The pathogenicity, immunogenicity and endogenous development of a precocious line of Eimeria brunetti

A precocious line of Eimeria brunetti with a prepatent period of 75 hours was compared with the parent strain (normal prepatent period of 120 hours) in a battery study. The precocious line was less pathogenic than the parent, and caused less weight depression and lower lesion scores in chickens given any dose of oocysts (1,000, 5,000, 25,000 or 125,000). No chickens receiving the precocious line died, whilst 19% and 25% mortality occurred in groups receiving 25,000 and 125,000 oocysts of the parent strain. The precocious line was also highly immunogenic. Chickens given 1,000 or 5,000 oocysts of the precocious line or parent strain developed immunity sufficient to protect against a challenge of the parent strain which caused 83% weight depression and 13% mortality in non-immunised, challenged controls. Histological observations showed that the endogenous development of the parent strain and precocious line was identical up to 48 hours. However, by 72 hours macrogametocytes and microgametes were seen only with the precocious line. The abbreviated asexual cycle of the precocious line would account for its lower reproductive potential.