Drug Resistance in Eimeria tenella. X. Restoration of Drug Sensitivity Following Exposure of Resistant Strains to Other Coccidiostats

Abstract

Twelve resistant substrains were derived from 4 resistant strains of Eimeria tenella by exposing the latter for 21 passages in chickens to coccidiostats to which they had no cross-resistance. Six of the substrains regained their sensitivity to the original drug. Thus, primary amprolium-, nicarbazin-, or zoalene-resistant strains became sensitive to these drugs again following subsequent exposure to a second coccidiostat. This was not the case for a glycarbylamide-resistant strain. The degree of tolerance remains virtually unchanged when drug-resistant strains of avian coccidia are passed serially through groups of unmedicated chickens (Gardiner and McLoughlin, 1963; Ball, 1968; McLoughlin and Chute, 1969). Moreover, in a study of field strains, Williams (1969) found that a strain of Eimeria acervulina and one of E. tenella remained resistant to zoalene after 59 weeks of exposure to clopidol and an additional 31 weeks to decoquinate. These strains also remained resistant to clopidol following the decoquinate medication. Williams also found that resistance persisted for varying periods in other species and strains following exposure to a second or third coccidiostat. In the present study, strains of E. tenella resistant to a specific coccidiostat were serially exposed to other coccidiostats to which there was no cross-resistance. Each strain was then tested against the drug to which it had developed the original resistance. The results obtained with four drug-resistant strains are the basis for this report. MATERIALS AND METHODS The drug-resistant strains, all of which were derived from the same parental stock, were developed experimentally under laboratory conditions. The individual strains resistant to amprolium, glycarbylamide, nicarbazin, and zoalene, respectively, are not cross-resistant to any of the others employed in these trials (McLoughlin and Gardiner, 1961, 1962, 1967, 1968, and unpublished data). Received for publication 28 July 1970. *A portion of this material was presented at a symposium on Anticoccidial Drugs for Use in Controlling Coccidiosis, Athens, Georgia, 26-27 May 1969. The resistant strains were assayed for tolerance to the coccidiostat to which they were resistant and were again tested for cross-resistance to the other coccidiostats used in these trials. Three substrains of the amprolium-resistant strain were developed by serial exposure to glycarbylamide (AMP-G), nicarbazin (AMP-N), or zoalene (AMP-Z); of the glycarbylamide-resistant strain by serial exposure to amprolium (GLY-A), nicarbazin (GLY-N), or zoalene (GLY-Z); of the nicarbazin-resistant strain by serial exposure to amprolium (NCB-A), glycarbylamide (NCB-G), or zoalene (NCB-Z); and of the zoalene-resistant strain by exposure to amprolium (ZOA-A), glycarbylamide (ZOA-G), or nicarbazin (ZOA-N). For the first 5 passages, the birds in which the substrains were propagated were fed mash containing 0.0062% amprolium, 0.003% glycarbylamide, 0.0062% nicarbazin, and 0.0062% zoalene, respectively. For the following 16 passages, the usual recommended levels of the coccidiostats, i.e., 0.0125%, 0.006%, 0.0125%, and 0.0125%, respectively, were used. After 21 passages through birds fed the specific coccidiostats, the substrains were assayed for tolerance to these compounds and, subsequently, to the one to which they had the primary resistance. A control strain that had had no exposure to drugs was also tested against each of the coccidiostats. All of the assays were replicated. Three-week-old chicks were grouped by weight, usually into lots of 10 or 20, as suggested by Gardiner and Wehr (1950). Each lot was started on the appropriate mash 24 hr before the birds in the infected groups were each given approximately 100,000 oocysts of the respective strain. On the 8th day after inoculation, the surviving birds were weighed and necropsied. The ceca were removed and cecal lesions evaluated. Oocyst cultures were made from the ceca and cecal contents, and the number of oocysts produced per surviving bird was estimated for each group. Weight gains, oocyst production, mortality rates, and cecal lesion scores obtained at each passage were used to calculate the Anticoccidial Indices as suggested by McManus et al. (1968). These Indices are the basis for comparison, with a point value of