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Pathology Case of the Month - Eastern Gray Squirrels

November 30, 2020

Case History: Six eastern gray squirrels (Sciurus carolinensis) were found dead below a tree in a residential yard in Wisconsin, U.S.A. 

Gross Findings: Two juvenile eastern grey squirrels in fair to poor body condition were examined at necropsy. Gross lesions in both animals were similar. Lung lobes contained multifocal firm, consolidated, dark red areas (Fig. 1A) ranging from 5-mm diameter to 30x15x13 mm; all sections sank in formalin. On cut section, affected areas were red to pale tan (Fig. 1B) and large airways contained abundant mucoid red to tan fluid. In addition, there were multifocal dark red (hemorrhagic) areas up to 14x10x5 mm.

Necropsy photos from eastern gray squirrel
Figure 1. Photographs from an eastern gray squirrel (Sciurus carolinensis) found dead in Wisconsin, U.S.A.  (A) The lung contained multifocal firm red areas (arrows) and multifocal areas of hemorrhage (arrowhead). (B) Firm areas are red to tan on cut section and airways contain mucoid red to tan fluid.

Histopathological Findings: Bronchioles are often expanded by large numbers of degenerate neutrophils which frequently extend into adjacent alveolar spaces (Fig. 2A, B). Brown and Hopps stain reveals low to moderate numbers of Gram-negative coccobacilli overlying apical bronchiolar epithelium (Fig. 2A inset), within bronchiolar lumens, and extending into adjacent alveoli. In the most severely affected areas, there is necrosis of bronchiolar walls and alveolar necrosis with filling of alveolar spaces with neutrophils, fibrin, and edema fluid. Bronchial-associated lymphoid tissue is multifocally hyperplastic and contains areas of necrosis and fibrin accumulation. Hilar and central portions of the lobes are most affected. Alveolar spaces without infiltrates contain low to moderate amounts of beaded to fibrillar eosinophilic material (edema and fibrin) (Fig. 2B).

Histology panel from eastern gray squirrel
Figure 2. Photomicrographs from the lung of an eastern gray squirrel (Sciurus carolinensis) found dead in Wisconsin, U.S.A. (A) A bronchiole (arrow) contains numerous neutrophils. H&E stain. Inset: Bronchiolar epithelium is overlain by Gram-negative bacteria. Brown and Hopps stain. Bar = 20 µm. (B) Approximately 30% of the pulmonary parenchyma is replaced by dense basophilic aggregates of inflammatory cells (diamond); the hilar region is most affected. Remaining alveolar spaces contain pale eosinophilic fluid (edema; asterisk). H&E stain.

Morphologic Diagnosis:

Lung: Bronchopneumonia, suppurative and fibrinonecrotizing, subacute, marked, with intralesional Gram-negative coccobacilli

Disease: Bordetellosis

Etiology: Bordetella bronchiseptica, a Gram-negative coccobacillus

Distribution: Worldwide

Host range: Bordetella bronchiseptica is an important pathogen of dogs, cats, pigs, rabbits, and laboratory animals, and has also been reported to cause disease in humans. In wildlife, B. bronchiseptica has been reported as a cause of pneumonia in red squirrels (Sciurus vulgaris) in Great Britain, and eastern gray squirrels are anecdotally reported to be susceptible, particularly in a rehabilitation setting. The bacterium can also occur as a commensal inhabitant of the upper respiratory tract.

Transmission: Direct contact, airborne (e.g. from sneezing or coughing), or via contaminated fomites.

Clinical signs: In domestic animals, clinical signs include coughing, sneezing, and nasal discharge, with lethargy or depression in more severe cases.

Pathology: Grossly, there are multifocal firm, red to tan, areas of pulmonary consolidation, primarily in the cranioventral regions. Both the upper and lower respiratory tract may contain mucopurulent exudate. Microscopic lesions are typical of bacterial bronchopneumonia, with filling of bronchioles and bronchi with neutrophils and areas of necrosis that affect adjacent alveoli. A distinguishing feature of B. bronchiseptica infection is the lining of bronchiolar epithelium by numerous Gram-negative coccobacilli.

Diagnosis: In conjunction with appropriate signalment, and gross and microscopic lesions, diagnosis is confirmed by bacterial culture or PCR. Because B. bronchiseptica often acts in concert with other pathogens, ruling out other pathogenic viruses or bacteria is warranted.

Public health concerns: B. bronchiseptica has rarely been reported as a cause of disease in humans and infections are thought to be limited to immunocompromised people. However, a majority of human cases are associated with animal exposure, suggesting zoonotic potential. The pathogen is closely related to B. pertussis, the causative agent of whooping cough in humans.

Wildlife population impacts: Likely minimal in free-ranging wildlife, although anecdotally B. bronchiseptica has been associated with mortality of squirrels in a wildlife rehabilitation setting.


Casey S, Goldthwait M. 2010. Bordetella in squirrels, Accessed September 2020.

Caswell JL, Williams KJ. 2016. Respiratory System. In: Jubb, Kennedy & Palmer’s Pathology of Domestic Animals, 6th edition, Maxie MG, editor. Elsevier, St. Louis, MO, pp 465–591.

Delaney MA, Treuting PM, Rothenburger JL. 2018. Rodentia. In: Pathology of Wildlife and Zoo Animals, Terio K, McAloose D, St. Leger J, editors. Academic Press, San Diego, CA, pp 499–515.

Goodnow RA. 1980. Biology of Bordetella bronchiseptica. Microbiological Reviews 44(4): 722–738

Simpson VR, Hargreaves J, Butler HM, Davison NJ, Everest DJ. 2013. Causes of mortality and pathological lesions observed post-mortem in red squirrels (Sciurus vulgaris) in Great Britain. BMC Veterinary Research 9:229.

Woolfrey BF, Moody JA. 1991. Human infections associated with Bordetella bronchiseptica. Clinical Microbiology Reviews 4(3): 243–255.



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