Pathology of wild Norway rats in Vancouver, Canada

Abstract

To achieve a contemporary understanding of the common and rare lesions that affect wild, urban Norway rats ( Rattus norvegicus), we conducted a detailed pathology analysis of 672 rats from Vancouver, British Columbia, Canada. Grossly evident lesions, such as wounds, abscesses, and neoplasms, were present in 71 of 672 rats (11%) and tended to be severe. The most common and significant lesions were infectious and inflammatory, most often affecting the respiratory tract and associated with bite wounds. We assessed a subset of rats (up to n = 406 per tissue) for the presence of microscopic lesions in a variety of organ systems. The most frequent lesions that could impact individual rat health included cardiomyopathy (128 of 406; 32%), chronic respiratory tract infections as indicated by pulmonary inducible bronchus-associated lymphoid tissue (270 of 403; 67%), tracheitis (192 of 372; 52%), and thyroid follicular hyperplasia (142 of 279; 51%). We isolated 21 bacterial species from purulent lesions in rats with bacterial infections, the most frequent of which were Escherichia coli, Enterococcus sp., and Staphylococcus aureus. Parasitic diseases in rats resulted from infection with several invasive nematodes: Capillaria hepatica in the liver (242 of 672; 36%), Eucoleus sp. in the upper gastrointestinal tract (164 of 399; 41%), and Trichosomoides crassicauda in the urinary bladder (59 of 194; 30%). Neoplastic, congenital, and degenerative lesions were rare, which likely reflects their adverse effect on survival in the urban environment. Our results establish a baseline of expected lesions in wild urban rats, which may have implications for urban rat and zoonotic pathogen ecology, as well as rat control in cities worldwide.

Keywords: Bacterial infections; Rodentia; parasitic diseases; pathology; trauma.

Figure 1.

Sample selection protocol for studying the pathology of urban Norway rats (Rattus norvegicus) from Vancouver, British Columbia, Canada.

Figures 2–7.

Microscopic lesions in urban Norway rats. Figure 2. Lymphocytic myocarditis, myocyte necrosis, and fibrosis indicative of cardiomyopathy. H&E. Figure 3. Medial hypertrophy of pulmonary blood vessels featuring thickening of the tunica media by disorganized smooth muscle cells. H&E. Figure 4. Eucoleus sp. adults and eggs in the non-glandular gastric mucosa in association with hyperkeratosis and mucosal hyperplasia. H&E. Figure 5. Two distinct esophageal lumens (double esophagus) are adjacent to the trachea. H&E. Figure 6. Multiple tortuous tan parasite tracts caused by Capillaria hepatica on the liver surface. Figure 7. An adult C. hepatica in oblique section and eggs efface hepatocytes and are surrounded by granulomatous inflammation and fibrosis in the liver parenchyma. H&E.

Figures 8–13.

Normal and diffuse thyroid follicular hyperplasia in the thyroid gland of urban Norway rats. Figure 8. Normal thyroid follicular cells are low cuboidal and surround follicles that contain abundant, homogeneous, eosinophilic colloid. H&E. Figure 9. Affected thyroid follicular cells are cuboidal to low columnar and surround follicles that lack colloid. H&E. Figure 10. Normal immunostaining that is intense and intracytoplasmic within follicular cells. Immunohistochemistry (IHC) for thyroglobulin. Figure 11. Within affected hyperplastic cells, immunostaining is weak, diffuse, and intracytoplasmic, consistent with follicular cells. IHC for thyroglobulin. Figures 12, 13. Scattered normal interstitial C cells have intense cytoplasmic immunostaining, whereas hyperplastic follicular cells are negative. IHC for calcitonin.

Figures 14–19.

Microscopic lesions in urban Norway rats. Figure 14. Lymphoplasmacytic tracheitis, featuring severe infiltration by lymphocytes and plasma cells within the submucosa; a few granulocytes are adhered to the mucosal surface. Figure 15. The bronchiole is cuffed by asymmetrical clusters of lymphocytes indicative of inducible bronchus-associated lymphoid tissue (iBALT). Figure 16. Blood vessels are cuffed by thick clusters of lymphocytes indicative of iBALT in the lungs. Figure 17. Arterioles are cuffed by a mixed population of granulocytes and lymphocytes indicating perivascular mixed inflammation. Figure 18. The interstitium adjacent to glomeruli and tubules is infiltrated by lymphocytes and plasma cells indicative of interstitial nephritis. Figure 19. Infection by the nematode parasite, Trichosomoides crassicauda, in the urinary bladder. Multiple adults in cross- and longitudinal sections are within the urinary bladder lumen.