Genetic mapping and coccidial parasites: past achievements and future prospects

Coccidial parasites including Cryptosporidium parvum, Cyclospora cayetanensis, Neospora caninum, Toxoplasma gondii and the Eimeria species can cause severe disease of medical and veterinary importance. As many as one-third of the human population may carry T. gondii infection, and Eimeria are thought to cost the global poultry production industry in excess of US$2 billion per annum. Despite their significance, effective vaccines are scarce and have been confined to the veterinary field. As sequencing and genotyping technologies continue to develop, genetic mapping remains a valuable tool for the identification of genes that underlie phenotypic traits of interest and the assembly of contiguous genome sequences. For the coccidian, cross-fertilization still requires in vivo infection, a feature of their life cycle which limits the use of genetic mapping strategies. Importantly, the development of population-based approaches has now removed the need to isolate clonal lines for genetic mapping of selectable traits, complementing the classical clone-based techniques. To date, four coccidial species, representing three genera, have been investigated using genetic mapping. In this review we will discuss recent progress with these species and examine the prospects for future initiatives.