The importance of transmission-blocking immunity in the control of infections by apicomplexan parasites.

Transmission-blocking immunity may have great potential for use in the control of diseases caused by apicomplexan parasites. In this review I will describe our work on the application of transmission-blocking immunity to the control of the Eimeria parasite and compare our results to those working on transmission-blocking immunity against Cryptosporidium and Plasmodium. Eimeria causes the disease known as coccidiosis in domestic animals. Coccidiosis is particularly problematic in the chicken industry, mainly due to the crowded rearing conditions under which chicks are raised. In our work we identified, isolated and characterized 3 major gametocyte antigens (230 kDa, 82 kDa and 56/54 kDa) of Eimeria maxima. We used these native glycoproteins to immunize laying hens that, via the egg yolk, provide large amounts of transmission-blocking maternal antibodies to offspring chicks. We demonstrated that hatchlings from immunized hens shed 60-80% fewer oocysts (i.e. the infective stage of the life-cycle of Eimeria) than those from control hens. Such a reduction in oocyst output acts to significantly reduce parasite numbers in the litter of chicks raised in floor pens. This reduction in oocyst output is comparable to that seen using the most effective coccidiostat drugs and is probably sufficient to control coccidiosis under field conditions. Based on our results together with those of other groups working on transmission-blocking immunity against Cryptosporidium and Plasmodium, it appears that this immunological approach holds great promise for the control of apicomplexan parasites that cause diseases in both animals and man.