Eimeria tenella: experimental studies on the development of resistance to robenidine.

The development of resistance by the Houghton strain (H) of E. tenella to robenidine has been studied in the laboratory, by serially passaging the strain in chickens fed increasing concentrations of drug. Resistance to robenidine developed more readily in experiments using larger numbers of birds with higher numbers of oocysts in the inoculum. Both these factors increased the parasite population and increased the chance of selecting parasites resistant to the drug. E. tenella (H) was made resistant to 264 ppm robenidine and showed no cross-resistance to other anticoccidial agents. Resistance arose in a series of 'steps' as the concentration of drug was increased. E. tenella (H) was continuously passaged at concentrations ranging from 2 to 33 ppm of robenidine. After 16 passages, lines passaged at 2,4 and 8 ppm were not resistant to 33 ppm robenidine, suggesting that the degree of resistance developed was dependent upon the drug selection pressure.