Parasite genetics, protection and antigen identification

The Eimeria species, causative agents of the disease coccidiosis, are largely controlled by prophylactic administration of anticoccidial drugs, but this approach is unsustainable. Live-parasite vaccines are available and used principally in the egg production sector. The development of subunit vaccines would revolutionize coccidiosis control, but this approach requires identification of the protective antigens expressed by the parasite. Traditionally, antigen selection is based upon recognition by the host immune system. However, many of the responses generated by infection are not protective and only a small subset of antigens is capable of stimulating protective immunity. Recognition of this aspect of the host-parasite relationship has driven the development of a new approach based upon parasite genetics, DNA fingerprinting and selection by immunity. This approach was developed with Eimeria maxima and the results have significant implications for the development of vaccines against other parasitic pathogens First, the relative ease in generating recombinant parasites that combine the phenotypes of drug resistance and strain-specific immunity indicates that few parasitic loci encode antigens that stimulate protective immunity. Secondly, the inheritance of relevant genomic regions can be followed using genetic fingerprinting of DNA from selected and non-selected parasites. Thirdly, these regions are highly likely to encode the protective antigens required for the development of an effective vaccine. At present, the genetic markers group into four regions of the genome, supporting our hypothesis that few parasite antigens are capable of stimulating protective immunity. These genomic regions are under scrutiny for identification of candidate antigens for inclusion in future vaccines. © CAB International 2006.