Eimeria tenella: clinical effects in partially immune and susceptible chickens.

Chickens made partially immune to E. tenella infection exhibited cecal lesions after challenge. Some of the chickens had cecal lesions of severity similar to those of birds having their first infection with the parasite. The presence of severe cecal lesions in the partially resistant birds was not accompanied by lowered body weight gain or packed erythrocyte volumes and prothrombin time was not extended. It is suggested that there are at least three stages of immunity to E. tenella; birds may be totally resistant to parasite and no development occurs, birds may be resistant to a degree where oocysts are discharged after challenge but no lesions occur, or birds may be resistant to the clinical effects of the disease despite having severe lesions.