Systemic isosporosis (atoxoplasmosis) in American robins (Turdus migratorius) in the southeastern United States

Systemic isosporosis (atoxoplasmosis) is a common disease of passerines globally. In July 2021, two free-ranging juvenile American robins ( Turdus migratorius ) from Kentucky, USA and a third of unknown age from Tennessee, USA died and were submitted to the Southeastern Cooperative Wildlife Disease Study for postmortem evaluation. On histopathology, high numbers of intracellular protozoal merozoites often associated with necrosis and granulomatous inflammation were observed in the lung (n=2), liver (n=2), spleen (n=2), heart (n=1), kidney (n=1), and brain (n=1). Isospora ( Atoxoplasma ) spp. was subsequently detected in all three birds. Comorbidities included poor nutritional condition (n=3), traumatic injury (n=1), avian pox (n=1), aspergillosis (n=1), and endoparasite infection (n=1). Disease development in young robins is likely due to a combination of factors, including incompletely developed immune systems, nutritional and/or environmental stress, coinfections, and anthropogenic influences. Continued monitoring of vector-borne and parasitic diseases in wildlife is important given rapidly changing landscapes and climatic conditions. • Systemic isosporosis in three free-ranging American robins ( Turdus migratorius ). • Merozoites in multiple organs (lung, liver, spleen, heart, kidney, and brain). • All three robins had multiple comorbidities.