Coccidiosis and recent progress in development of vaccines

Coccidiosis, caused mainly by Eimeria, is a major cause of economic loss in domestic livestock especially in poultry. This protozoan parasite multiplies in intestinal epithelial cells of wide range of hosts. Currently chemotherapy is extensively used and is exclusive means to control coccidiosis outbreaks, however the development of drug resistance and the complexity and expense of developing new anti-coccidial drug warrant alternative methods to prevent the disease in the fast growing poultry industry. The chemotherapy may cause to serious problem when drug development is becoming increasingly expensive and public awareness for the acceptance of drug residues in food animals is maximized. For these reasons, there is an increasing interest to develop vaccines for the control of coccidiosis. Live vaccines usually confer good immunity due to the fact that the immune system is exposed to the live, often attenuated, organism over a long period of time. A significant disadvantage to using live vaccines is that the attenuated organisms may overt back to a pathogenic wild form and there is also the potential risk of contamination of vaccine preparations by other pathogenic organisms. Killed vaccines are much safer in this sense but, unlike live attenuated vaccines, they are not able to generate cytotoxic T lymphocytes (CTL) response. Recombinant vaccines defined as biochemically purified or recombinantly produced proteins and consist of certain epitopes of an organism or metabolites of the parasite. Unlike live attenuated organism, recombinant vaccines do not carry much risk and generally are able to induce humoral and T cell responses. DNA vaccine, relatively new area in vaccinology, is to deliver the DNA of known antigenic molecules to host immune system and not many studies have established on its role and efficacy in avian coccidiosis. Successful development of vaccine for coccidiosis will depend on comprehensive understanding of the protective immunity and identification of important antigenic and immunogenic molecules. This review will underscore parasite life cycle, epidemiology, host immunity and progress toward vaccines against coccidiosis, probably one of the most economically important parasitic infectious diseases in the poultry industry.