Bordetella bronchiseptica is a Gram-negative, small rod-shaped bacterium which is a primary pathogen in the rabbit and the guinea pig. In rats, it is an opportunistic pathogen when associated with other primary pathogens such as Mycoplasma or viruses. The respiratory infection is characterized by suppu-rative rhinitis and bronchopneumonia.
Tyzzer's disease is caused by Clostridium piliformis, a Gramnegative spore-forming rod. It is an intracellular pathogen which cannot be cultivated on artificial media. It can be grown in embry-onated eggs or in cell culture. It is widely distributed in many species and clinical signs are not specific. Young adults are most often affected. The spores are relatively stable in the environment remaining viable in contaminated bedding for up to one year. This organism is susceptible to disinfectants such as sodium hypochlorite, and spores are inactivated at 80°C for 30 minutes.
• Clinical signs — Clinical signs include lethargy, weight loss, and distended abdomen, but clinically inapparent infections do occur. In rats, clinical signs usually are associated with stress or immunosuppression.
• Gross lesions — The major gross lesions involve the liver, ileum, and myocardium. The primary infective sites are the jejunum, ileum, and cecum. The intestinal lesion is flaccid segmental intestinal dilatation, with an edem-atous atonic ileum. Ileal lesions may extend to adjacent cecum and jejunum. In the liver, there are few to many scattered pale foci up to several millimeters in diameter. Circumscribed grayish foci may be seen in the myocardium in some cases. The mesenteric lymph nodes are usually swollen.
• Histopathology — Microscopically, in the liver there are foci of coagulative necrosis with a distinct boundary between the necrotic and adjacent normal tissue. The organism is found in viable hepatocytes on the periphery of the necrotic foci. Myocardial lesions vary in size, ranging from few fibers to complete transmural involvement. Inflammatory infiltrates are variable.
• Diagnosis — Definitive diagnosis of the organism is based on finding the organisms in the hepatocytes, intestinal epithelial cells, or myocardium. Special stains such as Gram stain, Giemsa, and Methylene blue are used for impression smears. For tissue stains, the Warthin-Starry, Giemsa, or PAS are valuable. An ELISA for antibody to the bacteria has been developed. One must differentiate the ileal distention of Tyzzer's disease from adynamic ileus, which is associated with intraperitoneal injection of the anesthetic chloral hydrate.
Streptococcal (Enterococcal) Enteropathy of Infant Rats
This is a disease of suckling rats with high morbidity and mortality. Enterococcus faeceum durans-2 is the causative agent.
• Clinical signs — The primary clinical signs are diarrhea in suckling rats and stunted growth.
• Gross lesions — Grossly there is abdominal distention and fecal soiling. Stomachs are distended with milk, with concurrent dilation of the small intestine.
• Histopathology — Microscopically there is little change in the intestinal villi and minimal or no inflammatory response. However, large numbers of Gram-positive cocci are present over the villar surface.
• Diagnosis — Bacterial culture, isolation, and identification of the organism is necessary to confirm the diagnosis.
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