INTRODUCTION

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Historical Perspective

Weisbroth (714), in an excellent review of the historical struggle against pathogens of laboratory rodents, divides the last 100 years of research involving laboratory animals roughly into three periods. The first, from 1880 to 1950, was the period of domestication, during which many rodent species became much-used research subjects. Many of these original stocks harbored a variety of natural, or indigenous, pathogens. However, during this period, many improvements were made in sanitation, nutrition, environmental control, and other aspects of animal husbandry. The result was a great reduction in the range and prevalence of pathogens found in laboratory animals. The second period, from 1960 to 1985, was the period of gnotobiotic derivation, when cesarean rederivation was exploited as a means of replacing infected stock with uninfected offspring. In this procedure, full-term fetuses are removed from an infected dam and transferred to a germ-free environment and foster care. This procedure was very successful in eliminating pathogens not transmitted in utero. Weisbroth has described the third period, from 1980 to 1996, as the period of eradication of the indigenous murine viruses. In this period, additional pathogens dropped from the scene or were found less and less often. These reductions were accomplished through serologic testing of animals for antibodies to specific pathogens and subsequent elimination or cesarean rederivation of antibody-positive colonies, in addition to continued advances in animal husbandry methods. Pathogen prevalence studies have been (475) and continue to be (234) conducted. Examination of several of these reports from past decades as well as the present one will confirm the steady decline in the range and extent of microbiological contamination in laboratory colonies.

To put things another way, someone has summarized the advances in laboratory animal disease control in the following way: At the turn of the century, an investigator might have said, "I can't do my experiment today because my rats are all dead"; at the midpoint of the current century, an investigator might have said, "I can't do my experiment today because my rats are all sick"; while today, an investigator might say, "I can't do my experiment today because my rats are antibody positive." Surely there has been a steady increase in the awareness of the varied and generally unwanted effects of natural pathogens in laboratory animals and there have been ever-greater efforts to exclude pathogens from research animals. Only when laboratory animals are free of pathogens which alter host physiology can valid experimental data be generated and interpreted.

In interpreting the microbiologic status of laboratory animals, it must be understood that infection is not synonymous with disease (475). Infection simply indicates the presence of microbes, which may be pathogens, opportunists, or commensals, of which the last two are most numerous (475). Few agents found in laboratory animals today cause overt, clinical disease. It is hoped that investigators will appreciate that overt disease need not be present for microorganisms to affect their research. Animals that appear normal and healthy may be unsuitable as research subjects due to the unobservable but significant local or systemic effects of viruses, bacteria, and parasites with which they may be infected. Microbiology and serology reports should be interpreted with the assistance of a veterinarian trained in laboratory animal medicine. Such a professional can assist the investigator in determining the significance of organisms reported.

As accrediting and funding bodies increase their scrutiny of pathogen status and, by inference, the experimental suitability of the animals used in sponsored research, investigators will also want to work with a laboratory animal veterinarian or animal facility manager to ensure that laboratory animals are obtained from reputable, pathogen-free sources and are maintained under conditions that preclude, as much as possible, the introduction of pathogens. It is far better to prevent the introduction of pathogens than to have to account for their presence when interpreting experimental results. At this point, it is appropriate to mention another valid reason for preventing pathogen entry into an animal facility: in some cases, the drugs used to clear pathogens will themselves alter the host physiology and interfere with research. For example, parasiticides with proven immune system-modulating activity include ivermectin (53), levamisole (82), and thiabendazole (676). Additionally, chlorpyrifos, an organophosphate occasionally used to treat mite infestations, has been reported to decrease brain acetylcholinesterase activity in mice (515).

This review is intended to add current information to the excellent body of literature previously published by others, for example Lussier (409) and, more recently, the National Research Council (475). In this regard, special recognition is due the many authors who contributed to the latter publication; it is an outstanding reference on the subject of infectious diseases of mice and rats. I have drawn heavily from that resource and wish to acknowledge this fact. The interested reader is directed there for additional information about the history, biology, and pathophysiology of specific pathogens, as well as for specific references pertaining to the effects of pathogens on mice and rats.

The reader may notice that considerably more information is presented on the effects of natural pathogens of mice and rats than on those of rabbits. This reflects the disparity between what is known about the pathogens of these species. Mice and rats have achieved a level of use in biomedical research unparalleled by other species, including rabbits. Consequently, far greater efforts have been made to identify the effects of pathogens in mice and rats. In addition, many of these infections serve as useful models of human infections or disease mechanisms and have therefore been more extensively studied than those of rabbits.

The review is organized first by the host species, then by the body system affected, and lastly by the pathogen. This organization of material best facilitates the finding of information concerning the pathogens that affect specific body systems, as well as information on specific pathogens.

MICE AND RATS

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Respiratory System

(i) Pneumonia virus of mice. Pneumonia virus of mice is a single-stranded RNA (ssRNA) virus of the family Paramyxoviridae, genus Pneumovirus. Transmission is via aerosol and contact exposure to the respiratory tract (450). Active infections are short-lived and generally without clinical signs in euthymic mice and rats, and there is no carrier state (61, 475). In contrast, athymic (nu/nu) mice develop chronic pneumonia and wasting and die (550). Pathologic lesions have not been reported in naturally infected mice or rats. Experimental intranasal infections of mice have resulted in mild rhinitis and interstitial pneumonia (91). The susceptibility of mice and rats may be increased by a variety of local and systemic stressors (475), and immune responsiveness is strain dependent (584). Experimentally infected athymic mice develop persistent interstitial pneumonia (92). While natural infections appear to be of little consequence in immunocompetent rodents, pneumonia virus of mice infection could alter the pulmonary architecture and interfere with immunological studies (409). Natural infection of athymic mice results in death and would therefore confound studies with such animals.

(ii) Sendai virus. Sendai virus (SV) is one of the most important pathogens of mice and rats (475). Hamsters may also be infected, although their infection is asymptomatic. SV is an ssRNA virus of the family Paramyxoviridae, genus Paramyxovirus, and species parainfluenza 1. Multiple strains have been described (565). SV is extremely contagious, and transmission is via contact and aerosol infection of the respiratory tract (302, 475). Natural infection of rats with SV is generally asymptomatic, with only minor effects on reproduction and growth of pups (415). Natural infections of mice present as enzootic or epizootic infections. Enzootic infections are those endemic to a colony, where the constant supply of susceptible animals maintains the infection. Mice are infected shortly after weaning as maternal antibody levels wane, and they show few clinical signs. Since there is no carrier state, cessation of breeding eventually results in elimination of the infection, although antibody titers remain in previously infected animals. Epizootic infections occur upon first introduction of the virus to a colony. Clinical signs may include teeth chattering, dyspnea, prolonged gestation, poor growth, and death of young mice (475). Where breeding is occurring, the enzootic pattern eventually takes over.

(i) CAR bacillus. Cilia-associated respiratory (CAR) bacillus is a relatively recently identified pathogen of wild (73) and laboratory rats and, to a lesser extent, mice and rabbits; it has been used in experimental infections of guinea pigs and hamsters (598). CAR bacillus is a gram-negative, filamentous rod of uncertain classification. Analyses of small-subunit rRNA sequences indicate that rat-origin CAR bacillus may be closely related to Flavobacterium ferrugineum and Flexibacter sancti (711). Recent studies suggest that CAR bacillus isolates of rat and rabbit origins may be distinct strains and suggest that, in mice, isolates of rat origin may be more virulent than those of rabbit origin (136). Transmission is probably via contact exposure to the respiratory system (323, 426). Current information suggests that CAR bacillus is usually a copathogen (135), most prominently of M. pulmonis in rats, and that it exacerbates lesions of murine respiratory mycoplasmosis (MRM) (254). However, primary infection of rats with CAR bacillus has been recently reported (439). The clinical signs following natural CAR bacillus infection that have been reported for rats are similar to those of severe murine respiratory mycoplasmosis (see the discussion of M. pulmonis, below) and include hunched posture, lethargy, rough coat, and periocular porphyrin staining (425, 475). Lesions of CAR bacillus infection are similar to those of MRM, and the reader is referred to that section for a full description. In addition, CAR bacillus infection produces severe bronchiolectasis, pulmonary abscesses, and atelectasis of entire lung lobes (425, 475). These lesions are due mainly to accumulation of pus in the airways. Large numbers of CAR bacilli can be observed between cilia on respiratory epithelial surfaces and cause the ciliated border to appear dense. Lesions may also be found on epithelial surfaces in nasal passages, larynx, trachea, and middle ears (475). Lesions have been observed in mice of the ICR strain experimentally infected with CAR bacillus (598), and lesions compatible with CAR bacillus infection have been reported in C57BL/6J-ob/ob mice, although these latter mice may have also been infected with SV and/or PVM (254). Information is lacking concerning effects of natural CAR bacillus infection on rats and mice. However, one might expect that CAR bacillus infection could contribute to the morbidity and mortality associated with MRM and could compromise studies of the respiratory system.

(ii) Klebsiella pneumoniae. Klebsiella pneumoniae is a gram-negative bacterium normally inhabiting the intestinal tract of rats, mice, and numerous other animals. Reports of clinical disease in immunocompetent rodents are rare (208, 275, 325, 579), and K. pneumoniae is therefore considered an opportunistic pathogen (475). The prevalence in rodent colonies is high and may increase with antibiotic treatment which eliminates other bacteria (272). Transmission is primarily fecal-oral; aerosol transmission is effective (58). Clinical signs in mice most commonly include dyspnea, sneezing, cervical lymphadenopathy, inappetance, hunched posture, and rough coat (208, 579), and those in rats include cervical and inguinal abscesses (275, 325). Following hematogenous spread, focal abscess formation can occur in any organ. In the lungs of mice, this results in granulomatous pneumonia. Clinical signs in immunocompromised rodents are generally more severe.

(iii) Mycoplasma pulmonis. M. pulmonis is, without question, one of the most important pathogens infecting laboratory rats and mice, and is the cause of MRM. M. pulmonis lacks a cell wall and has membrane-associated hemolytic activity (447). Prevalence rates can be high within animal facilities. Transmission is primarily intrauterine and by aerosol (302, 475, 618). The organism readily establishes infection by colonizing the nasopharynx and middle ears (145). Infection is usually asymptomatic, causing some researchers to consider M. pulmonis a commensal under ideal conditions (475). Its pathologic effects vary, depending on a variety of host, organismal, and environmental factors (314, 475, 580), including concurrent infection with copathogens (582). Levels of susceptibility differ according to host stock and strain (94, 96, 198, 380, 433). In this regard, the most resistant mouse strains include C57BR/cdJ, C57BL/6NCr, C57BL/10ScNCr, and C57BL/6J (93). Clinical signs typically follow chronic infection and include "snuffling" in rats and "chattering" in mice, dyspnea, weight loss, hunched posture, lethargy, and, in rats, periocular and perinasal porphyrin staining (475). Mice may be asymptomatic. M. pulmonis preferentially colonizes the luminal surfaces of respiratory epithelium lining the proximal airways. This characteristic gave rise to the earlier designation of "proximal airway disease" (475).

(iv) Streptococcus pneumoniae. Streptococcus pneumoniae is a gram-positive diplococcus commonly found in laboratory rodent colonies. More than 80 strains, grouped by capsular type, have been reported. Transmission is primarily via aerosol from infected humans. The organism is considered a commensal under most conditions, although host strain susceptibility differences have been reported (638). Typically, a carrier state is established in the nasal passages and middle ears. Clinical signs are uncommon, although natural outbreaks of disease have been reported (475). When present, clinical signs are nonspecific and may include dyspnea, weight loss, hunched posture, and snuffling (475). Virulence is related to several bacterial components, most prominently pneumolysin, a multifunctional toxin with distinct cytolytic and complement-activating activities (156, 561, 709). Infection begins in a bronchopulmonary segment and spreads centrifugally (475). The infection spreads from the lung to the pleura, pericardium, and, via septicemic spread, to the rest of the body. The affected lung is first edematous, then becomes consolidated and eventually is cleared of cellular debris (733). There may be suppurative or fibrinous lesions throughout the respiratory tract and adjacent structures. These most commonly include suppurative rhinitis and otitis media (475) but may also include similar changes in and around the deeper tissues of the respiratory tract. Septicemia may result in suppurative lesion establishment in virtually any organ, with death being a common sequela. Athymic mice are not more susceptible to disease (727). Host immunity is primarily humoral (16, 563, 677), with considerable help from the complement and mononuclear phagocytic systems (475), C-reactive protein (633, 634), TNF- (637), and pulmonary surfactant (651). IL-10 production is induced following S. pneumoniae infection and attenuates the proinflammatory cytokine response within the lungs, hampers effective clearance of the infection, and shortens survival (678). Rats and mice experimentally infected with S. pneumoniae serve as models of respiratory tract infection (638), peritonitis (220), meningitis (624), otitis media (410), and the effects of exercise on the course of bacterial infections (318). Natural infections of laboratory rats and mice with S. pneumoniae have been shown to alter hepatic metabolism, levels of biochemicals in serum, blood pH and electrolytes, thyroid function, and respiratory parameters (475) and could be expected to interfere with a variety of studies depending on the bacterial distribution following septicemic spread. The cost of eliminating the organism from colonies must be evaluated in light of the intended use of the animals.

(i) Pneumocystis carinii. Pneumocystis carinii, recently classified as a fungus (626), inhabits the respiratory tracts of laboratory mice and rats. It is a pathogen only under conditions of induced or inherent immunodeficiency. Transmission is via inhalation of infective cysts (608). Placental transmission does not occur (321). Recent studies have demonstrated differences in host specificity (38, 589, 712) and susceptibility (366). Clinical signs are absent in immunocompetent animals. Infection has been detected and clinical signs have been induced following several weeks of corticosteroid administration (629). Clinical signs in immunosuppressed or immunodeficient mice and rats include wasting, rough coat, dyspnea, cyanosis, and death (475). The lungs are enlarged, dark, and rubbery. Microscopic changes include alveolar septal thickening and alveolar filling with foamy, eosinophilic material consisting of organisms, dead host cells, serum protein, and pulmonary surfactant (104, 150, 186, 446). Pneumonia may be exacerbated by the presence of coinfecting pneumotropic pathogens (29, 559). The attachment of P. carinii to lung cells may play a role in the pathophysiology of P. carinii pneumonia (5) and may be enhanced by surfactant-associated protein A (725).

(i) Cytomegalovirus. Cytomegaloviruses (CMVs) are dsDNA viruses of the family Herpesviridae, subfamily Betaherpesvirinae. Mouse cytomegalovirus (MCMV) is commonly found in wild mice, principally in the submandibular salivary glands (475). Its prevalence in laboratory colonies is thought to be much lower, although survey results are affected by the screening method. Because the salivary glands are persistently infected, transmission is via contact with infectious saliva. Vertical transmission may also occur (662). Aside from the salivary glands, latent infections can occur in the kidneys, prostate, pancreas, testicles, heart, liver, lungs, spleen, neurons of the cerebral cortex and hippocampus, and cells of the myeloid lineage and are directly correlated with the extent of viral replication during acute infection (117, 448, 502, 532, 662). Natural infections of immunocompetent mice with MCMV are subclinical. Pathologic changes are limited to finding intranuclear inclusions in enlarged (cytomegalic) salivary gland cells (502). In addition, experimental infection results in adrenalitis without compromise of adrenal function (538). The effects of experimental infection are dependent on a variety of host factors, with newborn and immunocompromised mice being more susceptible than adult immunocompetent mice (176, 475). Lathbury et al. (387) reported that BALB/c and A/J mice are more susceptible to infection than are C57BL/10 and CBA/CaH mouse strains, whereas Dangler et al. (141) reported that C57BL/6 mice infected with MCMV develop inflammatory lesions affecting the ascending aorta and pulmonary artery more readily than do BALB/c mice. In addition, multiple natural and experimental strains of MCMV differing in virulence have been reported (63, 224). Immunity is primarily cell mediated, with CD8⁺ T cells and NK cells playing critical roles in controlling MCMV replication (387, 500). Monoclonal antibodies against MCMV antigens have been shown to cross-react with host proteins, suggesting a potential autoimmune component to immunity analogous to that described in humans (391). CMVs have recently been recognized as having superantigen activity (312). Natural MCMV infection has not been shown to interfere with research results. However, experimental infection may alter a variety of host physiologic functions, including depression of antibody and interferon production, major histocompatibility complex (MHC) class I-restricted antigen presentation, CD4⁺ lymphocyte numbers in bronchoalveolar lavage fluid, lymphocyte proliferation, cytotoxic lymphocyte responses, and allogeneic skin graft rejection; decreased fecundity; thrombocytopenia; exacerbation of normal cardiac calcification in BALB/c mice; formation of anticardiac autoantibodies; increased susceptibility to opportunistic infections; and induced reactivation of dormant Toxoplasma gondii infection (241, 475, 491, 534, 644).

(ii) Mouse parvovirus type 1. Mouse parvovirus type 1 (MPV-1), formerly known as orphan parvovirus, is a recently recognized and very important pathogen of laboratory mice. The prevalence of infection appears to be high within and among rodent facilities, although many colonies have yet to be screened. Three isolates (MPV-1a, MPV-1b, and MPV-1c) of one serotype have been reported (328). MPV-1 is an ssDNA virus of the family Parvoviridae. Like other parvoviruses, MPV-1 requires actively dividing or differentiating cells for survival. The virus is shed via urinary, fecal, and perhaps respiratory routes (605). Transmission is therefore most probably primarily direct, although extensive transmission studies have yet to be conducted (605). Transmission may also occur following experimental exposure to selected, infected T-cell lines (434). Natural infections of mice are generally asymptomatic and apathogenic, even for neonatal and immunocompromised mice (605). In immunocompetent mice, viral replication occurs in the pancreas, small intestine, lymphoid organs, and liver and may persist for several weeks (331, 605). Viral replication is more widespread in immunodeficient mice (605). MPV-1 has some antigenic cross-reactivity with minute virus of mice, another rodent parvovirus, due to two highly conserved nonstructural proteins (23, 49, 328). MPV-1 affects processes linked to cell proliferation. Reported effects include direct modulation and dysfunction of T lymphocytes and altered patterns of rejection of tumor and skin allografts (435). It is anticipated that additional effects will be reported as more studies are conducted on this important virus. Recently, a new parvovirus of rats, designated RPV-1, has been identified (327). To date, little is known about the virus. However, RPV-1 may suppress the development of lymphoid tumors (327).

(iii) Mouse rotavirus. The disease caused by mouse rotavirus, formerly known as epizootic diarrhea of infant mice, is commonly diagnosed in young laboratory mice with diarrhea. Rotaviruses are dsRNA viruses of the family Reoviridae. Mouse rotavirus is a member of the group A rotaviruses, which are known to infect a variety of vertebrate hosts, including humans. Multiple strains of mouse rotavirus have been identified (84, 316). Infection is highly contagious and is acquired through exposure to contaminated airborne dust and bedding and through contact with infected mice. There is no evidence of transplacental transmission (475). Mice are most susceptible from birth to about 2 weeks of age, possibly due to transient features of intestinal enterocytes (475). Virus is shed in the feces for up to about 10 days postinfection. It remains uncertain whether a carrier state, with persistent, low-level fecal virus shedding exists.

(iv) Rat rotavirus-like agent. Rat rotavirus-like agent (RVLA), like mouse rotavirus, is a dsRNA virus in the family Reoviridae. Unlike mouse rotavirus, however, RVLA has been tentatively classified as a group B rotavirus (696). The natural hosts of RVLA include rats and humans. It has yet to be grown in culture. Transmission is probably via direct contact with contaminated feces, fomite transmission, human contact, and possibly airborne spread of contaminated dust and bedding (475). Clinical signs are seen in rats 1 to 11 days of age and consist of poor growth, diarrhea, and perianal dermatitis (695). These signs led to the designation "infectious diarrhea of infant rats." Pathologic changes include watery, discolored proximal small-intestinal contents; villous atrophy and epithelial necrosis; increased crypt depth; and syncytial cell formation (569, 695). RVLA infection results in a net secretory state for water and in impaired sodium absorption (568, 569). Relatively little is known about immune mechanisms in RVLA infection, but it is likely that there are similarities to immunity to mouse rotavirus infection. In addition to acquired immunity, intestinal mucins may inhibit rotavirus replication and may be dependent on specific mucin-virus interactions (739). Natural infection of rats with RVLA would probably confound studies involving the intestinal system.

(v) Mouse thymic virus. Relatively little is known of mouse thymic virus (MTV) due to the inability to culture the virus in vitro. MTV is considered a member of the Herpesviridae, which are dsDNA viruses. Transmission appears to be via direct contact (623) and possibly via transmammary passage (464). Natural infections are subclinical. Pathologic changes are limited to transient lymphoid necrosis of the thymus, lymph nodes, and spleen of neonatal mice, followed by a diffuse granulomatous response with giant cells, which eventually resolves (732). The thymus is most severely affected, especially lymphocytes, epithelial reticular cells, macrophages, and lymphoepithelial cell complexes (thymic nurse cells). CD4⁺ CD8⁺ and CD4⁺ CD8 lymphocytes are selectively lysed by MTV (17). Both T-helper and T-cytotoxic lymphocytes may be involved (112). The virus also infects and persists in salivary glands. MTV infection has been shown to reduce T-cell responsiveness to concanavalin A and phytohemagglutinin and to reduce the graft-versus-host response (132). Natural infection of laboratory mice might therefore temporarily interfere with immune competence.

(vi) Reovirus type 3. Mammalian reoviruses are grouped into serotypes 1, 2, and 3. Reovirus type 3 is the most pathogenic reovirus of laboratory rodents (36). The primary importance of reovirus type 3 is as a contaminant of transplantable tumors and cell lines (475, 484). Reovirus type 3 is a dsRNA virus in the family Reoviridae. Transmission is thought to be primarily via direct contact. However, Barthold et al. (36) demonstrated that transmission of virus to cagemates or mothers of infected infants did not occur, indicating low contagiousness. The preponderance of the literature on the effects of reovirus type 3 reports on experimental infections. The effects of natural infections as well as relevant findings from experimental infections are reviewed here. Natural infection with reovirus-3 is nearly always asymptomatic. Cook (120) reported the following clinical signs in first litters of mice infected with reovirus type 3: stunting, diarrhea, oily coats, abdominal alopecia, and jaundice. Pathologic changes consisted of enlarged, black gallbladders; hepatic necrosis; and yellow kidneys (120). Experimentally inoculated mice have a wider scope of organ involvement (36, 475, 507).

(i) Helicobacter spp. The genus Helicobacter contains an ever-increasing number of recently identified, gram-negative, spiral, microaerophilic, gastrointestinal system pathogens that are known to infect mammals (127, 212). Species naturally infecting mice and/or rats include H. hepaticus, H. bilis, H. muridarum, H. trogontum, H. rodentium, and "Flexispira rappini," a Helicobacter sp. based on 16S rRNA analysis (212). Among these, H. hepaticus, a pathogen of mice, is most prominent. The prevalence of H. hepaticus is currently unknown but may be quite high (591). Rats, guinea pigs, and hamsters are not susceptible to infection (706). Transmission is via direct fecal-oral contact or fomites. Clinical signs are absent in immunocompetent mice but include rectal prolapse in immunodeficient mice (704). H. hepaticus selectively and persistently colonizes the bile canaliculi and cecal and colonic mucosae (211, 706). Pathologic changes include chronic, active hepatitis, possibly of autoimmune etiology (705); occasional enterocolitis; and hepatocellular neoplasms induced by as yet undelineated nongenotoxic mechanisms (88, 213, 706). Other, lesser known Helicobacter spp. include H. bilis, associated with multifocal chronic hepatitis and isolated from the liver, bile, and lower intestine of aged, inbred mice (214); H. muridarum, from the intestinal mucosa of rats and mice (394); H. rodentium and F. rappini, from the colons and ceca of mice (212, 576); and, lastly, H. trogontum, recently isolated from the colonic mucosa of Wistar and Holtzman rats (440). H. hepaticus has been associated with hepatic carcinomas and elevated levels of alanine aminotransferase in serum (215, 706). H. hepaticus serves as a model for H. pylori-induced chronic gastritis, gastric ulcers, and gastric adenocarcinoma (213). Natural infection of laboratory mice with H. hepaticus, and possibly other Helicobacter spp., could confound carcinogenicity research and research involving the gastrointestinal system. It is certain that much additional information concerning these and yet unknown murine Helicobacter pathogens will be published in the scientific literature in the near future.

(ii) Citrobacter rodentium. Citrobacter rodentium (577), formerly Citrobacter freundii biotype 4280, is the etiologic agent of transmissible murine colonic hyperplasia (31). C. rodentium is a gram-negative, facultatively anaerobic rod. Rats are not susceptible to infection (37). Transmission is via direct contact (71) or via contaminated food or bedding. C. rodentium is generally considered an opportunistic pathogen. For example, the use of antibiotics effective primarily against gram-negative rods may allow an overgrowth of C. rodentium in the mouse intestine (681). Clinical signs, when present, are nonspecific and may include ruffled coat, weight loss, depression, stunting, perianal fecal staining, and rectal prolapse (475). Nursing mice are most susceptible. Strain differences in susceptibility exist, with C3H/HeJ mice more susceptible than DBA/2J, NIHS (Swiss), or C57BL/6J mice (37). Infection is transient, and there is no carrier state. The hallmark pathologic lesion of C. rodentium infection is colonic hyperplasia. Generally, the descending colon is most affected. However, the entire colon and cecum may be involved, with crypt elongation, variable mucosal inflammation, crypt abscesses, occasional erosions and ulcers, and, with healing, goblet cell hyperplasia (32, 475). Transient colonization of the mouse small intestinal mucosa, followed by colonization of the large bowel, is dependent on the presence of the chromosomal eae gene (575). Once colonization has occurred, C. rodentium causes the formation of attaching and effacing (A/E) lesions. Outer membrane proteins, known as intimins, are required for formation of the A/E lesions (218). Immunity appears to be humoral and may be directed at least partially toward intimin antigens (218). Reported effects on research are few, but they include acceleration of carcinogenesis by 1,2-dimethylhydrazine (34). C. rodentium is used as a model of A/E lesions in vivo and in intestinal disease of humans. Natural infection of laboratory mice might severely, if only transiently, alter intestinal cytokinetics.

(iii) Clostridium piliforme. Clostridium piliforme (177), formerly Bacillus piliformis, is the causative agent of Tyzzer's disease. C. piliforme is a gram-negative, filamentous, endospore-forming bacterium. Prevalence remains high in laboratory rodent colonies (475). Possible explanations for this include the moderately contagious nature of the organism (61) and the wide range of susceptible and naturally infected host species (475). However, concerning the latter, Franklin et al. (219) have suggested that both cross-infective isolates and more host-specific isolates may exist. With this in mind, transmission is thought to be via ingestion of infectious endospores in contaminated food or bedding. Inadequate sterilization of feed or bedding components may facilitate the entry of the pathogen into an otherwise well-managed rodent colony.

(iv) Pseudomonas aeruginosa. Pseudomonas aeruginosa is a gram-negative rod that normally inhabits the nasopharynx, oropharynx, and lower digestive tract of many vertebrate species. The primary importance of P. aeruginosa is as an opportunistic pathogen (475). P. aeruginosa is commonly found in soil and organic waste and as a normal skin inhabitant, and it is frequently cultured from facility water systems. Active exclusion of the organism from the animal facility is achievable but costly. Transmission is via contact with contaminated water, feed, bedding, and infected rodents and humans (670).

(v) Salmonella enteritidis. Salmonella enteritidis and the roughly 1,500 serotypes of that species are gram-negative, non-endospore-forming bacteria that colonize the intestinal tracts of a wide variety of animal hosts. The primary importance of Salmonella spp. is as zoonotic agents and as pathogens in immunocompromised mice and rats. S. enteritidis serotype typhimurium is the most common serotype infecting laboratory rodents, although the prevalence of asymptomatic carriers is unknown but probably low. Transmission is via ingestion of contaminated feed ingredients and water and by contact with contaminated bedding and animal facility personnel (475). Reports of natural outbreaks of disease are rare in the literature (475), probably because most infections are asymptomatic in normal hosts. When clinical effects are observed, reproduction is most prominently affected, while other signs are nonspecific (398). Diarrhea is an uncommon finding (475). Many host, pathogen, and environmental factors determine the pathologic findings and severity of infection, including host age and genotype; makeup of the intestinal flora; nutritional state; immune status; presence of concurrent infections; bacterial serotype; and environmental stressors such as food and water deprivation, temperature, iron deficiency, and experimental manipulations (475). Inbred mouse strains have a wide range of susceptibility to S. enteritidis. Susceptible strains include DBA/1, BALB/c, C57BL/6, and C3H/HeJ. Relatively resistant strains include C3H/HeN, A/J, and DBA/2 (475). Susceptibility is determined by three distinct genetic loci (475).

(i) Giardia muris. Giardia muris is a flagellated intestinal protozoan. Infections are occasionally detected in laboratory rodent colonies. Strains of G. muris-infected mice and rats may be host specific (372). The life cycle is direct. Environmentally resistant and infectious cysts are passed in the feces. Excystation occurs following ingestion. The minimum infectious dose for a mouse is approximately 10 cysts (611). Shortly after excystment, trophozoites divide longitudinally and colonize the mucosal surface of the proximal small intestine, adhering to columnar cells near the bases of intestinal villi and moving within the mucus layer on the mucosa (475). Most infections are asymptomatic. When apparent, clinical signs are nonspecific and include weight loss, stunted growth, rough coat, and enlarged abdomen. In athymic or otherwise immunocompromised hosts, clinical signs may be more severe and may include diarrhea and death; and cyst shedding may be prolonged (60, 555). Pathologic changes include villous blunting; increased numbers of intraepithelial lymphocytes, goblet cells, and mast cells; and alterations in intestinal disaccharidase content (685).

(ii) Spironucleus muris. Spironucleus muris (formerly Hexamita muris) is a second flagellated protozoan commonly infecting laboratory mice and rats. Host-specific strains of S. muris have been identified (574). The biology of S. muris appears to be much like that of G. muris; however, due to the inability to culture S. muris, considerably less is known about this organism. Infectious cysts are passed in the feces. The minimum infective dose for a mouse is 1 cyst (611). Following ingestion, excystation occurs and trophozoites colonize the crypts of Lieberkühn in the small intestine. Infections with S. muris are asymptomatic in immunocompetent, adult mice and rats. However, weanling and immunodeficient mice may develop clinical disease. It has been reported by several investigators that young mice may develop diarrhea, dehydration, weight loss, rough coat, lethargy, abdominal distension, and hunched posture and may die (475, 720), although in none of the reported cases were other potential causes of the clinical signs excluded. In athymic (nu/nu) and lethally irradiated mice, S. muris causes severe chronic enteritis and weight loss (371, 441). In severe infections, the intestine is reddened and filled with fluid and gas. The crypts are hyperplastic and may be distended with trophozoites, microvilli and villi may be shortened, and enterocyte turnover is increased; inflammation is minimal (475, 720). S. muris has been shown to interfere with research in several ways, including increasing the severity of copathogen infection; increasing the mortality associated with cadmium treatment; and altering macrophage function and lymphocyte responsiveness to sheep erythrocytes, mitogens, and tetanus toxoid (475).

(iii) Oxyuriasis (pinworms). Pinworms commonly infecting laboratory rodents include the rat pinworm Syphacia muris and, in mice, Syphacia obvelata and Aspicularis tetraptera. S. obvelata has also been reported to infect humans (475). Life cycles are direct, with adult worms inhabiting the cecum and colon. Eggs are deposited in the perianal region of the host (Syphacia spp.) or are excreted with the feces (A. tetraptera). The eggs are very light and will aerosolize, resulting in widespread environmental contamination. Embryonated eggs are infective to another rodent and can survive for extended periods at room temperature. The prevalence of infection remains high (475, 527), even in well-managed animal colonies. Pinworm burden in an infected rodent population is a function of age, sex, and host immune status. In enzootically infected colonies, weanling animals develop the greatest parasite loads, males are more heavily parasitized than females, and pinworm numbers diminish with increasing age of the host. While infections are usually subclinical, rectal prolapse, intussusception, fecal impaction, poor weight gain, and rough coat have been reported in heavily infected rodents, although generally without adequate exclusion of other pathogens (475). Very heavy parasite loads may lead to catarrhal enteritis, liver granulomas, and perianal irritation. Athymic (nu/nu) mice are reportedly more susceptible to infection (326). Immunity is probably mostly humoral, as for many other helminthiases. In this regard, Syphacia-specific antibodies have been demonstrated in pinworm-infected mice (573). There are a few reports documenting the effects of pinworms on research. Pinworm infection resulted in significantly higher antibody production to sheep erythrocytes (573), reduced the occurrence of adjuvant-induced arthritis (512), and impaired intestinal electrolyte transport (408). While many consider pinworm infection irrelevant in laboratory rodents, specific research goals may justify the eradication of pinworms from an animal colony.

(i) Mouse mammary tumor virus. Mouse mammary tumor virus (MMTV) is a ssRNA type B retrovirus of the family Retroviridae. At least four major variants of the virus have been identified in laboratory mice, including MMTV-S ("standard"), MMTV-L ("low oncogenic"), MMTV-P ("pregnancy"), and MMTV-O ("overlooked") (428, 475). More recently, additional variants, including MMTV-SW and MMTV-C4, have been described (590). Mechanisms of transmission differ among the major variants. MMTV-O is endogenous to the genome of most mice, MMTV-S is transmitted via milk, MMTV-L is transmitted via germ cells, and MMTV-P is transmitted through both milk and germ cells (475). T cells are needed for transmission of milk-borne MMTV from the gut to the mammary gland (734). The variants also differ in oncogenicity, with MMTV-S and MMTV-P being highly oncogenic and MMTV-L and MMTV-O being less so (475). Depending upon the mouse strain and virus variant, MMTV may be expressed in mammary and many other tissues (698), including lymphoid tissues (345, 468, 663), or may exist as a provirus in the DNA of the host (680). Clinical signs of infection are generally limited to mammary tumors, which may arise several months after infection, although distant metastases can also occur with subsequent organ compromise. Most virus-induced tumors are adenocarcinomas (475). While the mechanism of tumor induction is unknown, it is thought that MMTV induces hyperplastic nodules which eventually become neoplastic (475). MMTV may integrate into and disrupt the Tpl-2/cot proto-oncogene (188). Various hormones (57), carcinogens (475), diet (204), and transforming growth factor  (467) may accelerate the development of tumors. Mouse strains differ in their susceptibility to MMTV to the point that mouse strain selection can be used as a control measure (475). Immunity involves both cellular and humoral components (47, 428). B cells are the primary targets of infection for MMTV. However, for productive retroviral infection, T-cell stimulation through the virally encoded superantigen (SAg) is necessary. SAg is expressed on lymphocytes (735); binds MHC class II molecules; stimulates T cells via interaction with the V domain of the T-cell receptor (3); activates B cells, leading to cell division and differentiation (99, 290); is involved in the transmission of milk-borne MMTV from virus-infected milk in the gut to the target mammary gland tissue (735); may initiate or aggravate graft-versus-host disease (444); and has the ability to destroy a large portion of CD4⁺ T cells (744). In addition, MMTV affects T-cell (392, 744) and B-cell (99) responses, activates NK cells through superantigen-dependent and -independent pathways (233), and lowers the amount of prolactin required to elicit -lactalbumin production from mammary epithelial cells (56). MMTV is used as a model for viral carcinogenesis. Natural infection of laboratory mice with MMTV will interfere with carcinogenesis studies and result in a shortened life span.

(i) Pasteurella pneumotropica. P. pneumotropica is a gram-negative, nonhemolytic bacterium. Multiple biotypes have been reported (62). While most species of rodents may harbor the organism (445, 475), reports of natural outbreaks are rare and are generally limited to rats and immunocompromised mice (61). McGinn et al. (432) reported otitis media in CBA/J mice used in hearing research. P. pneumotropica was isolated from infected otic bullae. However, in that report the primary pathogen was not clearly established. P. pneumotropica is frequently isolated from several sites on and within healthy rats and mice and is therefore considered an opportunistic pathogen (475). Transmission is probably via direct contact and fomites (475). Clinical disease, when apparent, is generally limited to lesions of the skin and adnexal structures, although ophthalmitis, conjunctivitis, and mastitis have also been reported (475). Lesions are characterized by suppurative inflammation (475). Natural infection of rats with P. pneumotropica could compromise research involving the skin.

(ii) Staphylococcus aureus. Staphylococcus aureus is a gram-positive, coagulase-positive coccus that commonly inhabits the skin, upper respiratory tract, and lower digestive tract of many animals, including laboratory rodents, in which it occasionally causes disease. Transmission is direct, and entry into the body is via breaks in normal barriers. Disease frequently occurs following physiologic changes in the host (e.g., stress and immunosuppression). A variety of clinical presentations have been reported in rats and mice. These include tail lesions, ulcerative dermatitis, and traumatic pododermatitis in rats; and facial abscesses, ulcerative dermatitis, preputial gland abscesses, and penile self-mutilation in mice (475). Lesions are more severe in immunocompromised hosts. In addition, rats inapparently infected during nonsterile surgical procedures are less active in open-field testing. Infected rats have alterations in the fibrinogen level in plasma, the glucose level in serum, total leukocyte counts, and wound histology scores (69). The hallmark of S. aureus infection is suppurative inflammation, with abscess formation in virtually any organ. Most commonly, infection occurs in the skin and subcutaneous tissues, but it may also be found in the upper airways, lungs, conjunctiva, and other tissues.