A laboratory method for the single‐passage selection of drug‐resistant mutants from populations of Eimeria species in chickens and its potential value in the choice of field use concentrations for new anticoccidial drugs

Abstract

Populations of Eimeria species in chickens frequently include mutants that are resistant to an anticoccidial drug without having been previously exposed to it. Such mutants are described here as ‘inherently resistant’. A novel laboratory method of selecting them from their parent populations is described. Selection may be achieved in a single in vivo passage in chickens. Decoquinate‐resistant mutants were selected from a previously unexposed E. tenella strain: they tolerated > 0.625% w/w decoquinate in the food of chicks, whilst the rest of the population was sensitive to < 0.002% w/w. This single‐passage method has a unique advantage over others; development of resistance to different drugs may be validly compared by using a single criterion, the lowest selective concentration (LSC), all other experimental factors remaining constant. Furthermore, the method is economical with time, facilities and experimental animals, and may have a predictive value in the choice of use concentrations for new anticoccidial agents. A potential selection index (PSI), the proposed or actual field use concentration of a drug divided by its LSC, is defined. It is suggested that a field use concentration for a new drug should be somewhat lower than the LSC, and the overall PSI for all Eimeria species and strains tested should therefore be < 1. In the present study, the PSI for decoquinate and E. tenella was 8.00, which is consistent with the very rapid development of resistance to quinolones that occurred in the field.