Applied strategies for the control of coccidiosis in poultry.

Apicomplexan parasites of the genus <i>Eimeria</i> are widespread in poultry flocks and if present in sufficient numbers can cause the intestinal disease coccidiosis. For many years, the disease has been controlled either by incorporating drugs in the feed (prophylaxis) or by immunization with live parasites (vaccination). Efficacy of anticoccidial drugs has been compromised by the acquisition of resistance which has been documented for all drugs introduced including those most widely used, the ionophorous antibiotics. A consequence of limited efficacy is that these drugs permit some parasite development thus allowing the acquisition of immunity. Combined with improvements in management, this may explain the continued use of drugs for the control of coccidiosis in broiler and turkey production. In recent years vaccination, utilizing live attenuated or non-attenuated parasites has made a significant contribution to the control of coccidiosis in chickens. Vaccines are administered on a single occasion by a variety of methods to newly hatched chicks and initiate an immune response in the bird. Full protective immunity requires secondary exposure to vaccinal oocysts in the litter, an aspect of vaccination over which the producer has little control. Exposure to wild-type parasites already present in the litter may cause clinical coccidiosis, as evident by characteristic intestinal lesions in birds, before protective immunity has time to fully develop. Several vaccines comprise parasites that have never been exposed to anticoccidial drugs and it has been shown that their use in a commercial setting may replace drug-resistant organisms resulting in a restoration of drug-sensitivity. Programs involving alternation of chemotherapy and vaccination have been introduced with the objective of prolonging the life of anticoccidial drugs that have been compromised by drug-resistance. This is the only known example of restoration of drug sensitivity in veterinary parasitology. So-called 'natural' products that include herbs, herbal and fungal extracts, and probiotics are claimed to alleviate <i>Eimeria</i> infections in poultry, but few have been thoroughly investigated. Advances have been made in our knowledge of the structure of proteins released during parasite invasion of host cells and may eventually result in novel vaccines based upon molecular technology. Hens immunized with proteins isolated from gametocyte stages of the life cycle of one species of <i>Eimeria</i> pass antibodies to developing embryos and this may provide short-term passive protection to young chickens' post-hatch. As with other approaches to vaccination, protection is dependent upon exposure to wild-type parasites present in the environment.