Proliferative visceral Isospora (atoxoplasmosis) with morbid impact on the Israeli sparrow Passer domesticus biblicus Hartert, 1904

House sparrows (Passer domesticus biblicus Hartert, 1904) caught in the Jordan valley, the coastal plain, and the desert region in Southern Israel were found massively infected with extraintestinal proliferative stages of Isospora, previously named Atoxoplasma. Infection coincided with Isospora spp. infections in the digestive tract. Prevalence of infection reached 70% among sparrows of all three regions; however, only in the Jordan valley did the severity of the sparrows compromised their survival. Healthy appearing captured birds showed symptoms of "going light" syndrome-diarrhea, emaciation, and death. Birds succumbed within 48 h to 15 days after confinement to cages. Merozoites accumulated predominantly in the spleen but were rarely found in the peripheral blood. The parasite stages in the visceral leukocytes propagated by merogony and yielded single large waiting-stage merozoites. Visceral infections resulted in multifocal necrosis. Proliferative visceral Isospora infection (atoxoplasmosis) is one of the more severe causes of mortality among captive birds, free-ranging birds appear to coexist with the infection but succumb under capture stress.