The mode of action of anticoccidial quinolones (6-decyloxy-4-hydroxyquinoline-3-carboxylates) in chickens.

The anticoccidial mode of action of quinolones (6-decyloxy-4-hydroxyquinoline-3-carboxylates) against Eimeria tenella and E. acervulina in chickens has been investigated, using decoquinate and M&B 15,584 as examples. The well known static effect on sporozoites of relatively high continuous drug concentrations in the food masked other components of the mode of action, newly described here. Lower concentrations of quinolones allowed sporozoites to continue their development. First-stage schizonts were susceptible to a secondary cidal effect, although later schizonts seemed to be rather refractory. Furthermore, the sporulation of oocysts produced by E. tenella that completed its life cycle in the presence of suboptimal concentrations of quinolones was inhibited: this probably reflects a drug effect on gametocytes. Quinolones were absorbed rapidly from the chicken intestine, probably in less than 1 h. Drug withdrawal experiments showed that quinolones persisted in chicken tissues at active concentrations for up to 48 h. Despite their static effect on sporozoites, they may nevertheless be expected to exert a therapeutic effect against drug-sensitive coccidia in interrupted regimes that allow the later cidal effect to come into play. This allows immunity to coccidiosis to develop in the presence of drug. These new results, with the previously available data have been combined in an updated account of the anticoccidial mode of action of quinolones in the chicken.