The effect of arprinocid on the reproductive potential of a field isolate of Eimeria tenella.

Oocyst production by an isolate of Eimeria tenella propagated from birds medicated with 50 ppm arprinocid was compared with that of the same isolate propagated from unmedicated birds. Statistically significant (P less than 0.05) reductions in oocyst production were found for the drug-exposed isolate for inocula levels of 64 and 250 sporulated oocysts per bird. Numerical reductions were observed for inocula levels of 1,000 oocysts per bird, but with higher levels of inocula, no differences were seen. Inhibitory drug effects for the inocula levels of 64 and 250 oocysts per bird appear to be meaningful because they coincide with the maximum reproductive potential (oocysts produced per oocyst inoculated). The results indicated that significant information can be masked by excessive inoculum with consequent crowding and reduction in reproductive potential.