Recent advances in the knowledge of the biology of the cyst-forming coccidia

Abstract

A review is given on recent knowledge of the genera Caryospora, Isospora, Cystoisospora, Hammondia, Toxoplasma, Besnoitia, Sarcocystis and Frenkelia. Caryospora hitherto considered to be monoxenous was found to have an optional intermediate host. Many Isospora species had to be transferred to the genus Cystoisospora because optional intermediate hosts were discovered. The obligatory two-host genus Hammondia could be confirmed to be distinct from the genus Toxoplasma. Even though the life cycles of Besnoitia wallacei and B. darlingi could be elucidated, the mode of transmission of B. besnoiti which is of considerable economic importance is still unknown. Since the discovery of the life cycle of the sarcosporidia in 1972 more than 600 papers have been published on this parasite. At present, 122 Sarcocystis species are named, and of 56 species both the definitive and intermediate hosts are known. It was shown that not only carnivorous and omnivorous mammals but also birds of prey, owls, and reptiles are definitive hosts of Sarcocystis species. the close relationship between the genera Sarcocystis and Frenkelia was confirmed by several investigators. Biology and Sarcocystis are of special interest. In the developmental cycle of most Sarcocystis species there are two schizogonic generations in endothelial cells of blood vessels, and in some species there is an additional asexual multiplication by endodyogeny in white blood cells. Some Sarcocystis species are highly pathogenic in non-immune intermediate hosts. Moreover, the sarcocysts which hitherto have been considered to be apathogenic may impair their hosts. In horses, they may cause myopathy, and in pigs, they have considerable influence on several parameters determining meat quality.