Drug Resistance in Eimeria tenella. III. Stability of Resistance to Glycarbylamide (1963)

A glycarbylamide-resistant strain of Eimeria tenella was serially propagated nine times in chickens fed unmedicated mash and then tested for glycarbylamide tolerance in chicks fed mash containing this coccidiostat. The strain showed no apparent increase in sensitivity to glycarbylamide as the result of serial propagation in the absence of the drug. The development of drug resistance has been observed in a variety of microorganisms. More than 50 years ago, Ehrlich and his students recognized and studied drug-resistant strains of trypanosomes. A sulfonamide-resistant field strain of Eimeria tenella was reported by Waletzky et al. in 1954. Cuckler and Malanga (1955) described field strains of Eimeria spp. resistant to sulfaquinoxaline, nitrofurazone, and nitrophenide and discussed the development of drug resistance by laboratory strains. Subsequently, laboratory-induced drug resistance has been the subject of a number of studies (Joyner, 1957; Horton-Smith, 1958; McLoughlin and Gardiner, 1961, 1962). In most instances, field and laboratory strains resistant to one coccidiostat have been tested for cross-resistance to others, but nothing, to our knowledge, has been published on the stability of drug resistance in coccidia. For this reason a glycarbylamide-resistant strain of Eimeria tenella was rapidly propagated for nine generations in unmedicated chickens and then tested for glycarbylamide sensitivity. MATERIALS AND METHODS The development of the resistant strain of coccidia used in this study has been described in an earlier paper (McLoughlin and Gardiner, 1961). The normal strain has been maintained in the Beltsville Parasitological Laboratory for many years. It has never been exposed to any drug. Three-week-old birds were used throughout the experiment. When more than one regimen was involved, the birds were divided by weight into Received for publication 10 January 1963. equivalent groups of 10 or 20 birds each. Each bird was inoculated with approximately 100,000 sporulated Eimeria tenella oocysts from either the resistant or the normal line, as the case required. The birds were weighed and killed on the 8th day after inoculation, and their ceca were removed and examined. Oocysts present in the ceca were recovered, sporulated, and, in the case of the resistant-line oocysts, used to inoculate the birds of the succeeding passage. Records were kept of mortality, growth, and severity of cecal lesions. The pathologic index was derived by the method suggested by Lynch (1961). The resistant strain was passed through ten successive groups of unmedicated birds. Other test and control groups were used concurrently at the first, fifth, and tenth passages (= tests A, B, and C). At the first passage (= test A), two groups of birds fed mash containing 0.003% glycarbylamide1 were inoculated with resistantand normal-line oocysts, respectively; two unmedicated groups were similarly inoculated. For test A, data for the experimental resistant and for the original resistant groups are from the same lots of birds. They are tabulated as separate groups in Table I for convenience in comparing the infections for the various tests. At the fifth and tenth passages (= tests B and C), resistant-strain oocysts recovered from the preceding serial passage were given to birds fed medicated mash as well as to those fed unmedicated mash. In addition, four other groups were used: medicated and unmedicated birds were given normal oocysts, and medicated and unmedicated birds were given oocysts from the original resistant culture, both of these cultures being the ones used in test A and since kept in cold storage. The additional unmedicated groups were included to provide a comparison of 1Glycarbylamide, under the trade name Glycamide, was supplied by Merck and Co., Rahway, N. J. At the time these experiments were initiated, the recommended level of glycarbylamide was 0.003%; subsequently, it has been increased. 657 This content downloaded from 157.55.39.221 on Mon, 03 Oct 2016 06:08:46 UTC All use subject to http://about.jstor.org/terms THE JOURNAL OF PARASITOLOGY TABLE I. Response of medicated and unmedicated chickens to inoculations of glycarbylamide-resistant and normal strains of Eimeria tenella.