THE PATHOGENIC EFFECTS OF A UK FIELD ISOLATE OF EIMERIA-TENELLA IN MODERN BROILER AND LAYER CHICKENS

Trials were conducted in groups of 14 day old Ross Broilers and Lohman n Brown laying birds experimentally infected with 350, 1,250, 5,000, 2 0,000, 80,000, 320,000, or 1,280,000 sporulated oocysts of a field str ain of Eimeria tenella. The oocyst output, weight gain and performance , clinical signs and mortality were recorded for 14 days post infectio n. Birds were necropsied for lesion scoring and histopathological exam ination. Performance was related to the level of challenge with mortal ity of up to 80% occurring in birds infected with 20,000 oocysts and a bove. There was good correlation between clinical signs and post morte m findings, with severe lesions present in birds in the more heavily i nfected groups. Caecae were engorged and distended with bloody caecal cores. Histopathological examination confirmed the presence of increas ing numbers of parasites from day 6 onwards, with massive epithelial d estruction and mucosal sloughing in the heavily infected groups.