Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 4, 1996
Publication Date: N/A
Interpretive Summary: Avian coccidiosis, an intestinal disease caused by protozoan parasites, is estimated to cost the U.S. poultry industry over $300 million annually. Control of these parasites is through use of anticoccidial compounds mixed in the feed, but within the last ten years, the coccidia have become more and more resistant to these medications. Compounding the difficulty of drug-resistant strains of these parasites is the large number of coccidial species (at east 8) that infect domestic chickens. The identification of coccidial species has been based on parasite morphology and the region of the digestive tract the parasite infects, but little work has been done at the molecular level for parasite identification and interpretation of the relationships among species. In this study, the gene sequences of 8 different species of coccidia were studied to look at phylogenetic relationships among the parasites. The 8 species of parasites were found to fall within 4 distinct groups genetically. In addition, it was speculated that two of the species that infect chickens probably arose from species originally infecting turkeys. These results add to the basic knowledge of the genetic makeup of the coccidia, which is necessary in devising new directions in parasite control.
The complete 18S ribosomal RNA gene sequences were determined for 8 Eimeria species of chickens and from Eimeria bovis of cattle. These sequences were aligned with each other and with sequences from 2 Sarcocystis spp., Toxoplasma gondii, Neospora caninum and 4 Cryptosporidium spp. The aligned sequences were analysed by maximum parsimony to infer evolutionary relationships among the avian Eimeria species. Eimeria bovis was found to be the sister taxon to the 8 Eimeria species infecting chickens. Within this clade of avian Eimeria species, E. necatrix and E. tenella were found to be sister taxa; this clade attached basally to the remaining chicken coccidia. The remaining Eimeria spp. formed three clades that corresponded to oocyst sizes and shapes. Eimeria mitis and E. mivati (small, near spherical oocysts) formed the next most basal clade followed by a clade comprised of E. praecox, E. maxima and E. brunetti (large, oval oocysts) which was the sister group to E. acervulina (small, oval oocysts). The 4 clades of avian Eimeria species were strongly supported in a bootstrap analysis. The basal rooting of a clade containing E. necatrix and E. tenella between the E. bovis and the Eimeria species infecting the upper intestinal tract and the apparent absence of coccidia that infect the caeca in jungle fowl all suggest that E. necatrix and E. tenella may have arisen from a host switch, perhaps from the North American turkey, Meleagris gallopavo.