SUMMARYNecrotic enteritis (NE) is a common disease that causes great economic loss to the broiler industry due to mortality and reduced performance. Although  Clostridium perfringens  (CP) is a necessary component of this disease, coccidia species are a well-defined predisposing factor that exacerbate the condition. Different  Eimeria  species have been reported to influence necrotic enteritis to different degrees. In a pair of experiments, six different  Eimeria  species were evaluated in the presence and absence of  Clostridium perfringens . Male broiler chicks were housed in battery cages for the duration of both experiments. Feed conversion, body weight gain, and NE mortality were reported in both experiments. Experiment 1 challenged birds with  E. maxima ,  E. acervulina ,  E. tenella ,  E. necatrix,  and  E. brunetti  at day 13 and subsequently inoculated birds with CP on days 18, 19, and 20. In the second experiment,  E. maxima ,  E. acervulina ,  E. tenella , and  E. praecox  were inoculated on day 15 and challenged with CP on days 17, 18, 19, 20, 21, and 22 of the experiment.  In the first experiment,  E. acervulina ,  E. brunetti ,  E. maxima , and  E. necatrix  with the addition of CP all stimulated necrotic enteritis mortality. In the second experiment,  E. praecox  had minimal impact on performance during the challenge (14-23 days) while  E. maxima  + CP decreased body weight gain and increased mortality compared to the CP alone control.  E. maxima  had the highest mortality (21.9% a ) in this experiment followed by  E. acervulina  (6.3% b ). The remaining  Eimeria  with added CP in the second experiment did not induce NE mortality. While the challenge with CP alone did not induce mortality, feed conversion was increased compared to the unchallenged control group. When using isolated  Eimeria  species in these experiments, disturbances created by  E. brunetti  and  E. maxima  resulted in the most severe challenges. These experiments highlight the necrotic enteritis risk of these species of  Eimeria  and give insight into how other species interact with the host in a controlled CP challenge model.

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