RAT BACTERIAL DISEASES STREPTOCOCCOSIS OCCURRENCE The organism is ubiquitous among humans & animals but is not considered to be zoonotic. It is frequently recovered from respiratory tract lesions in guinea pigs, nonhuman primates, and some domestic animals. URI of conventionally raised rats has been reported to be common. However, it is seldom present in barrier-maintained, commercial rat sources. ETIOLOGY The causative organism, Streptococcus pneumoniae, is a gram-positive coccus. A number of serological types have been associated with respiratory disease in rats. TRANSMISSION Via aerosol droplets. PREDISPOSING FACTORS Poor husbandry and general stress factors predispose to streptococcal infections. Biting facilitates transmission, as does the traumatic effect of overgrown teeth. PATHOGENESIS The infection often remains localized in the nasopharynx without the development of overt disease. A shift in the host-parasite balance due to stress or concurrent infection with another pathogen may result in bronchopneumonia and bacteremia. CLINICAL SIGNS The most common signs disease are serous to mucopurulent nasal and ocular discharge and "red tears" due to porphyrin pigments secreted from the Harderian glands, ruffled fur, anorexia, weight loss, dyspnea, rales, hematuria, and depressed activity. Torticollis may exist if the inner ear is affected, and abortions are associated with both general and uterine infections.Young animals are more apt to be clinically affected. Transmission is by aerosol droplet. Animals often die within a few days after onset of pneumonic signs. MORBIDITY & MORTALITY High in the acute phase. Variable in prolonged epizootics. LESIONS Gross: The most characteristic gross lesions are pulmonary consolidation and fibrinopurulent pleuritis and pericarditis. An extensive fibrinopurulent peritonitis, otitis media and interna, metritis, orchitis, or meningitis may occur as well. If a bacteremia occurs early, the disease may be acute with few gross lesions. Histo.: Streptococcus pneumoniae induces an outpouring of exudate rich in fibrin, neutrophils, and erythrocytes into the alveoli. Bronchioles are filled with neutrophils. Embolic lesions may occur in multiple tissues which include the spleen, liver, kidneys, joints, and brain. DIAGNOSIS Clinical signs and lesions. Recovery of S. pneumoniae on blood agar culture in the presence of 10% carbon dioxide confirms the diagnosis. The organism is alpha hemolytic and bile soluble and is inhibited by ethylhydrocupreine (optochin), an antibiotic. S. pneumoniae ferments inulin. PREVENTION There is no effective means to control S. pneumoniae infection once it is enzootic in the colony. Antibiotics will not eliminate the organism from rat colonies. Hysterectomy rederivation of breeding stock from infected colonies is an effective method of initiating new stock free from pneumococcal infection. TREATMENT Benzathine penicillin (30,000 units/200 gm BW) or Oxytetracycline (0.1 mg/ml in drinking water) for 7 days may be helpful in reducing the severity of the disease and as an aid in limiting infections to a subclinical mode in some animals. Selection of an antobiotic is based on sensitivity testing. A carrier state often develops following treatment, and rats may experience a relapse during a subsequent period of stress. SIGNIFICANCE TO RESEARCH Carrier animals are prone to clinical disease following experimental manipulations, e.g., peritonitis following intraperitoneal injections of any kind, encephalitis following placement of intracranial implants, and pleuritis following cardiac puncture. ZOONOSIS Streptococcus pneumoniae can cause respiratory and meningeal disease in man, esp. in the elderly and in people lacking spleens, and in some cases the serotypes that affect animals may also affect humans. PSEUDOTUBERCULOSIS (CORYNEBACTERIOSIS) OCCURRENCE Epizootics of pseudotuberculosis may occur in conventionally raised breeding colonies. Epizootics often can be retrospectively associated with an environmental stress (e.g., fluctuation in ambient temperature or ventilation). Mice & rats are affected. Rare isolations have been reported in the guinea pig. ETIOLOGY The causative agent is the gram-positive, nonmotile, diphtheroid bacillus, Corynebacterium kutcheri. After 48-hour aerobic incubation on 5% blood agar, colonies are circular, 1 to 4 mm in diameter, translucent, gray to yellow, smooth, and non-hemolytic. On occasion, other Corynebacterium species can cause similar syndromes in rats. TRANSMISSION Little is known concerning how C. kutscheri is carried or transmitted within a colony. It has been suggested that the organism is transmitted via aerosol droplet, fecal-oral, or direct contact. PREDISPOSING FACTORS Corynebacterium kutsheri infections in mice and rats are more often inapparent or latent and become overt disease following stressful manipulations. Nutritional deficiencies, concomitant infections, cortisone injections, and radiation exposure are predisposing stresses. Rats are more resistant to the spontaneous disease than mice. PATHOGENESIS The organism is an opportunistic pathogen uncommonly resident in the intestine and, possible, the upper respiratory tract. Once rats are infected, a hematogenous spread may be involved, since lung lesions are initially interstitial and not bronchial. CLINICAL SIGNS Typically, the organism causes inapparent infections in rats, with exacerbation of respiratory disease under conditions of stress. When clinically ill, the most commonly seen signs include serous oculonasal discharge, rough hair coat,abnormal gait, swollen joints, lethargy, dyspnea, anorexia, and loss of weight or retarded growth. In cutaneous infections the skin is abscessed, ulcerated, and underlaid with fistulous tracts. Most rats will have inapparent infections in which C. kutscheri cannot be isolated from internal organs. MORBIDITY & MORTALITY High in the acute clinical disease. Low in chronic infecton. LESIONS Gross: Lesions are characterized by a variable number of grayish-yellow foci surrounded by red zones, particularly in the lung. In longer standing cases, individual foci coalesce into raised lesions 1 cm or larger in diameter and become caseous, hence the term "pseudotuberculosis". Occasionally, fibrous adhesions occur between the lungs and thoracic walls. Similar lesions may be seen in other organs, including the liver, heart,skin, brain, and kidneys. The hepatic lesions resemble tubercles and have caseous centers and fibrous capsules. Prepucial adenitis, arthritis, and otitis media may also be caused by C. kutscheri. Histo.: The lesions in various target organs appear to be due to septic emboli. Pulmonary lesions initially consist of a polymorphonuclear cell and macrophage infiltrate of the bronchioles and interstitial tissue with a round cell infiltrate occurring later. Bronchi become impacted with PMN cells and necrotic leukocytes. Giemsa or Gram staining of infected tissues will reveal the rod-shaped C. kutscheri organisms. DIAGNOSIS Clinical signs, gross & microscopic lesions, serology (ELISA) and isolation of the bacterium from infected tissues. Corynebacterium kutscheri is easily recovered from lesions and upper respiratory tract exudates by culturing on blood agar plates incubated aerobically at 37 degrees C. DIFFERENTIAL DIAGNOSIS 1. Mycoplasma pulmonis- The rapidity with which C. kutscheri clinically affected rats succumb helps differentiate it from Mycoplasma. Whereas peribronchial lymphoid hyperplasia is a dominant lesion in Mycoplasmosis, it is unremarkable in C. kutscheri infections. 2. Streptococcosis- Fibrinopurulent pericarditis, peritonitis, and pleural effusion are not seen in Pseudotuberculosis. 3. Salmonellosis- There are usually no splenic lesions in Pseudotuberculosis. PREVENTION Culling of ill animals will not eliminate C. kutscheri from animals remaining in a colony. Isolation of the organism from animals with subclinical infections is not usually successful. For this reason, cortisone administration (10mg SC) has been advocated as a means for surveillance of infection in colonies prior to necropsy and culturing for C. kutscheri. Hysterectomy derivation is an effective means to establish a C. kutscheri-free colony. TREATMENT Antibiotic therapy will not eliminate C. kutscheri from a colony. The organism is sensitive to ampicillin (2-10 mg/100g BW PO bid), CHPC (20 mg/100g BW PO tid), or tetracycline (450-643 mg/L drinking water). SIGNIFICANCE TO RESEARCH Since C. kutscheri infection is, in most cases, inapparent and manifests itself whenever the host is sufficiently stressed, it can be a significant problem in experimentally stressed rats. ZOONOSIS Corynebacterium kutscheri is a rodent pathogen, but a case of chorioamnionitis has been observed in man. TYZZER'S DISEASE OCCURRENCE The number of species susceptible to infection by Bacillus piliformis continues to increase and presently includes mice, rats, hamsters, gerbils, rabbits, guinea pigs, horses, rhesus monkeys, cats, dogs, several wildlife species, and others. Tyzzer's disease is probably the most common disease of gerbils. It is also common in rabbits, foals, mice, and hamsters. It occurs occasionally in conventially raised rat colonies. It is widely distributed geographically. The acute, highly fatal disease is most often seen in weanling animals. ETIOLOGY Tyzzer's disease is caused by the gram-negative, spore-forming rod, pleomorphic, PAS-positive nonacid-fast Bacillus piliformis. This organism, which is not a true Bacillus, is an intracellular pathogen that has not been cultivated on artificial media, and is, as yet, taxonomically undefined. In the laboratory, B. piliformis is propagated on the yolk sac of embryonated chick eggs. TRANSMISSION Transmission is thought to be by the fecal-oral route. Infectious spore-like bodies may survive a year or more in bedding, soil, or contaminated feed. In utero transmission to the fetus has been demonstrated experimentally.- PREDISPOSING FACTORS Poor environmental sanitation, stress of shipping, and immunosuppressors such as radiation, corticosteroids, concurrent disease, thymectomy, and crowding contribute to the development of clinical disease. PATHOGENESIS The pathogenesis of the disease is believed to involve a primary intestinal infection with spread to the liver via the portal circulation. CLINICAL SIGNS Clinical signs associated with Tyzzer's disease are not particularly distinctive and, accordingly, only suggestive in making a diagnosis. Tyzzer's disease in weanling or stressed animals is an acute, enzootic disease causing rough hair coat, lethargy, and death within 48 to 72 hours. Chronically infected animals, in which hepatic lesions are more pronounced, exhibit weight loss, rough hair coat, distended abdomen and eventually death. Diarrhea is not a common sign in rats with B. piliformis infection. MORBIDITY & MORTALITY Animals displaying clinical signs generally die within several weeks. LESIONS Gross: The most consistent lesion of B. piliformis infection is an enlarged liver with few to numerous gray, white, or yellow foci, 1 to 2 mm in diameter on the surface or within the parenchyma. In more acute cases there may be edema, congestion, hemorrhage, and focal ulceration of the intestine, particularly around the iliocecal-colonic junction. The intestinal lesion has been termed "megaloileitis" due to a segmental dilatation and inflammation of the ileum. However, ileal distension is not always present. The gut is often atonic and filled with a yellowish fluid. Pale, circumscribed, gray myocardial foci have been noted in rabbits, rats, and hamsters with Tyzzer's disease. There are usually no splenic lesions. Histo.: Intracellular organisms are demonstratable in epithelium of crypts and villi. The necrotic foci in the liver are most often present near vessels. Surrounding these foci are varying numbers of leukocytes, macrophages, and fibroblasts. Intracytoplasmic bacteria may be seen in hepatocytes at the periphery of the lesions, but may be present in very small numbers and thus be hard to find. Organisms are also found in myocardium around foci of necrosis. The filamentous organisms may be seen within the cytoplasm of cells adjacent to the necrotic area, often in a "pile of sticks" arrangement. DIAGNOSIS A presumptive diagnosis can be made by the gross lesions, but a definitive diagnosis is dependent upon observation of the organism within hepatocytes, intestinal epithelium, or myocardium. Impression smears of liver taken at necropsy and stained with Gram, PAS, silver staining, Giemsa, or methylene blue stains may be useful for a rapid diagnosis. However, formalin-fixed specimens stained by Giemsa or Warthin-Starry methods are usually performed to confirm a diagnosis. Even with special stains, organisms in tissues may be difficult to find because of their paucity. Suspect material inoculated into cortisonized, susceptible strains of mice from a Tyzzer's-free colony may aid in detecting the disease. Similarly, cortisonizing suspect animals may aid in establishing the disease. Indirect fluorescent antibody tests are also a diagnostic aid. A CF test is widely used for screening in Japan. An intradermal skin test has been described in the United States but, as yet, does not appear to be widely used. DIFFERENTIAL DIAGNOSIS Chloral hydrate-induced lesions- causes ileal distension. PREVENTION Prevention of Tyzzer's disease in a colony is dependent upon a barrier that excludes entry of the agent by contaminated cages, equipment, and infected animals. Routine cage sanitation probably is ineffective in killing the spores of B. piliformis, but exposure of spores to 80 degree Centigrade for 30 min has been shown to inactivate them. Sodium hypochlorite (0.3%) and peracetic acid are effective disinfectants. Spores are resistant to ethanol and quaternary ammonia compounds. Filter cage covers aid in reducing transmission. TREATMENT The acute (1 to 4 day) course of the disease and the intracellular location of the organism reduce the effectiveness of treatment. Oxytetracycline in the drinking water at 0.1g/L for 30 days has reportedly suppressed an outbreak. Tetracycline at 10 mg/kg BW for 5 days "on-off-on" or at 400 mg/L for 10 days has also been used. Bacillus piliformis infection is also sensitive to penicillin, streptomycin, and erythromycin. Sulfonamides and CHPC appear not to affect the agent. SIGNIFICANCE TO RESEARCH Clinically evident Tyzzer's disease is usually associated with experimentation that compromises the immunocompetence of rats. ZOONOSIS No public health significance is known, but the report of B. piliformis infection in a rhesus monkey should be noted. Antibodies to B. piliformis have been found in pregnant women. Because B. piliformis affects such a wide spectrum of animal species, man may very well be susceptible to clinical disease under certain circumstances. PASTEURELLOSIS OCCURRENCE Rats, mice, and occasionally hamsters are susceptible to this disease. Only a relatively few reports document Pasteurella pneumotropica as a primary pathogen in cases of pneumonia, otitis media, and conjunctivitis. ETIOLOGY Pasteurella pneumotropica is a gram-negative, nonmotile, pleomorphic coccobacillus of very low virulence. It has been suggested that Pasteurella pneumotropica is essentially an enterotropic rather than a pneumotropic organism. TRANSMISSION Pasteurella pneumotropica often exists in a latent, carrier state in the upper respiratory or gastrointestinal or reproduction tracts, and may be disseminated by respiratory aerosol or fecal contamination, biting, licking, and intrauterine contamination. The intestinal tract is probably the primary site for localization of the organism in subclinical infections. PREDISPOSING FACTORS Since P. pneumotropica is an opportunistic pathogen, circumstances that lower a host's resistance, particularly other infections, may precipitate the clinical disease. P. pneumotropica may complicate Mycoplasma pulmonis infection in rats. CLINICAL SIGNS Pasteurella pneumotropica is widespread as a latent infection, but the bacterium causes clinical disease only sporadically. Signs associated with P. pneumotropica infection include chattering, labored respiration, weight loss, skin abscesses, conjunctivitis, panophthalmitis, mastitis, infertility, abortion, and internal and subcutaneous abscesses. LESIONS Gross & Histo.: Well-demarcated, red foci of consolidation in the lungs, with occasional scattering of abscesses. Suppurative reactions may also occur in the middle ears, orbital glands, uterus, skin, mammary gland, lymph nodes, accessory sex glands, and the urinary system. DIAGNOSIS Culture. On 24-hour incubation on blood agar the colonies are small (1 mm), circular, convex, smooth, and surrounded by a zone of slight greenish discoloration. Blood agar medium is satisfactory for primary isolation from nonenteric sites. However, for recovery from the intestinal tract, enrichment in a medium such as GN broth is recommended before isolation is attempted on blood agar plates. DIFFERENTIAL DIAGNOSIS Mycoplasma pulmonis- Pasteurella pneumotropica may have scattered areas of abscesses. PREVENTION Elimination of murine respiratory infection from a colony requires a known disease-free stock placed into a clean and barrier-sustained colony. Newly arrived animals should be quarantined until the microbial status of the rat has been determined. Caesarean derivation should be done with the knowledge that Pasteurella pneumotropica is a relatively common uterine inhabitant, as is M. pulmonis. TREATMENT Pasteurella pneumotropica is sensitive to several antibiotics. Among specific antibiotic regimens used with this organism are CHPC in the drinking water (0.25 mg/ml) for 2 weeks; ampicillin (9 mg daily for 5 days); oxytetracycline in the water at 3.5 mg per day, and streptomycin, although many isolants are resistant to the last 2 and streptomycin can be toxic for rodents. SIGNIFICANCE TO RESEARCH Since Pasteurella pneumotropica is frequently carried in the uterus, vertical transmission can occur, and, accordingly, this can compromise the microbial status of axenic and gnotobiotic colonies. ZOONOSIS A strain of Pasteurella pneumotropica can infect man, the possibility of rodent-to-man transmission is unlikely. SALMONELLOSIS OCCURRENCE Salmonellae are widespread in nature and affect a wide range of vertebrates. Guinea pigs are highly susceptible and develop severe clinical disease; mice and rats are also very susceptible and may carry subclinical infections for long periods. Rabbits, hamsters, and gerbils are less often infected, but severe outbreaks have occurred in these species. Infection in an immunologically naive colony typically results in an epizootic of clinically affected rats and a varying proportion of animals with inapparent infection. These latter animals act as subclinical carriers to render the infection as enzootic in a colony. However, salmonellosis is rarely reported today. ETIOLOGY Salmonella species that infect rats include Salmonella enteritidis, S. typhimurium, S. dublin, and S. meleagridis. Salmonella typhimurium and S. enteritidis are the species most often isolated from laboratory animal species. TRANSMISSION Transmission is by the fecal-oral route through ingestion of feces or fecal-contaminated feed or bedding. The organism can exist in the carrier state in the intestinal tract and be continually shed into the environment. PREDISPOSING FACTORS Among the factors predisposing to salmonellosis are youth or old age, nutritional deficiencies, concomitant diseases, genetic predisposition, serotype of organism involved, and environmental and experimental stresses. PATHOGENESIS Salmonellae penetrate the intestinal mucosa at the level of the ileum and cecum. The earliest lesions occur in this locale. In some infected animals, a bacteremic state occurs that results in the demise of the host before the development of further lesions. CLINICAL SIGNS Salmonellosis in laboratory animals is an enteric and systemic infection that may be enzootic or epizootic. Specific signs, when present, include anorexia, depressed activity, rough hair coat, weight loss, light soft feces, ocular discharge, small litters, dyspnea, and abortions. A colony of affected rats had conjunctivitis, anorexia, weight loss, and sporadic deaths. Chronic carriers exist and make elimination of the infection difficult. MORBIDITY & MORTALITY Affected animals die within 1 to 2 weeks. LESIONS Gross: Lesions that occur in salmonellosis differ depending on the stage of the disease. The earliest lesions occur in the intestinal mucosa of the ileum and cecum due to penetration of the organism. The lesions consist of mild dilatation, thickened intestinal walls, and a granular mucosal surface. Involvement of the reticuloendothelial system is reflected by enlarged Peyer's patches, mesenteric lymph nodes, and spleen. In some infected animals, a bacteremic state occurs that results in the demise of the host before the development of further lesions. However, in animals not succumbing to septicemia, ulceration of the ileal, colonic, and cecal mucosa occurs. Histo.: The villus epithelium of the ileum is markedly degenerated, and the lamina propria is infiltrated with neutrophils and macrophages. Concomitant with intestinal lesions is the development of focal necrosis and granulomas in the spleen and liver due to hematogenous spread of the organism. In rats who are intermittent or chronic shedders of salmonella, the most remarkable lesions are lymphadenitis of the mesenteric lymph nodes and ulceration of the cecal mucosa. Rats from which salmonella is chronically shed have more advanced lesions than do intermittent shedders of the organism. DIAGNOSIS While necropsy signs may be suggestive of salmonellosis, culture of feces or cecum is the method of choice for confirming an outbreak or a carrier state of salmonellosis. However, in asymptomatic carriers, organisms will be shed intermittently in the feces and recovery from tissues is difficult. The agents do not hydrolize urea and usually do not use lactose. Standard bacteriologic media are used in culture. These media may include selenite F or tetrathionate broth to selectively enhance growth of salmonella from fecal samples followed by culture on MacConkey's (MC), brilliant green (BG), agars, XLD, and HE. From these media, possible salmonella colonies are inoculated into triple-sugar-iron slants. A battery of additional biochemical tests is used to identify suspect cultures further. Suspected salmonella isolants must by confirmed by serology because several nonpathogenic enteric bacteria may have similar biochemical reactions. A final determination of serotype is best performed in a reference laboratory. PREVENTION Rigid, high husbandry standards and the screening of new arrivals, existing animals, especially dogs and subhuman primates, and animal care personnel will reduce the possibility of outbreaks. Birds, wild rodents, and contaminated feed must be excluded. Elimination of the infection from conventional colonies is extremely difficult; killing, disinfection, and restocking are more practical approaches. TREATMENT Treatment of salmonellosis may suppress an epizootic to an enzootic infection, but elimination of carriers is difficult. Because of the major public health concern, colonies infected with Salmonella should be eliminated, premises are sanitized, and clean animals are used for restocking. One therapeutic regiment that has been followed with some success is the additon of oxytetracycline to the drinking water at 10 g/L for 10 days or 250 mg/kg body weight per day. Treated rats may be isolated, the remainder of the colony killed, the room disinfected, and first litters monitored for Salmonella. Thus, infection in valuable rodents by minimally pathogenic strains of Salmonella may be controlled or eliminated by intense culture, isolation, and treatment. ZOONOSIS Salmonellosis occurs in man and can be contracted from, or given to, laboratory animals. Animal care personnel should be periodically inspected for latent infections of Salmonella. PSEUDOMONIASIS OCCURRENCE This ubiquitous bacteria found in soil and water, colonizes plants, insects, animals, and humans. ETIOLOGY Pseudomonas aeruginosa is a gram-negative bacterium. TRANSMISSION Transmission in laboratory rodents occurs primarily by direct contact and contaminated water bottles and automatic watering systems. PREDISPOSING FACTORS Infection with this organism in immunocompetent rats is nearly always inapparent. However, when rats are immunosuppressed, Pseudomonas aeruginosa invades the upper respiratory mucosa and cervical lymph nodes. PATHOGENESIS When rats are immunosuppressed, Pseudomonas aeruginosa invades the upper respiratory mucosa and cervical lymph nodes causing bacteremia and induces an acute, lethal disease. CLINICAL SIGNS In some cases, rats develop facial edema, conjunctivitis, and nasal discharge. In genetically thymic-deficient rats (nude), retro-orbital abscesses may occur prior to bacteremia. DIAGNOSIS Diagnosis of pseudomoniasis is based upon a history of immunosuppression associated with an epizootic of acute disease and isolation of Pseudomonas aeruginosa from the blood and organs of affected rats. Pseudomonas aeruginosa grows well on blood agar and most other standard laboratory media. Most strains are B-hemolytic and produce a bluish-green pigment, pyocyanin, as well as fluorescein. The use of specialized media (Pseudomonas P agar) enhances pigment production. The organism derives energy from carbohydrates via oxidation rather than fermentative metabolism. Identification of isolates as Pseudomonas aeruginosa is easily made by the above characteristics and appropriate biochemical reactions. DIFFERENTIAL DIAGNOSIS Facial edema in affected rats must be differentiated from viral sialodacryoadenitis. PREVENTION Phenolics are usually effective disinfectants, but quaternary ammonium compounds may actually support its growth. Infection can be relatively well controlled in a colony by hyperchlorinating drinking water at 12 ppm or by acidification of water to a pH of 2.5-2.8. In a closed colony, it is advisable to remove rats that remain culturally positive after water treatment has been instituted. In studies requiring pseudomonas-free rats, isolators are useful in which a gnotobiotic environment can be achieved. Alternatively, laminar flow units may suffice if supplies and equipment are sterilized and personnel wear sterile garments. SIGNIFICANCE TO RESEARCH In most research applications, Pseudomonas aeruginosa-free rats are not necessary for the conduct of the work. It is a major problem, however, in rats used for burn research and in studies in which drugs or radiation induce immunosuppression. STREPTOBACILLOSIS OCCURRENCE Streptobacillus moniliformis is considered a commensal of low pathogenicity for rats but is capable of producing disease in mice, guinea pigs, and humans. ETIOLOGY Streptobacillus moniliformis, a pleomorphic, gram negative rod or filamentous bacterium. PATHOGENESIS Streptobacillus moniliformis is a commensal bacterium often present in the nasopharynx of conventially raised rats. It may be involved occasionally as a secondary invader within inflammatory lesions of the rat. DIAGNOSIS Culture- media requires blood, serum, or ascites fluid and growth is enhanced with 10% carbon dioxide. PREVENTION Gnotobiotic derivation and segregation from infected animals and proper management of air and personnel flows. ZOONOSIS The chief importance of S. moniliformis is that it is the principal agent causing rat-bite fever in humans. The other bacterium associated with this clinical syndrome is Spirillum minus. Clinical signs in humans usually occur within 10 days of a rat bite and consist of headache, weakness, fever, a generalized rash, and arthritis. Often clinical signs subside in several days but then recur at irregular intervals for weeks or months. MYCOPLASMAL DISEASES MURINE RESPIRATORY MYCOPLASMOSIS (MRM) OCCURRENCE Rats and mice are the principal hosts of Mycoplasma pulmonis and M. arthritidis. Rabbits, guinea pigs, and other rodents may carry the organism, but they are not clinically affected. The disease has many synonyms: murine pneumonitis, infectious catarrh, enzootic bronchietasis, chronic respiratory disease (CRD), endemic murine pneumonia , viral pneumonia of rats, labyrinthitis, and chronic murine pneumonia. By far, this is the most common disease of laboratory rats. ETIOLOGY Mycoplasmas are very small pleomorphic organisms with no distinctive cell wall and a diameter between 0.2 and 1.0 um. Mycoplasma pulmonis, which may accompany Sendai virus or bacterial infections, is a mucosal pathogen responsible for the clinical signs and lesions of murine mycoplasmosis. The bacteria that may accompany M. pulmonis in respiratory disease include Pasteurella pneumotropica, Actinobacillus spp, Streptococcus pnemoniae, Bordetella bronchiseptica, and Corynebacterium kutscheri. Mice, like rats, may carry the agent as an inapparent infection. TRANSMISSION Transmission of the extremely contagious mycoplasmal infections is by direct contact between mother and young, respiratory aerosol over short distances, sexual transfer, animal carriers, and in utero passage. Some of the unexplained appearances of mycoplasmosis in established, isolated, pathogen-free colonies may arise from infections acquired in utero and undetected, minute foci of infection in a few individuals. There is little evidence to indicate that transmission occurs thru fomites such as caging equipment and garments worn by personnel. PREDISPOSING FACTORS Agents that damage the protective capacity of the respiratory epithelium predispose to M. pulmonis infection. Such agents include ammonia, sulfur dioxide, and Sendai virus. Ammonia gas in an animal colony is generated from urine and feces by urease-positive bacteria. Factors involved in ammonia accumulation include poor ventilation and sanitation, cage crowding, bacterial growth, and excessive populations. The metaplastic and ciliary inhibiting effects of ammonia can extend an innocuous upper respiratory infection into a bronchopneumonia. Older animals and animals of certain strains with diminished immune function are more susceptible to the clinical manifestations of the chronic disease. Lewis strain rats are, for example, more susceptible to clinical murine respiratory mycoplasmosis than F344 rats. Vitamin A or E deficiencies may also be predisposing factors. PATHOGENESIS The basis for the pathogenicity of M. pulmonis is not well understood. Mycoplasma pulmonis adsorbs to the cell membrane of the ciliated, columnar or cuboidal epithelia in the respiratory tract. It has been suggested that adsorption is a means by which mycoplasmas damage host cells by uptake of essential cellular metabolites; release of cytotoxic products, such as hydrogen peroxide; or cross reaction of antibody with cell membrane components that are antigenically similar to or altered by mycoplasmas. Infection severely distorts or ablates ciliary structures, interfering with mucociliary clearance mechanisms. Unilateral or bilateral otitis interna occurs because of an extension of the infection up the eutachian tube into the middle ear and then to the inner ear. The disease is usually slowly progressive, with young animals only mildly affected but clinical signs becoming more severe in older animals. CLINICAL SIGNS Clinical signs are typically not evident until an advanced stage of disease is reached. Acute death is usually the result of secondary bacterial infections. The upper respiratory disease, involving the nasal passages and middle ears, is indicated by sniffling, occasional squinting, rough hair coat, serous or mucopurulent oculonasal discharge, and sneezing. If the inner ear becomes involved, torticollis may occur. When rats with labrinthitis are held by the tail in a vertical position, they typically rotate their bodies rapidly; whereas rats without inner ear involvement remain rigidly suspended. The bronchopulmonary syndrome, initiated or exacerbated by ammonia, bacterial infections, or Sendai virus infection, is characterized by lethargy, rough hair coat, hunched posture, chattering, weight loss, labored breathing, and eventually death. Unless the respiratory infections are complicated by bacteria, the terminal clinical stages of mycoplasmosis may last weeks or months. LESIONS Gross: In the earliest stage, pulmonary lesions are limited to peribroncholar lymphoid hyperplasia, visible only on microcopic exam. As the disease progresses, the upper respiratory infection is characterized by serous to purulent inflammation in affected tissues. In murine mycoplasmosis unilateral or bilateral otitis media is a common finding, often the only gross abnormality. The pulmonary lesions in the early stages of the disease are well-demarcated foci of firm red or gray atelectasis and consolidation or lymphoid aggregation. As the disease progresses, inflammatory debris accumulates in the air passages, resulting in bulging, mucopurulent areas of bronchiectasis. The content of these lumps is viscid to caseous and yellow-gray. Histo.: Microscopically, the inflammatory response is characterized by a lymphocyte and plasma cell infiltrate in the submucosa and neutrophilic leukocyte response within the lumina of the nasal cavity, eustachian tubes, middle ears, and tracheobronchial tree. A consistent and prominent lesion in the lung is the peribronchial lymphoid hyperplasia that often becomes quite massive. Within the lumina of the bronchi and bronchioles, mucin and neutrophil exudation increases during the course of the disease to the point of bronchiectasis. DIAGNOSIS Diagnosis is based on gross and microscopic lesions and on the isolation of M. pulmonis from the nasal pharynx, tympanic bullae, trachea, uterus, or lungs. The organism may be carried in the upper respiratory passages in the absence of clinical disease. Culture of M. pulmonis requires special media enriched with yeast extract and 10% swine or horse blood. The plates are incubated at 37 degrees C in an atmosphere of normal or reduced oxygen and increased humidity. Mycoplasma pulmonis spp require sterols for growth. Mycoplasma arthritidis requires arginine. In mycoplasma broth media, moderate to heavy growth is reflected by pH and color of the broth. In broth cultures in which the titer is low, a perceptable pH change may not occur. Tissue and washing samples should be placed in broth rather than agar media, since recovery of the organism is more likely in those samples containing few mycoplasmas. Samples from broth cultures are transferred to agar media when a pH change is readily evident or at 7-10 days if no pH change occurs. Mycoplasma colonies are evident in 3-4 days by observation with 40X stereoscopic microscopy. ELISA tests for detecting mycoplasmal infections has eliminated some of the uncertainty associated with the diagnosis of mycoplasmosis by cultural means, e.g., culture negatives, but false positives and cross reactions with other Mycoplasma do occur. DIFFERENTIAL DIAGNOSIS 1. Streptococcosis-nasal exudate is also present in this disease. 2. Sialdacryoadenitis-causes reddish porphyrin deposition around the nares which can be confused with exudate. 3. Water deprivation- same as 2. 4. Pseudotuberculosis-lung lesions may mimic each other. 5. Sendai-often superimposed on MRM. Histo. and serology will differentiate the two from each other. 6. Unknown filamentous bacterium-recently a filamentous bacterium has been associated with bronchietasis in wild and laboratory rats. However, the causal relationship of this organism with lesions is undefined . This filamentous bacterium has not been successfully grown on artificial media, and its presence is best verified by either histology, using the Warthin-Starry Stain, or electron microscopy. Although a definitive diagnosis of MRM is made by isolation of M. pulmonis from involved tissues, it is evident that the existence of other agents must be evaluated to determine if copathogens are contributory to lesions. PREVENTION Prevention of murine mycoplasmosis involves placing Mycoplasma-free rodents into a barrier-sustained facility. Strict husbandry standards, exclusion of wild rodents, serologic and postmortem monitoring, good ventilation, and low population densities in cage and room contribute to the maintenance of a Mycoplasma-free colony. Hysterectomy derivation is the only means of establishing an M. pulmonis-free breeding colony from a previously infected stock. Due to the frequent localization of this microorganism in the uterus, it is necessary to ensure that neonates taken by hysterectomy have not been infected in utero. Rats used in research animal facilities are obtained from various commercial and institutional sources. Accordingly, it is essential that the mycoplasma status of these sources is known and that the rats are housed by vendor or in groups with a similar microbial status. For assessment of whether a group of rats is M. pulmonis-free, the best sites for isolation in animals without gross lesions are the nasal cavity, middle ear, trachea, and uterus-oviduct. Localized, low level infections with Mycoplasma spp may be more common than suspected. Such infections are very difficult to detect and constitute a continuous source of the organism in a colony. TREATMENT Elimination of a mycoplasmal infection from affected rats and mice is, for all practical purposes, impossible. Antimicrobials placed in the drinking water, however, may suppress infection and clinical signs. Bacterial culture and sensitivity testing are necessary to determine the most appropriate choice of antibiotics. Tetracycline hydrochloride at 2 to 5 mg/ml given fresh daily for 5 days in deionized, sweetened drinking water (5% sucrose) often suppresses clinical signs if rats drink the concentrated solution. Some tetracycline solutions at this concentration in tap water form a scale that blocks the sipper tube. Lower levels of tetracycline may have an effect on secondary bacterial complications. Sulfamerazine at 0.02% in the drinking water or 1 mg/4 g feed; tylosin at 66 mg/L (2.5 g/10 gal) for 21 days, and CHPC at 30 mg/kg BW for 5 days are other treatment suggestions, but the prognosis for recovery remains poor, and treatment should never be advocated as a method to eliminate Mycoplasma from a colony. Labyrinthitis does not usually respond to treatment, but animals with pronounced head tilt will often survive for months. SIGNIFICANCE TO RESEARCH Of all the pathogens occurring in laboratory rats, M. pulmonis has had the greatest negative impact on studies. Long-term studies in areas of toxicology, carcinogenesis, nutrition, and gerontology, in particular, are affected. Mycoplasma pulmonis and arthritidis may contaminate transmissible tumors and caution should be exercised to ensue transplanted tissues are not contaminated. ZOONOSIS Mycoplasma pulmonis does not affect man, although the organism may be carried in the human nasal passage. MURINE GENITAL MYCOPLASMOSIS OCCURRENCE Mycoplasma pulmonis recently has become recognized as an important pathogen in the female genital tract of rats. Genial mycoplasmosis in the male rat has not been well documented. However, it is known that experimental inoculation can include an inflammatory response in the ductus efferens and epididymis. Moreover, it is known that M. pulmonis is capable of adherence to spermatozoa in an in vitro system. ETIOLOGY Same as MRM. TRANSMISSION Same as MRM. PATHOGENESIS Infection of the oviduct and uterus occurs frequently in rats who have respiratory mycoplasmosis. It is unknown whether localization in the genital tract occurs due to a hematogenous spread or to an ascending infection of the genital tract. It has been shown that subsequent to intravenous inoculation, M. pulmonis almost invariably localizes in the female oviduct-uterus. CLINICAL SIGNS Infertility, embryonic resorptions, and small litters occur. LESIONS Gross: The genital infection, which may exist independently of the respiratory infection, is an ascending process that can involve the entire reproductive tract. Older females are more often affected. The LEW strain is particularly prone to develop gross lesions. Metritis, pyometra, and purulent oophoritis and salpingitis characterize the serious genital infection. The lesions in the ovarian bursa include edema and inflammation. Uterine lesions can vary from a mild inflammatory change to pyometra. Histo.: M. pulmonis adsorbs to the epithelial cells in the genital tract in a manner similar to that seen in the respiratory tract. Salpingitis occurs most frequently and is characterized by exudation of neutrophils into the lumen, hyperplasia of oviductal epithelium, and a lymphoidal response in the submucosa. DIAGNOSIS Same as MRM. DIFFERENTIAL DIAGNOSIS 1. Pasteurella pneumotropica- induces similar lesions in the female rat. 2. Neoplasia- caseous lesions in the ovary and oviduct can be mistaken for neoplasia if microscopy is not done. PREVENTION Same as MRM. TREATMENT Same as MRM. SIGNIFICANCE TO RESEARCH Same as MRM. ZOONOSIS Same as MRM. MYCOPLASMAL ARTHRITIS OCCURRENCE This mycoplasma species colonizes the pharynx, middle ears, and lungs of rats, although few studies have been done to document the relative frequency of this mycoplasma in rat sources. Although it is often considerd to be the principal agent involved in arthritis in rats, the disease has been rarely reported. ETIOLOGY Mycoplasma arthritidis. PREDISPOSING FACTORS It has been suggested that poor cage sanitation and abrasions of the extemities are involved in entry of the organism to the joints by hematogenous spread or extension from surrounding tissues. Since the organism appears to be of low virulence, the immunocompetence of the host may be a major factor in the outcome of infection. CLINICAL SIGNS Arthritic animals limp and move with difficulty due to pain associated with the polyarthritis. Affected joints are hyperemic and swollen. Any of the joints in the limbs and vertebrae can be affected, but the tibiotarsal and radiocarpal joints are most often involved. LESIONS Within the respiratory tract, M. arthritidis colonization is thought to induce negligible lesions, and it has been shown to coexist with M. pulmonis. Gross: Incised joints reveal a purulent exudate in both articular and periarticular tissues. Histo.: There is exudation of neutrophils into the synovial spaces, and lymphocyte and plasma cell infiltration in the synovial membranes. Destruction of the articular cartilage occurs subsequent to the inflammatory response. DIAGNOSIS Same as MRM. DIFFERENTIAL DIAGNOSIS Pseudotuberculosis- polyarthritis can occur subsequent to septicemias associated with other bacteria, particularly C. kutscheri. TREATMENT Same as MRM. SIGNIFICANCE TO RESEARCH Same as MRM. RICKETTSIAL DISEASES HEMOBARTONELLOSIS OCCURRENCE The rarity of reported cases would indicate H. muris is no longer a significant problem in barrier-maintained colonies. However, conventionally maintained colonies may be exposed to infected wild rats and P. spinulosa (see "Transmission") and, accordingly, the disease still is of importance in the laboratory rat. ETIOLOGY The causative agent of this rickettsial disease is Hemobartonella muris. This organism is an extracellular parasite of erythrocytes and induces inapparent infections that may persist for long periods. TRANSMISSION Transmission of H. muris involves the blood-sucking louse, Polypax spinulosa. Transmission can occur during a blood meal or when rats crush infected lice and are inoculated via pruritis-induced abrasions. The organism can also be transmitted inadvertantly with transplantable tumors and other biological products. PREDISPOSING FACTORS PATHOGENESIS The ability of the host to restrict the infection to a subclinical mode rests with the integrity of the retculoendothelial system. CLINICAL SIGNS H. muris infections in rats are almost always inapparent unless an animal is splenectomized or splenic function is otherwise impaired. Evidence of infection is usually limited to splenomegaly and laboratory findings of mild parasitemia and reticulocytosis. DIAGNOSIS Diagnosis of hemobartonellosis is dependent upon identification of the organism in the peripheral blood of infected animals. However, in the latent state, organisms are not seen in peripheral blood. The usual method of detection is by splenectomizing rats suspected of harboring the organism. In these rats, severe parasitemia with hemolytic anemia, reticulocytosis and frequently hemoglobinuria and death occurs within 2 weeks after surgery. Hemobartonella muris can be visualized on the surface of erythrocytes in Romanowsky-stained blood smears as coccoid bodies arranged singly, in clusters, or chains. TREATMENT None. SIGNIFICANCE TO RESEARCH The disease has had a negative impact on investigations of various types, but principally with those in which the host's immune competence has been impaired. Infection with H. muris can alter host response to transplantable tumors, generally making rats more resistant to tumor growth. Latent infection causes marked elevation in serum levels of immunoglobulin. Infection causes severe alteration in the RE system. The phagocytic index of experimentally infected rats is markedly elevated. In mice, infected animals show an almost completely blocked response to interferon induction. Experimental plasmodium infection in rats is alterd by coincidental infection with hemobartonella. Concurrent infection with eperythrozoon potentiates the virulence of ectromelia virus, lymphocytic choriomeningitis virus and mouse hepatitis virus. The effect appears to be related to altered phagocytic activity. FUNGAL DISEASES ASPERGILLOSIS OCCURRENCE Pulmonary aspergillosis is occasionally observed in rats and mice. ETIOLOGY Aspergillus sp. PREDISPOSING FACTORS Immunocompromisation due to stress, debilatation, ammonia, cortisone treatment, etc. CLINICAL SIGNS Dyspnea. LESIONS Gross: Pulmonary lesions consist of miliary granulomas are the primary lesions. However, other organs such as liver and kidney may be affected due to hematogenous spread. Histo.: Granulomas contain Langhans giant cells, macrophages, epitheloid cells, and eosinophils in areas with characteristic uniformly branched, septate hyphae. Congestion, microscopic hemorrhage, and septal thickening occurs in surrounding parenchyma. DIAGNOSIS Aspergillus can be readily isolated or observed in affected lung tissue with selected histochemical techniques. Sabouraud's agar with bacterial inhibitors (CHPC, penicillin, and streptomycin) is satisfactory for this purpose, but care must be used to avoid modern media designed for isolation of dermatophytes, since the cycloheximide used as inhibitor will prevent growth by "contaminants" such as Aspergillus. Isolated Aspergillus cultures should be speciated according to the critieria outlined in the "Manual of Medical Microbiology" by Austwick, which emphasizes subculture on Czapek-Dox medium for optimal growth of differential features useful in speciation. Aspergillus is seen quite distinctly in hematoxylin and eosin stained material, but Gridley's fungus stain or Gomori-Grocott should be used for careful study. DIFFERENTIAL DIAGNOSIS Phycomycosis- The Aspergillus species may be differentiated in tissue from those causing phycomycosis by the more uniform and smaller hyphae (2.5 um in diamter), regular branching at 45 degree angles, and prominant septae. In contrast, phycomycotic fungi in tissue have thicker hyphae (10-15 um in diameter), irregular branches, and are nonseptate. PHYCOMYCOTIC ENCEPHALITIS OCCURRENCE Phycomycotic encephalitis has been reported as a natural disease in 2 to 4 week old rats. The disease is quite rare. ETIOLOGY Fungi of the group Phycomycetes, including species of Absidia, Mucor, Rhizopus, Hyphomyces, Basidiobolus, Entomophora, and Mortierella. PREDISPOSING FACTORS The disease has most often been reported in association with primary conditions lowering resistance , e.g., diabetis mellitus, cortisone treatment. LESIONS Gross: Lesions present as purulent, necrotizing foci. Histo.: Phycomycotic fungi have thick, irregularly branched, nonseptate hyphae (10-15 um in diameter) in tissue. An acute inflammatory response will be present. DERMATOMYCOSIS OCCURRENCE Dermatomycosis is probably more likely to be encountered than deep mycosis, but is also rare in laboratory rats. Dermatophytes affect a wide range of animals. ETIOLOGY Trichophyton mentagrophytes. Occasionally, Microsporum. TRANSMISSION Although clinical infections are uncommon, asymptomatic carriers of the dermatophyte are not and pose a continuing threat to both animals and caretakers. Dermatophytes are transmitted easily by direct contact with spores on hair coats, bedding, and soil. PREDISPOSING FACTORS Husbandry, nutritional, and environmental or internal stress factors that increase exposure or reduce resistance predispose to dermatophytoses. Genetic background, overcrowding, heat and humidity, ectoparasitism, youth, old age, and pregnancy have all been implicated as predisposing factors. CLINICAL SIGNS Infected rats can be lesionless carriers or display patchy alopecia and erythema with scale, papule, or pustule formation. Lesions are generally over the back as opposed to mice which have tail lesions. LESIONS Fungal elements are visable within the stratum corneum and hair follicles. Ecothrix spore formation in hair shafts can be present. DIAGNOSIS The disease can be diagnosed by examination of skin scrapings treated with 10% potassium hydroxide, histological sections of skin prepared with fungal stains (periodic acid Schiff or silver), or isolation. Dermatophyte test medium can be used. PREVENTION Maintenance of high-level husbandry standards, particularly with the young, aged, pregnant, or otherwise stressed, is a protective measure. Cultural screening for dermatophytes, proper adjustment of temperature and humidity, removal of ectoparasites, culling carriers, and sterilization of contaminated equipment are other preventive measures. TREATMENT Successful treatment of ringworm involves elimination of the organism, not just the lesions. In the case of T. mentagrophytes infection, treatment is more a textbook exercise than a practical enterprise. Topical and systemic antifungal agents are available, but treatment should be undertaken only after consideration of the public health significance of the fungus and the probably ineffective but prolonged therapeutic regimen. Topical antifungal creams are applied twice daily for a minimum of 4 weeks. Griseofulvin is administered at 25 mg/kg BW daily in the water or feed (0.375 g/lb feed) for 14 days. Animals may also be dipped in a mixture of 67 g of 25% tetraethylthiuram monosulfide in alcohol made to 1 liter in water. The animals are dipped at 3-week intervals. A single local application of 10% polyyvinyl iodine solution may be effective. Another reported treatment is 1.5% griseofulvin in DMSO applied topically SID or BID for 5 to 7 days. ZOONOSIS Trichophyton mentagrophytes may infect the caretakers before it is noticed on the animals. The fungus is highly infectious for man, particularly children and the infirm. REFERENCES Holmes, D.D. 1984: Clinical Laboratory Animal Medicine. Iowa State University Press, Ames. Baker, H.J., Lindsey, J.R., & Weisbroth, S.H. 1979: The Laboratory Rat. Academic Press, New York. Fox, J.G., Cohen, B.J., & Loew, F.M. 1984: Laboratory Animal Medicine. Academic Press, New York. Harkness, J.E., Wagner, J.E. 1983: The Biology and Medicine of Rabbits & Rodents. Lea & Febiger, Philadelphia. VanHoosier, Jr., G.L. 1977: The Laboratory Rat. ACLAM.