Development of Sarcocystis falcatula in its intermediate host, the Brown-headed Cowbird (Molothrus ater)

Sporocysts of Sarcocystis falcatula obtained from experimentally infected V irginia opossums (Didelphis virginiana) were inoculated orally to 60 wild-c aught Brown-headed Cowbirds (Molothrus ater), Another 30 Brown-headed Cowbi rds were not challenged and served as uninfected controls. Two inoculated a nd one control cowbird were necropsied every 2 weeks and the pectoral and t high muscles were examined grossly for cyst development. Stained histologic sections of pectoral muscle, thigh muscle, and lung were examined by light microscopy and presence, density, and size of sarcocysts were determined. Sarcocysts were present by 6 weeks post-inoculation (PI) and were still gro wing at 30 weeks PI. The sarcocysts from birds 40 weeks post-infection were infective to an opossum. The morphology of the sarcocyst wall by transmiss ion electron microscopy substantiated the identification as S. falcatula. L ung sections were examined for the presence of schizonts, but were seen onl y at 2 weeks PI. This evaluation was complicated by the presence of unident ified microfilariae. These birds are migratory and the continued growth and development of muscle cysts would allow them to be a source of infection a t both extremes of their geographic range, regardless of which end of the m igration at which they were infected. (C) 2001 Elsevier Science B.V. All ri ghts reserved.