Mortality Due to Toxoplasmosis in Suburban Eastern Fox Squirrels (Sciurus niger) in Michigan, USA

Three separate mortality events affecting wild eastern fox squirrels (Sciurus niger) were investigated in suburban areas within southeastern Michigan, US over a 3-yr period from the summer of 2015 through the winter of 2017. A total of seven squirrels were submitted for investigation. The squirrels were generally in fair to good body condition with moderate fat deposits. The tissues that most commonly exhibited gross or histologic lesions included the lungs, liver, and heart, whereas spleen and brain exhibited lesions less frequently. Lung lesions in all seven squirrels consisted of moderate interstitial pneumonia with necrosis and moderate to high numbers of protozoal organisms. Livers in four out of seven squirrels had multifocal necrosis associated with low to moderate numbers of protozoal organisms. Three out of seven brains examined had mild nonsuppurative meningoencephalitis with widely scattered protozoal cysts. Protozoal organisms observed in various tissues were strongly immunoreactive to Toxoplasma gondii antibody by immunohistochemical staining. Other primary disease conditions tested for included West Nile virus infection, pesticides, and anticoagulants. Toxoplasma gondii can cause disease and mortality in a variety of wild squirrel species, especially near human settlements, and would merit more attention.