Rats are susceptible to a number of important bacterial pathogens. Recognizing that any scheme is imperfect and overlapping, the following text has been organized into 3 sections:

pathogens that are (i) primary enteric infections, (ii) primary respiratory infections, and (iii) other bacterial infections. Alphabetical listing is used within each category. This approach can best assist the pathologist with differential diagnoses.

Primary Respiratory Infections Bordetella bronchiseptica Infection

Bordetella bronchiseptica is an uncommon, typically opportunistic pathogen that has been associated with respiratory disease in laboratory rats. The organism is a more common inhabitant of the upper respiratory tract of species such as the guinea pig and domestic rabbit. The organism tends to colonize the apices of respiratory epithelial cells, resulting in impaired clearance by cili­ated epithelial cells.

Pathology

Aerosol exposure to B. bronchiseptica in laboratory rats has resulted in lesions characterized by suppurative rhi­nitis, multifocal bronchopneumonia with polymorpho­nuclear cell and lymphocytic infiltration, and peribronchial lymphoid hyperplasia. In animals exam­ined at 2 or more weeks postinoculation, there was fibroblast proliferation and mononuclear cell infiltra­tion. Spontaneous cases of bronchopneumonia associ­ated with B. bronchiseptica infection in rats feature a suppurative bronchopneumonia with consolidation of affected anteroventral areas of the lung. Frequently, there is an identifiable concurrent infection, such as rat coronavirus or Mycoplasma infection. Experimental infection of SPF rats resulted in transient infection with­out spread to contact rats, lending credence to coinfection as a critical element for natural disease.

Diagnosis

Isolation of the organism in large numbers from affected tissues is required for definitive diagnosis. Identification of copathogens should also be considered.

Cilia-Associated Respiratory (CAR) Bacillus Infection

Naturally occurring respiratory disease in rats has been associated with Cilia-associated respiratory (CAR) bacil­lus, a filamentous, argyrophilic bacterium with gliding motility that colonizes the ciliated epithelium of airways.

Pathology

Infection may be subclinical with minimal or no micro­scopic lesions. Chronic suppurative bronchitis and bron­chiolitis, with peribronchiolar cuffing with lymphocytes and plasma cells, are typical microscopic findings when disease is present. There is marked leukocytic infiltration in the lamina propria of affected airways.

Diagnosis

The typical slender CAR bacteria are best demonstrated in tissue sections by silver stains, such as the Warthin-Starry stain. The organisms are also readily seen as electron- dense bacilli oriented between the cilia of respiratory epithelial cells by electron microscopy (Fig. 2.20). Serol­ogy can be used to detect seropositive animals. However, the use of CAR bacillus whole cell lysates as antigen for serology is problematic, in that this bacterium has a number of cross-reacting antigens among other bacteria. Other diagnostic techniques include the demonstration of the CAR bacillus in silver-stained tracheal scrapings using Steiner's silver stain, or in nasal swabs using PCR. The organism has been grown in embryonated chick eggs, cell culture, and, more recently, in cell-free media. Differ­ential diagnoses include M. pulmonis infection and pneu­monia due to conventional bacteria, with possible complications due to concurrent infections with other

FIG. 2.19. Bronchiole from a rat infected with CAR bacillus. The cilia have been completely effaced by overgrowth of CAR bacilli (Warthin-Starry stain).

FIG. 2.20. Electron micrograph of bronchiolar epithelium colonized by CAR bacilli. Note the end-on attachment of the bacilli (arrowheads) among the cilia (arrows). The inset represents a cross section of bacilli among the cilia. (Source: Ganaway et al. 1985. Reproduced with permission from American Society for Microbiology.)

respiratory tract pathogens (Sendai virus, PVM, rat coro­navirus, or Pneumocystis).