Medical Parasitology

The Coccidian Parasites

Coccidia are complex unicellular parasites of vertebrates and invertebrates. They parasitise their host intracellularly. Coccidian parasites are responsible for several of the most severe diseases known in animals and man. For example in domestic animals, Eimeria tenella is responsible for considerable decrease in growth and development of domestic poultry flocks by damage caused to the intestinal lining during infection. The traditional coccidians (Eucoccidia) have been described in all classes of vertebrates such as in fish, reptiles, birds, and mammals including humans. However the second coccidians (Adeleorina) includes species associated with invertebrates as well as parasites of vertebrates. The latter group infects blood cells of vertebrates and is collectively known as "haemogregarines". Veterinary important parasites in the genus Haepatozoon belong to this group. Parasites within the coccidian group can be either monoxenous, parasitising a single host throughout their lifecycle, or heteroxenous whereby the parasite will parasite multiple hosts. For instance, species belonging to the genera Eimeria and Isospora are well known monoxenous parasites, while those of Toxoplasma make use of an intermediate host. It is thought that coccidians that have heteroxenous lifecycles can have a wide range of intermediate hosts, but they commonly have a narrow range for the final host. As with the rest of the phylum Apicomplexa, classification within the coccidian groups is largely a 'mess' due to incorrect or misinterpreted morphological data gathered over the years. Increasingly, clade support is being sought to characterise parasites with the advent of molecular biology. Due to the escalating amount of information being made available on the apicomplexans, such standardisation is vital to enable arrangement of this information for further use.