, 153, 175, 433), indicating that antigenic sites on the virion may also be involved in other virus functions. Regardless of the mechanism, analysis of both genome sequence and antigenic variation has been invaluable in epidemiological studies of outbreaks and analysis of virus within countries where the disease is enzootic, and, in the case of a possible deliberate introduction of virus, it will also have forensic value in tracking the source (17, 206, 210, 255, 258, 288).

Antigenic sites on the surface of the FMD virion have been identified for five of the seven serotypes of the virus (South African Territories 1 and 3 being the only exceptions) (41, 43, 68, 80, 113, 204, 247, 264, 288, 298, 305, 453, 484). At least four antigenic sites have been identified, involving one or more of the capsid proteins, VP1, VP2, and VP3; however each serotype may not contain all four sites. Interestingly, three of the sites have elements located within the flexible loops which connect the ß-sheets of the viral proteins, and at least two sites include the C terminus of VP1 (134). While all of the sites appear to be necessary for a complete immunologic response to either infection or vaccination, the major antigenic site, to which most of the immune response is directed and which is common to all of the serotypes, is located within the G-H loop of VP1 (Fig. 3d) (see "Virus structure" above) (69, 299). This site also contains the RGD receptor-binding recognition sequence (see "Early interactions: adsorption, penetration, and uncoating" above). While this site is clearly the major antigenic site, FMDV antigenic variation is associated with mutations leading to amino acid replacements within all of the known antigenic sites (154, 300). Nevertheless, even though there is extensive antigenic variation within FMDV, the changes are limited to very specific regions of the viral surface. This may be because changes within other regions of the capsid would compromise either viral structural integrity or virus identity (134). The antigenic variation within FMDV makes control extremely difficult, since even the best vaccine may induce immunologic pressure within the population that results in the emergence of a new variant. Furthermore, the observation that antigenic variation can also occur in tissue culture has implications for vaccine production, since a number of tissue culture passages are required to produce vaccine for a new variant, leading to the possibility that the virus eventually utilized as antigen may not provide the antigenic coverage needed.


   PATHOGENESIS
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While FMD affects a wide variety of cloven-hoofed animals, pathogenesis has been studied mainly in cattle and pigs. Infection of cattle generally occurs via the respiratory route by aerosolized virus (137). Infection can also occur through abrasions on the skin or mucous membranes, but is very inefficient, requiring almost 10,000 times more virus (137). Virus is excreted into the milk of dairy cattle (78, 219, 378) as well as in semen, urine, and feces (137, 243), and calves can become infected by inhaling milk droplets. Infected cattle also aerosolize large amounts of virus, which can infect other cattle in addition to other species (438). A number of studies have suggested that the lung or pharyngeal areas are the sites of initial virus replication (72, 79, 444), with rapid dissemination of the virus to oral and pedal epithelial areas (72, 79, 444), possibly mediated by cells of monocyte/macrophage origin (72). In cattle experimentally infected via aerosol, it was found, by in situ hybridization (ISH), that within the first 24 h, virus was present in respiratory bronchiolar epithelium, subepithelium, and interstitial areas of the lung (74). By 72 h, signal was detected in epithelial cells of the tongue, soft palate, feet, tonsils, and tracheobronchial lymph nodes (74). Other studies, however, have suggested that the pharynx, and not the lungs, may be the initial site of viral replication in infected cattle (8, 79, 489). The conflicting observations about the region of the respiratory tract that is initially infected in cattle exposed to aerosols may be the result of a number of variables, including aerosol particle size, strain of virus, or how the aerosol was generated (8).

Vesicles develop at multiple sites, generally on the feet and tongue, and are usually preceded by fever. Severe lesions often occur in areas subjected to trauma or physical stress, and most animals develop viremia. The incubation period can be between 2 and 14 days, depending on the infecting dose and route of infection (163).

Pigs usually become infected either by eating FMDV-contaminated food, by direct contact with infected animals, or by being placed into areas that had once housed FMDV-infected animals. They are, however, much less susceptible to aerosol infection than cattle (5, 6), yet they excrete far more aerosolized virus than cattle or sheep (6, 7). As in cattle, the incubation period is dependent on the amount of infecting virus and the route of infection, but it is generally 2 days or more. Animals develop fever, viremia, and lesions on the feet and tongue. Foot lesions are the most common finding in pigs, while lesions at other sites occur less frequently (244). Tongue lesions are usually small and less noticeable than those in cattle (244). In young piglets, the infection may be fatal due to myocarditis. Initial replication of the virus occurs at the site through which the virus gains entry, followed by rapid dissemination to most of the epithelial sites within the animal (73, 346). Interestingly, virus can be found at sites where clinical lesions either were not present or do not form (73, 346). While pigs excrete large amounts of aerosolized virus, recent evidence suggests that much more viral replication takes place in the nasal mucosa than in the lungs (346).

Of all of the important livestock species, sheep played the major role in the United Kingdom outbreak of 2001 (see below). Because it is very difficult to make a clinical diagnosis of FMD in sheep (R. De la Rua, G. H. Watkins, and P. J. Watson, Letter, Vet. Rec. 149:30-31, 2001), the disease can be spread to other livestock prior to detection (245). Sheep are highly susceptible to virus infection via aerosol and can excrete airborne virus; however, during outbreaks they are most likely infected by contact with infected animals (245). Clinical disease in sheep is characterized by lesions on the feet and mouth, fever, and viremia. It has been reported, however, that up to 25% of infected sheep may fail to develop lesions, and an additional 20% may form only one lesion (171, 218).

FMDV can also infect a wide variety of wildlife. The risk of spread of the infection by wildlife is controversial and is discussed in two recent review articles (443, 455).

Virulence Factors

In theory, any of the viral structural and NS proteins, elements of the viral RNA, and host proteins and membranes that participate in the viral replication cycle can be considered a virulence factor, since defects in the factor or its absence in the cell may lead to the virus' inability to replicate and cause disease in the host species. Table 1 lists viral and host factors that might be involved in virulence. Some of the factors listed in Table 1 have been shown to play a role in virulence either in the FMDV system or in other picornaviruses (295). It is beyond the scope of this review to examine the roles of all of these factors, so we will discuss only those that have been shown to be directly involved in virulence in susceptible animals.


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  		TABLE 1. Viral and host factors that may determine virulence

 
It has been recognized for many years that viral receptors play a role in tissue tropism and disease pathogenesis (112, 151, 429, 476). In "Early interactions: adsorption, penetration, and uncoating" above, we discussed the viral integrin receptors that have been identified and the ability of FMDV to utilize alternative receptors. It is known that virus which has had the RGD sequence of the VP1 G-H loop either mutated or deleted cannot replicate in tissue culture or cause disease in animals (268, 297, 309). A type O virus variant adapted to utilize the HS receptor in vitro (see "Early interactions: adsorption, penetration, and uncoating" above) has shown the importance of the virus-integrin interaction in vivo. This virus was shown to be relatively avirulent in cattle, while the wild-type virus, which required only integrin receptors to initiate infection in vitro (334), was virulent in bovines (409). More interestingly, two bovines inoculated with large amounts of the HS binding virus eventually showed signs of FMD. Virus isolated from these animals could only utilize the integrin as a receptor and had lost the ability to interact with HS in vitro (334, 409). It is not clear why FMDV should need three integrin receptors to cause disease or whether it utilizes all or some the integrins in the susceptible hosts. We have recently shown in tissue culture that different serotypes of the virus exhibit altered efficiencies of integrin utilization (144). It is possible that the virus uses different receptors during various stages of the disease. While it appears that the disease process in susceptible animals is mediated by the virus-integrin interaction, a type C virus containing an RGGD sequence has been isolated from a bovine which was not protected from virus challenge following immunization with an experimental peptide vaccine (447, 448). In addition, a tissue culture-adapted type C virus with a genetically engineered RGG sequence, which was unable to bind to heparin, was able to infect cells expressing both HS and integrin receptors and cells which do not express FMDV integrin receptors and HS (26, 27). The ability of these two viruses to cause disease in susceptible animals has not been demonstrated. More interestingly, a tissue culture-adapted derivative of a type O1 virus, isolated in China, was able to replicate in tissue culture in both an integrin- and HS-independent manner and to cause mild disease in pigs (490). Thus, there is a possibility that nonintegrin receptors may be involved in disease pathogenesis. However, with the exception of the virus isolated after challenge from the peptide-vaccinated bovine (447, 448), no natural isolate of FMDV which does not contain the RGD sequence within the VP1 G-H loop has been identified.

Lpro was shown to be a virulence determinant based on experiments with animals with genetically engineered type A12 virus with Lpro deleted (leaderless) (98, 296, 356). It was thought that this virus would be less virulent than the wild-type virus, since the lack of Lpro would lead to the inability of the virus to cleave eIF4G and shut off cellular protein synthesis. While this virus replicated at only a slightly lower rate than wild-type virus in BHK-21 cells (356), it was markedly avirulent when injected into cattle and pigs and was unable to spread to cohoused animals (98, 296). The mechanism of attenuation of leaderless type A12 virus was examined by aerosol exposure of cattle. Wild-type-infected cattle had histologically altered respiratory bronchioles and virus-specific ISH signals in bronchioles by 24 h, and by 72 h they developed clinical disease, including fever and vesicles on the feet and positive ISH signals in epidermal sites corresponding to visible lesion development. In contrast, cattle infected with leaderless virus showed no clinical disease at 72 h and no pulmonary changes at either 24 or 72 h. These animals had only limited positive virus-specific ISH signals in respiratory bronchioles by 24 h and had no evidence of lesions or ISH signals in epithelial tissue by 72 h (74). Thus, the leaderless virus did not appear to replicate well at the site of primary infection and was not able to spread to other sites within the host. It was subsequently shown that infection with wild-type or leaderless virus induced the synthesis of alpha/beta interferon (IFN-{alpha}/ß) mRNA both in tissue culture (96, 99) and in lung mononuclear cells from aerosol-exposed cattle (71), but in tissue culture, IFN activity was detected only in leaderless-virus-infected cells (96, 99). The latter observation can be attributed to the inability of the leaderless virus to inhibit host translation, including IFN synthesis, and the production of IFN within the infected animal probably inhibited initial amplification and spread of the virus. In contrast, wild-type virus infection blocks capped IFN mRNA translation, allowing the virus to rapidly spread to neighboring cells and systemically prior to the induction of the adaptive immune response.

The role of the 3A protein in viral virulence was demonstrated during studies of the FMDV isolate responsible for an outbreak in Taiwan in 1997 (designated O/Taw/97) (see below). This outbreak was unusual in that only pigs, and not cattle, were affected, and the disease had an unusually high mortality rate in pigs (143, 216). Molecular characterization of the virus revealed that it contained a 10-codon deletion in the C-terminal half of the 3A protein (47). The location of this deletion was similar to that of a 19- to 20-codon deletion found in FMDV passaged in chicken embryos. This virus also exhibited reduced virulence in bovines (172, 410). The role of this deletion in the bovine-attenuated phenotype was confirmed by using reverse genetic analysis (47), and an analysis of viruses circulating in the region for the last 30 years suggested that in addition to the deletion, mutations in the 3A protein in the region surrounding the deletion may also be responsible for the observed phenotype (254). The molecular basis for the porcinophilic phenotype appears to be related to a reduction in viral RNA synthesis, which is manifested to a greater degree in bovine cells than in swine cells (345). The 3A proteins from either the porcinophilic or a bovine-virulent isolate colocalized to RNA replication complexes in either bovine or porcine cells and also caused a disruption of the Golgi apparatus (345). However, as yet, there is no clear picture as to why the 3A deletion should affect FMDV replication in bovine cells more than in swine cells. Nunez and coworkers have also shown that a single amino acid change in the 3A protein was responsible for adaptation of FMDV to guinea pigs; however, the mutation was located in a different region than the deletion associated with the porcinophilic phenotype (342).

It has been suggested that IRES elements, and possibly the host factors that bind to them, affect the pathogenicity and virulence of other picornaviruses (152, 238, 285, 324, 344, 436). In FMDV, a virus rescued from persistently infected BHK-21 cells had two mutations within the IRES, which the authors suggest might have resulted in increased virulence of the virus in tissue culture (292). Some recent preliminary results suggest that the FMDV-specific IRES-binding protein ITAF45 may play a role in virus virulence by controlling virus tropism within tissues of susceptible species (V. O'Donnell, E. Pilipenko, E. Viktorova. R. Roos, and P. Mason, Abstr. 12th Meet. Eur. Study Group Mol. Biol. Picornaviruses, abstr. K11, 2002).

Host Response

The virus elicits a rapid humoral response in either infected or vaccinated animals. Virus-specific antibodies protect animals in a serotype-specific manner against reinfection, or against infection in the case of vaccination, and protection is generally correlated with high levels of neutralizing antibodies (reviewed in references 306 and 415). The response is directed to epitopes on the three external structural proteins (see "Antigenic variation" above), and good protective immunity is apparent between 7 and 14 days after either infection or vaccination. In cattle, the immunoglobulin G1 (IgG1) response predominates over IgG2 (86, 326, 417), and antibody, including IgA, can be detected in upper respiratory secretions early in infection (415, 417). The neutralization of virus within the host may occur by mechanisms similar to those occurring in in vitro neutralization; however, there is a suggestion that macrophages may play a role in clearing the virus from the infected animal by phagocytosis of opsonized virus (306, 307, 383).

The role of cellular immunity in the protection of animals from FMD is still a matter of some controversy. Specific T-cell antiviral responses, involving CD4+ and CD8+ cells, have been observed in cattle and swine following either infection or vaccination (36, 94, 104, 166, 173, 412), and it has been suggested that cell-mediated immunity is involved in clearance of virus from persistently infected animals (see below) (94, 224). The induction of anti-FMDV antibody correlates with a lymphoproliferative response in cattle and swine (104, 412) and is T-cell dependent in mice (105). A recent study of the early acute phase of FMDV infection of swine, prior to the detection of antibody, demonstrated a transient lymphopenia by 2 days after infection involving CD4+, CD8+, and CD4+/CD8+ T cells, which does not appear to be related to either infection of T cells or apoptosis and thus may be caused by alteration of lymphocyte trafficking (36). In addition, T-cell function, as measured by response to mitogens, is either reduced or eliminated (36). Both the number of lymphocytes and the altered T-cell function return to normal levels by 4 days after infection. These results suggest that T cells play a role in virus protection and that the reduction of both T-cell numbers and function enhances viral pathogenesis by allowing the virus to spread within the host, leading to increased viral shedding into the environment. In addition, immunization of both cattle and swine with a replication-competent human adenovirus 5 (Ad5) vector expressing the P1 capsid precursor did not result in the generation of virus-specific neutralizing antibody in the serum but partially protected animals from FMDV challenge (423, 424). The swine developed FMDV-specific T-cell responses (423), but these assays were not performed on the immunized cattle. These results may further indicate a role for cellular immunity in protection from FMDV infection; however, it also possible that innate immune responses may be responsible for the protection seen in these studies.

There has been increasing interest recently concerning the role of the innate immune response of the host to both FMDV infection and vaccination. A number of studies have shown that IFN-{alpha}, -ß, and -{gamma} may be involved in the host defense against FMDV infection (7, 71, 96, 99, 488) (see "Antiviral approach" below). In addition to the IFNs, other cytokines may also play a role in the host response. In studies of swine which were immunized with a conventional FMD vaccine, it was shown that vaccinated pigs did not appear to exhibit a systemic inflammatory response, but chemotactic activity of plasma on peripheral blood leukocytes increased within the first week after immunization (383). Furthermore, in pigs that either were only vaccinated or were vaccinated and challenged, levels of interleukin-6 (IL-6), IL-8, and IL-12 in plasma increased after vaccination and/or challenge, suggesting monocyte/macrophage activation (29). Although the levels of IL-6 and IL-8 did not appear to be related to protection of pigs upon challenge, IL-12 levels were higher in vaccinated pigs, which were protected from contact challenge, suggesting a role for cytokine-induced monocytic cell activity in protection from acute-phase disease (29).

Carrier State

Following the acute phase of FMDV infection in ruminants, some animals may experience a long asymptomatic persistent infection. In addition, animals which have been successfully vaccinated may also become persistently infected if exposed to infectious virus. These animals are referred to as carrier animals, and the carrier state is a complication which can occur during outbreak situations. In this section we briefly discuss what is known about the carrier state. For more in-depth information, a number of excellent reviews on this topic have been written (7, 415, 416).

Van Bekkum and colleagues first demonstrated that live FMDV could be recovered from esophageal-pharyngeal fluids of cattle during the convalescent phase of FMD (464). Currently, carrier animals are defined as those from which live virus can be isolated at 28 days, or later, after infection (445). In domestic cattle, the carrier state can last as long as 3.5 years, and it has also been identified in sheep and goats but not in pigs (7). African buffalo have been reported to carry live virus for up to 5 years (106), and other cloven-hoofed wildlife may become carriers (see reference 7 and references therein). The number of carrier animals in a population depends on the species, the incidence of infection, and the immune state of the herd (i.e., vaccinated or not vaccinated). In the African buffalo, the carrier rate can be as high as 50 to 70% in the field (106), and rates in cattle and sheep can vary widely, from 15 to 50% (7). In general, the titer of virus in the esophageal-pharyngeal fluids of carrier animals is low, and virus is not consistently recovered from individual animals. Currently, virus isolation from esophageal-pharyngeal fluids is the most sensitive method to detect carrier animals, but reverse transcription-PCR (RT-PCR) assays are being developed to attempt to increase sensitivity. The recovered virus probably originates in the pharynx, which appears to be the target region for persistent infection in cattle (7, 329, 489).

The role of carrier animals in the spread of virus in the field is still controversial. The only direct evidence is that of transmission from African buffalo to cattle during outbreaks in Zimbabwe in the late 1980s and early 1990s (120). In addition, transmission from buffalo to cattle has been obtained experimentally (121), and it has recently been proposed that such transmission might occur through sexual contact (35). There has been no experimental evidence to date indicating that carrier cattle or sheep can transmit virus to uninfected animals. The presence of live virus in esophageal-pharyngeal fluids, however, does make this a real possibility. In addition, the long persistence and replication of the virus in the host animals can lead to genetic variation in the field, possibly being responsible for the generation of new viral variants (167, 411, 457).

The mechanisms for the establishment and maintenance of the carrier state are not well understood, since persistence can occur in animals exposed to virus after either acute disease or vaccination. It does appear that the immune status of the animal probably controls the level of virus replication (7). Alexandersen and colleagues (7) have proposed two mechanisms for the development of persistence in the pharynx. One suggests that FMDV can infect immune system cells, such as macrophages, or other immunologically privileged sites, leading to evasion of the immune response. Baxt and Mason (42) examined viral replication in porcine peripheral blood macrophages and found that virus can infect such cells only when presented as an immune complex, presumably by Fc receptor-mediated adsorption. Furthermore, the infection was abortive and did not lead to the production of new infectious virus. In a more recent study, Rigden and colleagues (384) found that porcine alveolar macrophages also were not able to support viral replication; however, the virus bound to macrophages in the absence of specific antibody. In addition, virus appeared to be internalized by phagocytosis but remained infectious for at least 12 h. The second mechanism proposes that the virus exploits the host response to provide favorable intracellular conditions for long-term persistence, possibly by utilizing cytokine signaling. Studies on the innate immune response (see "Host response" above) should help to define the mechanisms involved in this phenomenon and possibly help in the development of methods to eliminate persistence.


   DISEASE OUTBREAKS AND CONTROL MEASURE