Some aspects of the biology and host-parasite interactions of Isospora spp. (Protozoa: Coccidiida) of passerine birds

In wild bird populations parasites are at least as important as predators. The subject of this work was Isospora, a genus of intestinal coccidia that has been rarely studied in passerine birds. We established a method of estimating the intensity of Isospora infection and showed diurnal periodicity in oocysts output. We studied fauna of Isospora species and their host specificity. Ground feeders are more frequently and intensively infected than bird species feeding in the air. The young birds are more intensively infected than adult ones because of higher level of chronic infection and higher susceptibility to re-infection. During autumn migration there was no difference in prevalence of infection between birds on the Courish Spit and on the island of Helgoland, whereas the intensity of infection was lower on the island than on land, and this difference was more pronounced in long-distance migrants. Under natural cirumstances most birds are likely to be able to tolerate Isospora infection. However, in young birds, by infection with high doses, re-infection, or particular environmental constraints these parasites may have profound effects on fitness and survival.