Eimeria tenella: experimental development of resistance to monensin in the chicken

The development of resistance by the Houghton strain of Eimeria tenella (H) to monensin has been studied by serially passaging the strain in groups of 30 birds, each inoculated with 10(6) oocysts and given 100 p.p.m. of drug in the food. After 16 passages a line partially resistant to this concentration of monensin was produced. Passage of the line in birds medicated with 200 or 300 p.p.m. of monensin did not result in the acquisition of a greater degree of resistance. A further reduction in sensitivity to 100 p.p.m. monensin was obtained, however, when the line was serially passaged in 15 groups of 6 birds each given 3 X 10(6) oocysts. Five passages in the absence of monensin resulted in restoration of sensitivity, suggesting that the resistance developed was not stable. The pathogenic effects of the line that had been passaged in chickens given 300 p.p.m. of monensin were less than those of the parent line.