EIMERIA-TENELLA INFECTIONS IN CHICKENS - ASPECTS OF HOST-PARASITE - INTERACTION

Intestinal coccidiosis, caused by various species of Eimeria, has beco me an economically important disease of poultry and livestock througho ut the world. Infection of chickens starts after ingestion of oocysts when sporozoites penetrate the epithelium of the villi. After passage through the lamina propria, they enter crypt epithelial cells where th ey undergo several rounds of asexual and sexual proliferation, thus fo rming merozoites and later, gametocytes. When macrogametes are fertili zed by microgametes, oocysts are formed that are shed in the faeces. N owadays, coccidiosis is prevented by anticoccidial drugs that are adde d to food, but the prolonged use of these drugs leads inevitably to th e emergence of resistent Eimeria strains. During infection, there are three stages when the chicken immune system can inhibit parasitic deve lopment. The first is when the sporozoite searches for a site of penet ration and binds to the epithelium. The second is when the sporozoite is in the villus epithelium amongst intra-epithelial leucocytes. The t hird is during its passage through the lamina propria to the crypt epi thelium. To investigate this, the decisive factors in the induction an d effector phase of immunity against coccidiosis have been investigate d in situ, Our studies have revealed that three phenomena are responsi ble for immunity against Eimeria infections. First, the actual passage and presence of parasites in the lamina propria to induce immunity, S econd, the sporozoite seems to be the most important parasite stage fo r immunity, and third, cytotoxic T cells are necessary to inhibit para sites.