INTRODUCTION

Top Previous Next References

Because Q fever is rarely a notifiable disease, the incidence of human Q fever cannot be assessed in most countries. Current epidemiological studies indicate, however, that Q fever should be considered a public health problem in many countries, including France, the United Kingdom, Italy, Spain, Germany, Israel, Greece, and Canada (Nova Scotia), as well as in many countries where Q fever is prevalent but unrecognized because of poor surveillance of the disease. Q fever remains primarily an occupational hazard in persons in contact with domestic animals such as cattle, sheep and, less frequently, goats. Persons at risk from Q fever include farmers, veterinarians, abattoir workers, those in contact with dairy products, and laboratory personnel performing Coxiella burnetii culture and more importantly working with C. burnetii-infected animals. However, there has been an increase in reports of sporadic cases in people living in urban areas after occasional contact with farm animals or after contact with infected pets such as dogs and cats.

C. burnetii infection in humans usually is asymptomatic or manifests as a mild disease with spontaneous recovery. However, Q fever may lead to serious complications and even death in patients with acute disease, especially those with meningoencephalitis or myocarditis, and more frequently in chronically infected patients with endocarditis. Patients at risk from chronic Q fever include persons with previous cardiac valve defects and to a lesser extent immunocompromised hosts and pregnant women. Q fever during pregnancy has been associated with abortion, premature birth, and low weight in newborn babies.

The clinical manifestations of Q fever may be so variable that the disease is often diagnosed only if it has been systematically considered. However, when evoked, a definite diagnosis of the disease is easy and remains based upon serology, with phase I and phase II antibodies distinguishing acute from chronic disease. However, cell culture systems (especially the shell vial method) have led to the more frequent isolation of C. burnetii from human sources.

The possibility of studying larger series of clinical C. burnetii strains by molecular biological techniques has improved genetic and antigenic characterization of the bacterium and helped to develop a better understanding of the pathophysiology of Q fever. In particular, recent experimental data indicate that host factors rather than specific genetic bacterial determinants are the main factors influencing the clinical course of C. burnetii infection.

Tetracyclines are still the best for treating acute Q fever. Although the prognosis of Q fever endocarditis has recently been improved by the use of the combination of doxycycline with chloroquine, a definite antibiotic regimen has still to be established for treating Q fever endocarditis. Therefore, prevention of chronic Q fever in the "at-risk" population has to be considered. Effective vaccines exist for humans but are currently not available in most countries. We have recently reviewed diagnostic techniques for Q fever (106). The present review deals with recent advances in the microbiological, clinical, epidemiological, diagnostic, and therapeutic aspects of Q fever.

The term "Q fever" (for query fever) was proposed in 1937 by Edward Holbrook Derrick to describe febrile illnesses in abattoir workers in Brisbane, Queensland, Australia (75). In 1935, as the Director of the Laboratory of Microbiology and Pathology of the Queensland Health Department at Brisbane, he was invited to investigate an outbreak of undiagnosed febrile illness among abattoir workers in Brisbane. Since sporadic cases of the illness continued to occur regularly, he first carefully described the disease. He then attempted to isolate the etiological agent of the disease by inducing a febrile illness in guinea pigs. However, he did not succeed in isolating or even visualizing the etiological agent and speculated that the Q fever agent was a virus. The probable rickettsial origin of the disease was hypothesized by Macfarlane Burnet and his associate Mavis Freeman, to which Derrick had sent some infectious material. They reproduced the disease in guinea pigs and also in other animals including mice and monkeys. Examining hematoxylin-and-eosin-stained spleen sections from infected mice, Burnet and Freeman observed intracellular vacuoles filled with granular material, whereas staining by Castaneda's method or Giemsa allowed visualization of numerous small rods which appeared rickettsial in nature (45). With these results, Derrick and his collaborators investigated the epidemiology of the disease, especially the potential role of an arthropod vector. They concluded that wild animals were the natural reservoir of Q fever, with domestic animals being a secondary reservoir, and that the disease may be transmitted by ticks or other arthropods.

In 1935, and independently of Derrick's work, Gordon Davis, at the Rocky Mountain Laboratory in Hamilton, Mont., was investigating the ecology of Rocky Mountain spotted fever. Ticks collected in Nine Mile, Mont., were allowed to feed on guinea pigs, and a febrile illness was established in some animals (70). However, the observed symptoms in these animals, including a lack of marked testicular swelling, were not suggestive of Rocky Mountain spotted fever. In addition, the disease could be transmitted to uninfected guinea pigs by intraperitoneal inoculation of blood collected from infected animals, and the etiological agent could not be grown in axenic media. In 1936, Herald Rea Cox joined Davis at the Rocky Mountain Laboratory to further characterize the "Nine Mile agent." Burnet and Freeman, as well as Davis and Cox, demonstrated that the etiological agent was filterable and displayed properties of both viruses and rickettsiae (63, 70). A major advance was obtained in 1938, when Cox succeeded in propagating the infectious agent in embryonated eggs (64).

The connection between the groups in Montana and Brisbane arose when a laboratory-acquired Q fever infection occurred in the Rocky Mountain Laboratory in 1938. Rolla Eugene Dyer, Director of the National Institutes of Health, went to Hamilton to verify the possibility of growing the Nine Mile agent in eggs. He then became infected with the organism that the laboratory was working with. A febrile illness was reproduced in guinea pigs inoculated with Dyer's blood, and rickettsiae were identified in spleen samples from the infected animals. Also, cross-immunity was demonstrated between microorganisms isolated from Dyer's blood and the Nine Mile agent. Dyer then established a definitive link between the Nine Mile agent and the Australian Q fever agent. Burnet sent him some spleen samples which had been removed from mice infected with the Q fever agent. After inoculation of the Q fever agent into guinea pigs, Dyer demonstrated that such animals were protected from a new challenge with the strain isolated from his blood. Such cross-immunity was highly indicative that the Q fever agent, Dyer's blood isolate, and the Nine Mile agent were in fact isolates of a single microorganism. The etiological agent of Q fever was first named Rickettsia burnetii. However, in 1938, Cornelius B. Philip proposed the creation of a new genus called Coxiella and the renaming of the etiological agent as C. burnetii, a name which honours both Cox and Burnet, who had identified the Q fever agent as a new rickettsial species.

C. burnetii is an obligate intracellular, small gram-negative bacterium (0.2 to 0.4 µm wide, 0.4 to 1 µm long). Although possessing a membrane similar to that of a gram-negative bacterium, it is usually not stainable by the Gram technique. The Gimenez method (120) is usually used to stain C. burnetii in clinical specimens or laboratory cultures. Since C. burnetii cannot be grown in axenic medium and has long been recovered from ticks, it has been classified in the Rickettsiales order, the Rickettsiaceae family, and the Rickettsiae tribe together with the genera Rickettsia and Rochalimaea (396). However, recent phylogenetic investigations, based mainly on 16S rRNA sequence analysis, have shown that the Coxiella genus belongs to the gamma subdivision of Proteobacteria (347, 393, 394, 410), with the genera Legionella, Francisella, and Rickettsiella as its closest relatives (Fig. 1). Bacteria of the Rickettsia genus belong to the alpha-1 subgroup of Proteobacteria, whereas species of the genus Rochalimaea have recently been reclassified within the genus Bartonella and the family Bartonellaceae and belong to the alpha-2 subgroup of Proteobacteria.

View larger version (16K): [in this window] [in a new window]   FIG. 1.   Phylogenetic tree showing the relationships of C. burnetii to other species belonging to the Proteobacteria. The tree was constructed by the neighbor-joining method with 16S rRNA gene sequences.

C. burnetii expresses a low degree of genetic heterogeneity among strains by DNA-DNA hybridization (384). However, when DNA from 38 C. burnetii isolates was examined by restriction fragment length polymorphism (RFLP) analysis, six genomic groups (I to VI) were described (146). Later, analysis of NotI and SfiI C. burnetii DNA restriction fragments by pulsed-field gel electrophoresis (PFGE) resulted in the characterization of four different DNA fragment patterns representing isolates from genomic groups I, IV, V, and VI (140). Genomic variation in C. burnetii has been more recently further characterized (359, 402). By using PFGE and NotI, 16 additional restriction groups were identified among 80 C. burnetii isolates collected worldwide. By using different banding patterns and the unweighted pair group method with arithmetic mean (UPGMA), a phylogenetic tree, involving strains from various sources including goats, sheep, and humans with acute or chronic Q fever, was constructed. The dendrogram showed that most French isolates (groups 12 to 16) formed a separate cluster from the Russian isolates (groups 10 and 11). The other isolates were located in one main cluster, with large genetic distances, ranging from 7 to 58%, between the different groups. Thus, considerable heterogeneity was observed among C. burnetii total genomic RFLP patterns, an unexpected result. Restriction site redistribution may correspond to significant chromosomal rearrangements such as translocations, inversions, insertions, or deletions.

The genome size of C. burnetii Nine Mile strain is 2.1 Mb (403); the genome size is highly variable among different C. burnetii strains, ranging from 1.5 to 2.4 Mb (403). A locus structurally and functionally suggestive of the origin of replication (oriC) of the C. burnetii genome has been isolated (52). However, it is considered only putative at this time because, although functionally expressed in plasmids in an Escherichia coli host, it did not initiate DNA synthesis in the C. burnetii chromosome (354). The inability to localize origin function by standard methods could well be related to the fact that C. burnetii probably has a linear rather than a circular chromosome and thus may not have conventional bidirectional replication (403). A physical macrorestriction map of C. burnetii Nine Mile phase I was constructed by Willems et al. (402, 403). Twenty-five DNA fragments were distinguished by PFGE after restriction of total DNA with NotI. Such a physical map may serve as a basis for constructing a genetic map and comparing gene loci and genetic organization among different C. burnetii isolates (402, 403). The inability to clone the ends of the 240- and 7.3-kb fragments from C. burnetii Nine Mile phase I strain into a compatible vector, after excision of these fragments from a PFGE gel and digestion with a second enzyme (402, 403), favored the hypothesis that the chromosome of this organism may be linear.

Eleven chromosomal genes have been cloned and expressed in E. coli: gltA, the citrate synthase gene (139); sodB, the superoxide dismutase gene (141); htpA, the 14-kDa heat shock protein gene (385); htpB, the 62-kDa heat shock protein gene (385); omp, a 27-kDa surface antigen gene (145); pyrB, the aspartate carbamoyl transferase gene (151); qrsA, a sensor protein gene (237); dnaJ, a heat shock protein gene (424); mucZ, the capsule induction protein gene (425); serS, the seryl-tRNA synthase gene (402); and algC, the phosphomannomutase gene (402). C. burnetii gene sequences partially or completely available in the GenBank or EMBL databases include 23 chromosomal sequences and 17 plasmid sequences. Genes organized into operons have been described previously (151, 385). Genetic transformation has been achieved in C. burnetii (355). Plasmid pSKO(+)1000, containing a previously characterized C. burnetii autonomous replication sequence (354) cloned into a ColE1-type replicon encoding -lactamase, was introduced into C. burnetii by electroporation. Transformants stably maintained the pSKO(+)1000 bla DNA sequence in the chromosome, and the bla gene was expressed in C. burnetii during acid activation (355).

The C. burnetii genome comprises facultatively a 36- to 42-kb plasmid, whose function remains undetermined. The first to be described was the QpH1 plasmid (36 kb, one to three copies per cell) (313). The entire nucleotide sequence of this plasmid has been determined (361). Such a plasmid was found in genomic groups I, II, and III. A 39-kb plasmid was then characterized in genomic group IV (197). This plasmid, designated QpRS, was found in C. burnetii Priscilla, obtained from an aborted goat fetus. The QpRS plasmid was also found in human Q fever endocarditis isolates, all belonging to the genomic group IV. A third plasmid, of 42 kb, designated QpDG, was found in genomic group VI isolates, obtained from feral rodents (197). Finally, plasmidless C. burnetii isolates from Q fever endocarditis patients were found to contain DNA sequences with homology to the QpRS plasmid (316). This chromosome-integrated plasmid DNA fragment has recently been cloned and sequenced (402). Such strains corresponded to genomic group V. More recently, a new 33-kb plasmid has been found in a French strain isolated from an endocarditis patient and designated QpDV by Valkova and Kazar (380). These plasmids all share a 30-kb region (196), whereas plasmid type-specific regions have been characterized (235, 236).

C. burnetii displays antigenic variations similar to the smooth-rough variation in the family Enterobacteriaceae. Phase variation is related mainly to mutational variation in the lipopolysaccharide (LPS) (129-131). Phase I is the natural phase found in infected animals, arthropods, or humans. It is highly infectious and corresponds to smooth LPS. In contrast, phase II is not very infectious and is obtained only in laboratories after serial passages in cell cultures or embryonated egg cultures. It corresponds to rough LPS. Compared to phase I, phase II displays a truncated LPS and lacks some protein cell surface determinants (8). The sugar composition of LPS is also different in the two phases. LPS in phase I contains sugars such as L-virenose, dihydrohydroxystreptose, and galactosamine uronyl--(1,6)-glucosamine, which are lacking in phase II LPS (10, 132, 321, 322). Although the genetic evidence supporting phase variation in C. burnetii remains unsettled, large chromosomal deletions were demonstrated in the attenuated phase II C. burnetii Nine Mile strain (383). Recently, a NotI C. burnetii DNA restriction fragment common to all C. burnetii isolates has been cloned and sequenced (402). One open reading frame displayed significant homology to the algC gene encoding phosphomannomutase (PMM) in Pseudomonas aeruginosa. In the later species, PMM is involved in the synthesis of LPS and the surface polysaccharide alginate (423). P. aeruginosa with PMM activity corresponds to the smooth phenotype, whereas P. aeruginosa with no PMM due to mutations in algC corresponds to the rough phenotype. However, more experimental data are needed before making a connection between the P. aeruginosa alginate pathway and the C. burnetii LPS.

Genetic variability among different C. burnetii strains, as demonstrated by different RFLP-based genomic groups (146), specific plasmid regions (314), and LPS variations (130), were tentatively related to virulence. Genomic groups I, II, and III were associated with animal, tick, or acute Q fever human isolates, referred to as acute strains, whereas groups IV and V were associated with human Q fever endocarditis isolates, referred to as chronic strains. Group VI isolates, obtained from feral rodents in Dugway (Utah), were of unknown pathogenicity. QpH1 plasmid was found in genomic groups I, II, and III and thus was associated with acute C. burnetii strains, whereas QpRS plasmid was found in genomic group IV and was associated with chronic strains (197, 313, 314). Comparison of the various isolates for LPS variations, using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting, resulted in isolates being placed into groups similar to the genomic groups (130). These findings, however, were not confirmed in larger series of C. burnetii strains by using genomic RFLP analysis (360), plasmid typing with specific primers (348), or LPS analysis with specific monoclonal antibodies (421). Recent investigations suggest that predisposing host factors are more important than genomic strain variation in the explanation of the occurrence of acute or chronic Q fever diseases in humans (181, 351, 421). Moreover, recent data shows that genetic variation has an apparently closer connection with the geographical source of the isolate than with the clinical presentation.

LIFE IN THE HOST CELL

Top Previous Next References

Susceptible Cells

C. burnetii can be grown in vitro in a number of cell types, including mouse macrophage-like cells (including the P388D1 and J774 cell lines), fibroblast cells (including L929 cell line), and Vero cells (5, 21, 23, 127, 221, 287, 306). Embryonated eggs and laboratory animals such as mice and guinea pigs have been used extensively for in vivo propagation of C. burnetii. In humans (41, 124, 217, 281, 343) and animals (183, 219), however, monocytes-macrophages are the only known target cells. When infection occurs via the respiratory route, alveolar macrophages in the lungs are supposedly the primary cells to be infected during acute Q fever. Kupffer cells in the liver are also susceptible and may be infected via the bloodstream or from the digestive route; only a few patients are supposedly infected via the digestive route.

Microorganisms, including intracellular microorganisms, use specific eukaryotic receptors such as integrins to invade host cells. Many of these agents, including Legionella pneumophila and Mycobacterium tuberculosis, use CR3 as a receptor for entry into phagocytes. C. burnetii phase II enters human monocyte-derived macrophages by engaging the CR3 receptor (233). In contrast, the infectious phase I C. burnetii blocks entry via the CR3 receptor and binds to human monocytes via the complex of LRI (leukocyte response integrin, _v3) and IAP (integrin-associated protein) (233). The natural phase I C. burnetii is only poorly internalized by monocytes and macrophages but can survive within these cells. In contrast, phase II C. burnetii is readily internalized by monocytes and macrophages (233) but is rapidly killed via the phagolysosomal pathway. The receptor used by each C. burnetii phase for entry into monocytes and macrophages is probably critical for its survival within these phagocytic cells.

After passive entry into the host cell, C. burnetii is internalized within eukaryotic cells in phagosomes, which fuse rapidly with lysosomes to form phagolysosomes. The early phagolysosomes fuse progressively to form a large unique vacuole (127). The lysosomal origin of C. burnetii-containing vacuoles is evidenced by the presence within this compartment of lysosomal markers such as proton-ATPase, acid phosphatase, cathepsin D, and the lysosomal glycoproteins LAMP-1 and LAMP-2 (143). C. burnetii has adapted to the phagolysosomes of eukaryotic cells (127), multiplying in acidic vacuoles (pH 4.7 to 5.2) (5, 221). Such acidity was stable in L929 fibroblast cells persistently infected with C. burnetii Nine Mile over a period of 6 months (221). C. burnetii is an acidophilic bacterium whose metabolism is enhanced at acidic pH. Its multiplication can be stopped by raising the phagolysosomal pH using lysosomotropic agents such as chloroquine (5, 289). The only microorganism known to have similarly adapted to such an acidic compartment of monocytes-macrophages is the amastigote form of Leishmania species (14). Acidity is needed for C. burnetii to assimilate nutrients necessary for its metabolism, including synthesis of nucleic acids (53) and amino acids (128, 144, 426). For example, the penetration of glutamate (128) and proline (144) into C. burnetii was shown to be pH dependent. It has been hypothesized that the protein encoded by the qrsA gene, classified as a sensor protein, may be involved in the adaptation mechanism of C. burnetii to the acidic milieu of the phagolysosomes (402).

C. burnetii displays a complex intracellular cycle, leading to the formation of spore-like forms (227). McCaul and Williams (228) have proposed the terms "small-cell variant" (SCV) and "large-cell variant" (LCV) to differentiate the two C. burnetii cell forms observed in persistently infected cells (48, 251, 388, 398). SCVs and LCVs correspond to different intracellular development stages of C. burnetii (227). They can be differentiated by their morphology, size, peptidoglycan content, and resistance to osmotic pressure (9, 228, 229). SCVs are 204 by 450 nm in size and rod shaped, with densely stained walls and electron-dense nucleoids. LCVs are up to 2 µm long, more pleomorphic, rounded, granular, sometimes with fibrillar cytoplasm and with dispersed nucleoid filaments. Heinzen and Hackstadt (142) characterized in SCV but not in LCV a 20-kDa DNA-binding protein (termed Hq1) with primary amino acid sequence similarities to the eukaryotic histone H1. Hq1 is supposed to control nucleoid structure in the different C. burnetii cell variants. Both LCVs and SCVs have a typical eubacterial gram-negative cell wall with two layers separated by the periplasmic space. However, a dense material fills the periplasmic space in SCVs. This material corresponds to proteins and peptidoglycan and may explain the increased resistance of SCVs to environmental conditions.

SCVs are metabolically inactive and resistant to osmotic pressure and correspond to the extracellular form of the bacterium. SCVs attach to the eukaryotic cell membrane to enter phagocytic cells. After phagolysosomal fusion, acid activation of the metabolism of SCVs may lead to the formation of LCVs. Thus, LCVs correspond to the metabolically active intracellular form of C. burnetii. A sporogenic differentiation has been characterized in LCVs, leading to the formation of resistant, spore-like forms of bacteria (227, 228). Both activated SCVs and LCVs divide by binary fission. This sporulation-like process in LCVs may be distinguished morphologically from the cell division process by the asymmetrical position of the septum, close to one of the LCV pole in the former process (227, 230). McCaul showed that DNA was present in both the spore and the mother cells, suggesting the presence of newly replicated chromosome on each side of the septum (227). The endogenous spore-like forms undergo further development to the metabolically inactive SCVs, which are then released from the infected host cell either by cell lysis or possibly by exocytosis. Physical or biochemical factors which may induce the sporulation-like process in C. burnetii are unknown. However, nutrient deprivation is commonly the signal for the initiation of sporulation in sporulated gram-positive bacteria such as Bacillus or Clostridium species. The extracellular forms of C. burnetii resist environmental conditions such as desiccation and low or high pH and chemical products such as ammonium chloride, disinfectants such as 0.5% sodium hypochlorite, and UV radiation (20, 324). Only exposure to high concentrations of formalin (i.e., 5%) for a prolonged time (at least 24 to 48 h) may allow killing of C. burnetii (324). We have tested the resistance of C. burnetii to formalin in our laboratory (184). A C. burnetii inoculum of 10⁸ IU/ml was exposed overnight to different concentrations of formalin and then subcultured in uninfected HEL cells grown in shell vials to determine the residual viable bacterial inoculum. Undetectable viability in shell vial cultures (i.e., less than 1 microorganism per ml) was obtained only after overnight incubation in 10% formalin.

C. burnetii has the ability to induce persistent infections both in humans and animals (22). Chronically infected animals shed bacteria in feces and urine. Such persistent infections are mostly asymptomatic but may occur in pregnant females in the form of massive contamination of the placenta with C. burnetii, leading to abortion or low fetal birth weight. C. burnetii also induces chronic infections in humans, especially in immunocompromised patients or during pregnancy (350). In vitro, C. burnetii-infected eukaryotic cells may be maintained in persistent cultures for several months or even years, provided that the cell incubation medium is changed regularly (5, 21, 23, 46, 306). The slow intracellular multiplication of C. burnetii, with a doubling time of ~20 h, which is similar to that of eukaryotic cells, may partly explain why the bacterium does not damage infected cells despite prolonged infection. Roman et al. (306) have proposed a model for persistent cell infection in which when an infected cell divides, one daughter cell receives the unique C. burnetii vacuole whereas the other remains uninfected. The finding that uninfected cells are still present in infected cell cultures after several months of infection is compatible with such model.

EPIDEMIOLOGY

Top Previous Next References

Mode of Transmission

The aerosol route (inhalation of infected fomites) is the primary mode of human contamination with C. burnetii (212). Contamination by C. burnetii aerosols may occur directly from parturient fluids of infected animals, which may contaminate newborn animals, placenta, or wool (365). C. burnetii is very resistant to killing in nature and may survive for several weeks in areas where animals have been present; the organism may also be spread by the wind (220, 367). Thus, Q fever may occur in patients without any evident contact with animals. We have recently infected amoebae with C. burnetii Nine Mile and demonstrated that C. burnetii remained viable in amoebae for 6 weeks, as demonstrated by the ability to subculture bacteria in HEL cells (184). Thus, amoebae may serve as a reservoir for C. burnetii in nature, as has been demonstrated for Legionella species (308), and could represent a protected niche in the wild.

Ingestion (mainly drinking raw milk) is probably a minor factor in the transmission of C. burnetii (30, 102, 177) and is now even a point of controversy. Person-to-person transmission is probably extremely rare. Although infrequent, sporadic human Q fever cases have occurred following contact with an infected parturient woman (in an obstetrician who performed an abortion on the pregnant woman) (297), via transplacental transmission resulting in congenital infections (350), during autopsies (135, 200), via intradermal inoculation (13a), or via blood transfusion (13b). Although C. burnetii has been isolated from arthropods, mainly ticks, arthropod-borne transmission of Q fever in humans is unlikely to be significant (25, 92). However, we have reported two cases of coinfection with Rickettsia conorii and C. burnetii diagnosed in patients living near Montpellier and suspect that they were caused by a tick bite (160). Sexual transmission of C. burnetii was demonstrated experimentally in infected mice (377); however, this mode of transmission remains to be established in humans and wild animals.

Q fever is a zoonosis with a worldwide distribution. The reservoir is large and includes many wild and domestic mammals, birds, and arthropods such as ticks (20). Babudieri, in a large review, reported that C. burnetii was detected in virtually all the animal kingdoms (20). However, domestic ruminants represent the most frequent source of human C. burnetii infection (212). Animals are often chronically infected but do not experience symptoms of C. burnetii infection. The uterus and mammary glands of females are sites of chronic C. burnetii infection (20). Shedding of C. burnetii into the environment occurs mainly during parturition; over 10⁹ bacteria per g of placenta are released at the time of delivery (20). Milk may also contain large amounts of C. burnetii, although this is probably a minor route of Q fever acquisition.

Livestock. Early studies have shown that coxiellosis in livestock is widespread. However, most seroepidemiological studies in livestock were performed in the 1960s, and in most areas the real prevalence of C. burnetii infection in such animals is currently unknown. Recent seroepidemiological studies available for cattle have indicated that C. burnetii antibody seroprevalence in these animals is higher nowadays than 20 or 30 years ago (175, 179). Cattle, goats, and sheep are considered the primary reservoirs from which human contamination occurs. Infected mammals shed C. burnetii in their urine, feces, milk, and birth products, from which humans may be contaminated. Although C. burnetii infection is usually not harmful in infected animals, abortions in sheep and goats (261, 390) and lower birth weight and infertility in cattle, have been associated with chronic C. burnetii infection (149, 319). Epidemiological data indicate that dairy cows are more frequently chronically infected than sheep and thus may represent the most important source of human infection. Studies performed in California in 1951 showed that when imported into an area of endemic infection, 40% of uninfected cows became C. burnetii infected within 6 months, as evidenced by seroconversion to C. burnetii antigens (156). Prolonged detection of specific antibodies in the sera of infected dairy cows and the longer shedding of C. burnetii in the milk of these animals compared to sheep have been reported (20, 38, 95, 179). Goats share a predisposition with dairy cows to remain chronically infected (179). Transmission of C. burnetii to humans from infected goats may be significant in areas where they replace cows as a source of milk.

Pets. Cats and dogs may represent reservoirs of C. burnetii. Dogs may be infected by tick bite (198), by consumption of placentas or milk from infected ruminants, and by the aerosol route. C. burnetii infection in parturient dogs may lead to the early death of pups (44). The possibility of human Q fever acquired from infected dogs has been reported (44, 185, 205, 302), and we have isolated two strains from the uterus of Canadian dogs (282). Human Q fever cases were described in Nova Scotia after contact with parturient cats (171, 180, 205, 210); these included 12 patients who developed a febrile illness 2 weeks after playing poker in a room where a cat had given birth to kittens (171). All the infected persons had handled the cat or its litter, and specific antibodies were demonstrated in the cat serum.

Ticks and other arthropods. In many animals, a transient bacteremia with C. burnetii occurs early after infection. Thus, ticks have the opportunity to become infected with C. burnetii during feeding. Over 40 tick species are naturally infected with C. burnetii, including Rhipicephalus sanguineus found in dogs (198), Haemaphysalis humerosa found in the marsupial bandicoot (334), Amblyomma triguttatum found in kangaroos (278), and several ticks collected in different parts of the United States. These included Dermacentor occidentalis, Amblyomma americanum, Haemaphysalis leporis-palustris, Ixodes dentatus, and Otobius magnini (63, 65, 71). Experimental transmission of C. burnetii from infected to uninfected guinea pigs via tick bite has been performed with Ixodes holocyclus, Haemaphysalis bispinosa, and Rhipicephalus sanguineus (271, 335, 336). Experimental infection with C. burnetii has also been performed in Dermacentor andersoni (262). C. burnetii multiplies in the cells of the middle gut or stomach of infected ticks. These arthropods expel heavy loads of C. burnetii with their feces onto the skin of the animal host at the time of feeding. Infection of tick ovaries has been demonstrated and may lead to germinative infection of the offspring, allowing C. burnetii infection to persist in the tick population (20). C. burnetii organisms in ticks, as in mammals, are in phase I and thus are highly infectious. However, ticks are not considered essential in the natural cycle of C. burnetii infection in livestock (20), and animals which live in close contact have many other opportunities to become infected with C. burnetii. In contrast, ticks may play a significant role in the transmission of coxiellosis among the wild vertebrates, especially in rodents, lagomorphs, and wild birds (20, 179, 207). Anecdotal reports indicate that C. burnetii may be isolated from other arthropods including chiggers (20), lice (121), and flies (274). However, other extensive investigations of lice, fleas, mites, flies, mosquitoes, and other arthropods collected from cows, sheep, and rodents have not resulted in isolation of C. burnetii from these arthropods. The role of these arthropods in the natural cycle of C. burnetii remains unknown. The possibility of C. burnetii being transmitted to humans via a tick bite has seldom been reported (92), and human Q fever as opposed to other rickettsial diseases is rarely if ever an arthropod-borne disease.

Others. C. burnetii infection has been reported less frequently in a number of other domestic or wild mammals, including horses, rabbits, swine, camels, water buffalo, rats, and mice (20). A recent seroepidemiological study of rats in the United Kingdom has shown anti-phase II antibody seroprevalences ranging from 7 to 53% among wild brown rat populations (391). The authors hypothesized that wild rats may represent a major reservoir of C. burnetii from which domestic animals, especially cats, which are natural predators of these animals, may become contaminated. Birds may also be infected, and C. burnetii was isolated from pigeons, chickens, ducks, geese, and turkeys (20). Humans may acquire Q fever from infected domestic poultry by consumption of raw eggs or inhalation of infected fomites. Anti-C. burnetii antibodies have been found in snakes and tortoises in India, but C. burnetii has not been isolated from these animals (20).

Q fever has been described in almost every country (20), with New Zealand remaining an exception (148). In most countries, Q fever is not included in the list of nationally notifiable diseases. Thus, its epidemiology may only be extrapolated from investigations of defined outbreaks, from serosurveys conducted in humans or in animals in some areas, or from data obtained from public health laboratories or reference laboratories for rickettsial diseases. In this review, only recently reported epidemiological situations are addressed.

France. Although Q fever cases are reported to occur sporadically in various parts of France (365), the disease is diagnosed predominantly in the south, near Marseille. However, this probably reflects the influence of the presence of the National Reference Center (NRC) for rickettsial diseases in this area rather than a higher prevalence of the disease. In a serosurvey reported by Tissot Dupont et al. in 1992 (365) with 942 serum samples collected from blood donors in Marseille, specific anti-C. burnetii antibodies were detected in 38 samples, corresponding to a seroprevalence of 4.03 per 100 inhabitants. The highest prevalence reported in France was in a rural population in the Alps, with specific antibodies detected in 30% of the village population (43). Of the 22,496 serum samples tested at the NRC from January 1982 to December 1990, anti-C. burnetii immunoglobulin G (IgG) phase II was detected in 5,166 (23%) at titers of 1:25 and in 1,754 (7.8%) at titers of >1:200 (365). During the same period, 323 acute Q fever cases were diagnosed at the NRC. Among the 149 acutely infected Q fever patients whose occupation was known, only 9.4% were farmers and 29.8% lived in a rural area; contact with farm animals and ingestion of raw milk or unpasteurized cheese were rarely reported. However, among reported risk factors, contact with sheep or placenta from ewes or goats was predominant. Thus, Q fever is common in France. Since the rural population has decreased extensively over the last few decades, the disease is now often found in the urban population, often after occasional exposure to infected animals or potentially after ingestion of contaminated raw milk (365). Most cases are diagnosed in spring or early summer.

United Kingdom. From 1975 to 1995, 67 to 169 Q fever cases were reported annually to the Communicable Disease Surveillance Center by laboratories in England and Wales (364). This represents a stable incidence ranging from 0.15 to 0.35 case per 100,000 population per year. Among 641 Q fever cases reported between 1991 and 1995 in England, Wales, Northern Ireland, and the Channel Islands, Q fever most frequently involved adult men (485 cases versus 151 in women; sex ratio, 4:1) with a mean age of 46.2 years and occurred most frequently in May (364). Most cases were reported in Northern Ireland and southwestern England. In a series reporting 90 cases, endocarditis represented 11% of Q fever cases diagnosed (261). Q fever accounts for about 3% of all endocarditis cases in England and Wales (261).

Spain. From 1981 to 1985, 249 Q fever cases from different Spanish regions were serologically diagnosed at the Centro Nacional de Microbiologia, Virologia e Immunologia Sanitarias (357). Most cases were sporadic, although cases from two outbreaks (51 cases) were included in the study. These included 234 and 15 acute and chronic Q fever cases, respectively (6%), including 14 cases diagnosed as endocarditis. Most acute cases occurred in hospitalized patients with atypical pneumonia (75%) or a febrile illness (18%). Hepatic involvement was documented in 7.4 and 19% of patients with pneumonia or febrile illness, respectively. The majority of cases were in men (77.1%) between 15 and 44 years old, which supposedly reflected occupational risk.

Switzerland. Only 30 to 90 Q fever cases are reported annually to the Federal Office of Public Health in Switzerland. A large outbreak of Q fever occurred in the Val de Bagnes (Valais, Switzerland) in the autumn of 1983 (86). This outbreak was investigated after eight Q fever cases were diagnosed concomitantly at the Marigny hospital in patients who lived in the valley. Epidemiological investigation revealed that the outbreak had started 3 weeks after about 850 to 900 sheep descended from the alpine pasture and crossed several villages of the Val de Bagnes. Between October and December 1983, Q fever occurred in 21.1% of the more exposed population residing in villages situated in the lower part of the valley but in only 2.9% of the population of the higher villages away from the road crossed by the sheep. Altogether, Q fever was diagnosed serologically in 415 of the 3,036 inhabitants examined, including 240 men and 175 women. Most cases (224 of 415 [54%]) were asymptomatic, whereas more than 75% of the 191 patients with acute Q fever examined by physicians presented with prolonged fever, shivering, and headaches (84, 86). However, only 8 patients (4%) required hospitalization, and no Q fever endocarditis cases had been diagnosed in 1987 at the time of the outbreak. A high prevalence of antibodies to C. burnetii antigens was found (38%) in sera from 448 sheep examined.

Israel. Q fever is endemic in Israel. Between 1981 and 1990, 758 Q fever cases were reported to the Ministry of Health (415). A series of 34 patients with Q fever endocarditis was reported more recently (332). Patients were from 25 to 74 years old, and most were males (23 of 34 [68%]). The national incidence of Q fever endocarditis in Israel was estimated to be 3.5 cases per year or approximately 0.75 cases per 1 million population per year. Possible exposure to cattle or sheep was recorded in 10 patients (29%), 8 of whom lived in rural areas.

Greece. A study performed in 1990 by Alexiou-Daniil (7) in northern Greece demonstrated that 4.7% of 3686 patients with "atypical pneumonia" had antibodies against C. burnetii antigens. The high seroprevalences of anti-C. burnetii antibodies found in two villages in Crete (38.1% in 1987) encouraged physicians from the island to send sera to the National Reference Center of Parasitology, Zoonoses and Geographical Medicine in Heraklion (Crete) (372). From 1989 to 1993, 98 Q fever cases were diagnosed serologically by IFA. Most patients were men (72 of 98 [73.5%]). Patients aged 20 to 39 years and 80 to 89 years had a higher risk of acquiring Q fever. A seasonal occurrence was noted, with the majority of cases being diagnosed between January and June. Contact with animals or ingestion of unpasteurized milk or fresh cheese was recorded for 35.4% of the patients. Most patients presented with fever (91.7%) and respiratory symptoms (88.5%), whereas hepatitis was present in 52%. Interestingly, 11 patients (11.5%) presented with neurological symptoms and 2 (2.1%) had cutaneous rash.

Italy. A large outbreak of Q fever occurred in the summer and autumn of 1993 near Vicenza in northeastern Italy (326). The outbreak followed the crossing of populated areas near Vicenza by several flocks of sheep on their way to higher prealpine pastures. Whereas only 3 Q fever cases were officially reported in the province of Vicenza between 1983 and 1992, 58 cases were diagnosed serologically by the complement fixation test during the 5-month study period. Most cases were men (sex ratio, 2.8:1). The majority of patients presented with fever (100%), weakness (81%), headaches (76%), and chills (72%). Cough was recorded in 47% of patients, whereas abnormalities were present in 39 (81%) of the 48 chest X-rays performed. Hospitalization was necessary in 48% of patients. The only risk factor for acquisition of Q fever in this population was exposure to the migrating flocks of sheep. Of the 100 flocks investigated, 30 were found to be infected with C. burnetii, with seroprevalences ranging from 12 to 55% in a given flock.

Germany. Q fever is a notifiable disease in Germany, and 27 to 100 cases are reported annually (13). In May 1996, a Q fever outbreak occurred in Rollshausen and five surrounding towns in the district of Lohra (13, 194). In this rural area, two flocks of sheep (1,000 to 2,000 and 20 animals, respectively) had been kept near Rollshausen before the Q fever outbreak. Lambing occurred in December 1995 and January 1996. The Robert Koch Institute was invited to investigate the outbreak. A retrospective cohort study was conducted in Rollshausen residents who were older than 15 years of age. Sera from 200 inhabitants were tested by enzyme-linked immunosorbent assay for the presence of anti-C. burnetii antibodies. Of the 200 residents, 45 (23%) were considered to be infected with C. burnetii on the basis of clinical and/or serological investigation. The attack rates were similar in men and women and in the different age populations. Most patients suffered from fatigue (80%), fever (78%), malaise (76%), and chills (71%). All 35 symptomatic patients had pneumonia confirmed by chest X rays, and 4 (11%) were admitted to hospital. Living near the flocks of sheep was significantly associated with an increased risk of acquiring Q fever, and the predominant mode of contamination was considered to be by air. Anti-C. burnetii antibodies were found in the sera of 15 of 20 sheep from the largest flock investigated.

Russia. Official Russian statistics indicate that between 1957 and 1995, 11,058 Q fever cases were reported in 37 administrative territories, including 39% in Povolzhje, 31% in West Siberia, and 14% in central Chernozemje, mostly in the regions of Astrakhan, Novosibirsk, and Voronezh (309). However, Q fever is underreported in Russia because of diagnostic difficulties and insufficient laboratory equipment. As in other countries, cattle, sheep and goats are the main reservoirs from which humans become contaminated, especially at the time of parturition. Q fever cases acquired from goat and sheep reservoirs have been reported over recent years mainly in the European and Asiatic parts of Russia, including outbreaks in the regions of Novosibirsk, Voronezh, and Altai. Reports of goats as sources of Q fever in Russia have increased over recent years, mainly due to their increased number.

United States. The prevalence of C. burnetii infection in humans and animals is poorly defined in the United States. The first major Q fever outbreaks were reported in 1946 in packing houses in Amarillo, Tex. (371), and in Chicago, Ill. (328). Studies performed between 1947 and 1950 (27, 56, 371) showed that California was an area of endemic infection for Q fever. Between 1948 and 1977, a total of 1,169 human Q fever cases were reported to the Centers for Disease Control (67), including 785 (67%) from California. Fewer than 30 cases were reported annually between 1978 and 1986 (318).

Nova Scotia. Q fever was first reported in Nova Scotia in 1981 (204). The disease was unexpectedly diagnosed while investigating the usual causes of atypical pneumonia in this province of Canada (204). Marrie recorded 174 Q fever cases between 1980 and 1987 (208). The mean age of the 174 patients was 40.1 years (212), although most cases occurred in patients aged from 20 to 49 years. The sex ratio (men/women) was 2:1. Most Q fever cases recently reported were acquired from exposure to parturient cats (171, 180, 210). Exposure to infected dogs, wild hares, and deer has also been reported as a risk factor (44, 185, 207). Q fever cases are not seasonal in Nova Scotia (220). Eleven Q fever endocarditis cases were diagnosed between 1979 and 1993 (220) in Nova Scotia, which represents an incidence of 0.73 per million inhabitants per year. C. burnetii is the etiological agent of approximately 3% of all endocarditis cases in this province of Canada (213).

Japan. Serosurveys have shown that Q fever is endemic in animals in Japan (154, 241, 414, 420). C. burnetii was first isolated from a patient's blood in 1989 (253). In 1996, To et al. (368) retrospectively investigated the role of C. burnetii in children with "atypical pneumonia". Acute-phase sera collected between 1982 and 1983 in Gifu Prefecture from 58 children (from 2 to 10 years old) were tested for the presence of phase II C. burnetii antibodies and for the presence of C. burnetii DNA by a PCR-based assay. Specific IgM antibodies were found in 20 of 58 patients (34.5%) who were diagnosed with Q fever pneumonia, whereas IgG antibodies were found in 7 of 58 (12%). The PCR-based assay was positive in 23 of 58 patients (39.6%). In all patients, C. burnetii infection was confirmed by infection of mice with the patient serum samples. Since convalescent-phase sera were not available, diagnosis was essentially based upon PCR. In our experience, PCR-based methods to diagnose Q fever are not adequate for blood samples, and results obtained with such a technique should be interpreted with caution (282). In the same year, Nagaoka et al. (250) reported the isolation of C. burnetii in sera from children with influenza-like symptoms. Paired sera (acute phase and convalescent phase) collected from 1992 to 1993 in 55 schoolchildren (7 to 11 years old) in Shizuoka Prefecture during an influenza epidemic were examined for the presence of phase II C. burnetii antibodies by IFA. Of the 55 convalescent-phase sera, 18 (32.7%) were positive. C. burnetii was isolated from 13 children by inoculation of acute-phase serum samples into mice. Thus, both studies indicate that Q fever may be widespread and present at a high incidence in Japan. The prevalence of Q fever among children with "atypical pneumonia" (i.e., 39.6%) (368) and those with influenza symptoms (i.e., 32.7%) (250) seems high and should be confirmed by further studies.

Australia. Since the description of Q fever by Derrick in 1937 in abattoir workers in Brisbane (Queensland), the disease has continued to be prevalent in Australia. Q fever has been a notifiable disease in Australia since 1977. In 1982, Spelman (341) reported 111 consecutive Q fever cases occurring between June 1962 and June 1981. All but one were in males. The only woman in the series may have been infected outside Australia, since she presented symptoms 6 days after returning from a 3-month holiday in Greece. Most of the infected men (102 of 110) had recently worked in abattoirs. Most of the infected patients presented with an acute febrile illness (81 of 111 [72.9%]), but prolonged fever was recorded in 18 (16.2%), "atypical pneumonia" was found in 8 (7.2%), and acute hepatitis was found in only 3 (2.7%). Only one patient (0.9%) was diagnosed with Q fever endocarditis.

New Zealand. A seroepidemiological study was conducted in 2,181 cattle and 12,556 sheepdogs in New Zealand in 1993 (148). Sera collected from these animals were examined for the presence of anti-C. burnetii antibodies. Since cattle and sheep are the major ruminant population of New Zealand, the study was considered to be reliable indicator of the presence of C. burnetii in this country. All sera tested were negative, and the authors argued that New Zealand may be considered to be free from coxiellosis and thus from human Q fever.

Although the number of Q fever cases reported in humans may vary from one area to another, this may not accurately reflect the true incidence of the disease, especially since the clinical manifestations of the disease are often nonspecific or even absent. Of note, the incidence of Q fever is often very high in areas where rickettsiologists are working (75). In Europe, acute Q fever cases are more frequently reported in spring and early summer (365). The maximum incidence of Q fever during spring is supposedly related to the "outside" lambing season, which corresponds to heavy contamination of the environment with C. burnetii. The "major" lambing season in October is not related to a higher incidence.

Q fever is usually an occupational hazard. Persons at greatest risk are those in contact with farm animals and include farmers, abattoir workers, and veterinarians. Laboratory personnel are also at risk for Q fever infection, especially when manipulating animal or human products containing phase I C. burnetii. However, persons in contact with pets (e.g., cats and dogs), especially when they give birth, are also at risk. The seroprevalence of Q fever was found to be three times higher in HIV-positive patients than in blood donors (295), suggesting higher susceptibility or higher exposure of this population to C. burnetii infection. The fact that the population investigated consisted mainly of drug addicts also suggests the possibility of blood transmission. Patients at risk for chronic Q fever include those with previous valvulopathy (75, 286, 299, 300), immunocompromised patients (294, 295), and pregnant women (297).

The sex ratio and age of infection with C. burnetii may vary from one area to another according to the most predominant animal reservoir from which patients become contaminated and the opportunity for exposure to these infected animals. In areas where exposure to infected cattle is the predominant risk factor for Q fever, including most European countries, California, and Australia (69, 114, 238, 315, 341, 357, 362, 363), the disease most often supervenes in the active population of from 30 to 60 years old and more frequently in men. In contrast, exposure to parturient cats is the primary mode of C. burnetii contamination in Nova Scotia, and a sex ratio of 1:1 was reported in this province of Canada (216). However, the sex ratio of patients with Q fever illness may not accurately reflect the risk of C. burnetii contamination. Of the 323 hospitalized patients with acute Q fever diagnosed in France from 1982 to 1990, infection was more frequently reported in men than in women (sex ratio, 2.5:1) (365). However, among 942 sera collected from blood donors in the same country (365), anti-C. burnetii antibodies were detected in 38, with a sex ratio of 1:1. Thus, although men and women may have been at equal risk from C. burnetii contamination, Q fever was more severe in men than in women.

PATHOGENESIS AND PATHOLOGY

Top Previous Next References

Humans

The aerosol route is the principal mode of acquisition of C. burnetii infection in humans (20). Ingestion of high doses of C. burnetii via the digestive route (especially by consumption of contaminated dairy products) is considered a rare alternative for acquiring infection (102). The incubation period of acute Q fever may range from 1 to 3 weeks, depending on the C. burnetii-inoculating dose (211). In the majority of cases, C. burnetii infection remains asymptomatic or presents as a nonspecific flu-like illness; thus, the disease remains undiagnosed (86). Acute Q fever has two primary clinical presentations: atypical pneumonia and hepatitis. It has been hypothesized that the route of acquisition C. burnetii infection may influence the clinical presentation of the disease in a particular area. Pneumonia is the primary clinical manifestation of acute Q fever in Nova Scotia, where C. burnetii infection is supposed to occur predominantly via contaminated aerosols from infected cats (210). In contrast, ingestion of raw milk supposedly plays a more significant role in many parts of Europe, where acute Q fever manifests mostly as a granulomatous hepatitis (102). Most infected patients, however, will experience a transient bacteremia with C. burnetii, usually late in the incubation period. Thus, whatever the mode of acquisition of C. burnetii, hematogenous spread of the pathogen may lead to involvement of other organs including the liver, spleen, lungs, bone marrow, and female genital tract. Life-threatening complications may occur, including meningoencephalitis, myocarditis, or pericarditis. C. burnetii infection is usually controlled by the T-cell immune response. However, it is likely that cell-mediated immunity usually does not lead to eradication of C. burnetii from the infected host. The possibility of Q fever resurgence has been documented in patients with acquired immunosuppression, including those with cancers, lymphomas, or HIV infection, and in pregnant women (286, 294, 295, 297, 350). In patients with previous valvulopathy (290) and, to a lesser extent, in pregnant women (350) and immunocompromised patients (138, 286, 294, 295), Q fever may become a chronic and often spontaneously fatal disease. Chronic Q fever may be defined by a clinical evolution lasting longer than 6 months, and is biologically characterized by the presence of IgG and IgA to phase I C. burnetii antigens (263). Q fever endocarditis is the most frequent clinical manifestation of chronic Q fever. Typically a disseminated disease, it is often associated with multiorgan involvement including chronic hepatitis. Less frequently, chronic infections of vascular aneurysms or prosthesis (40, 93, 97, 105, 299), chronic osteomyelitis (61, 93, 277) and osteoarthritis (267, 285), lung tumors (161, 191), pneumonic fibrosis (3), and chronic hepatitis without endocarditis (397, 419) have been described.

Pathological lesions of Q fever in humans have been previously described in organ biopsy specimens or in autopsy specimens performed for diagnostic purposes (214). Liver biopsy specimens were frequently obtained in patients with hepatitis. In contrast, because Q fever pneumonia is rarely fatal, pathological descriptions of the disease in humans are scarce. Organ biopsies are no longer performed for Q fever diagnosis which may be established accurately by serological methods. Typical pulmonary histopathological lesions in patients with Q fever pneumonia correspond to a gross consolidation, microscopic interstitial pneumonia, and alveolar exudates (189, 268, 412). Interstitial infiltrates are composed of mostly macrophages and lymphocytes and to a lesser extent polymorphonuclear leukocytes (161, 268). Fibrin and erythrocytes, together with mononuclear cells, are found in alveolar exudates. Such pathological findings are not specific to Q fever pneumonia and may be encountered with other etiological agents of atypical pneumonia, including Legionella pneumophila, Chlamydia pneumoniae, and Chlamydia psittaci. By using specific antibodies, C. burnetii may be revealed intracellularly within alveolar macrophages.

Because a granulomatous hepatitis is the most frequent indication of the presence of C. burnetii in the liver (33, 72, 83, 117, 150, 265, 275, 281, 333, 374, 386, 392), Q fever hepatitis was frequently confused with tuberculosis. Other less specific hepatic lesions, including portal triaditis, Kupffer cell hyperplasia, and moderate fatty change, have been described. Kupffer cells are considered to be the target cells for C. burnetii infection in liver tissue. This may initiate local inflammation and the formation of granulomas. Histological examination of liver tissue sections reveal the presence of focal hepatocellular necrosis and cell infiltrates composed of macrophages, lymphocytes and polymorphonuclear leukocytes. Macrophages with epithelioid morphology, multinucleated giant cells, and fibrin may also be present. A characteristic feature is noted when a central clear space and a fibrin ring are observed within the granuloma or at its periphery, which is referred to as a doughnut granuloma (83, 124, 265, 281, 343) (Fig. 2). Although doughnut granulomas have occasionally been found in patients with Hodgkin's disease, typhoid fever, cytomegalovirus infection, infectious mononucleosis, or allopurinol hypersensitivity, it is considered to be characteristic of Q fever. C. burnetii is not generally detected in the liver, although isolation or visualization by direct immunofluorescence of bacteria has occasionally been successful. Although liver involvement is often associated with Q fever endocarditis, granulomas have rarely been reported in chronically infected patients and the typical doughnut granulomas have never been reported (290, 397). The predominant histological pattern of the liver in chronic Q fever patients corresponds to a nonspecific reactive hepatitis with lymphocytic infiltration with foci of spotty necrosis (161). The lack of T-cell immune response in chronically infected patients may explain the absence of typical granuloma.

View larger version (117K): [in this window] [in a new window]   FIG. 2.   Doughnut granuloma (arrow) in a liver section from a patient with acute Q fever hepatitis. Hematoxylin-phloxin-saffron stain. Magnification, ×250.

Bone marrow lesions usually correspond to granulomas similar to those found in the liver (59, 74, 150, 254, 333, 343, 386). Doughnut granulomas with macrophages, lymphocytes, polymorphonuclear leukocytes, and multinucleated giant cells surrounding a central clear space and a fibrinoid ring may also be found in bone marrow. Indeed, liver and bone marrow granulomas are concurrently present in many Q fever patients (150, 333, 386).

Q fever endocarditis usually involves the aortic and mitral valves, although prosthetic valve endocarditis is increasingly reported (98, 240, 307, 332). The typical vegetation is small and often difficult to visualize by transthoracic echocardiography. Gross examination of cardiac valves may reveal the infectious vegetation with variable destruction of the relevant cardiac valve. Perforation of valvular cusps and the Valsalva sinus and the formation of aneurysms at the site of the valvular annulus, the membranous portion of the interventricular septum, or the subvalvular myocardium have occasionally been described. Histological findings in the infected cardiac valve are nonspecific. They include the presenc