Restoration of sensitivity to salinomycin in Eimeria following 5 flocks of broiler chickens reared in floor-pens using drug programs and vaccination to control coccidiosis.

Five successive flocks of broilers were reared in floor-pens and given different drug programs or were vaccinated against coccidiosis. Oocysts of Eimeria were isolated from the litter of pens during the fifth flock and their sensitivity to salinomycin (Sal) investigated by measuring new oocyst production following infection of medicated and unmedicated birds. Parasites obtained following 5 flocks given Sal were not well-controlled and it was concluded that they were partially resistant to the drug. Parasites obtained following 4 unmedicated flocks and one medicated flock were better controlled by Sal and it was concluded that in the absence of continuous medication there had been an improvement in drug efficacy. Sal almost completely suppressed oocyst production of isolates from treatments in which medication was followed by vaccination, indicating that when a drug program is followed by vaccination, restoration of sensitivity to Sal had occurred.