Resistance of field isolates of Eimeria species to anticoccidial drugs.

A study was made of the incidence of coccidial oocysts in the litter of a broiler house used for commercial trials. The following anticoccidial drugs were under investigation: zoalene, Pancoxin, Lerbek and monensin. Monensin was the only drug able to suppress the numbers of oocysts appearing in the litter. Two species of Eimeria were isolated from this site and their response to anticoccidial agents studied in the laboratory. The isolate of E. tenella was not controlled by zoalene, clopidol or monensin but was controlled by amprolium, sulphaquinoxaline, methyl benzoquate and robenidine. The isolate of E. acervulina was not controlled by zoalene, amprolium, sulphaquinoxaline, methyl benzoquate, clopidol or robenidine. The only treatment successful in controlling the isolate of E. acervulina was monensin. Results obtained indicate that the lack of activity of certain drugs was due to drug resistance of the Eimeria species concerned.