Eimeria tenella: experimental studies on the development of resistance to amprolium, clopidol and methyl benzoquate

Abstract

SummaryThe development of resistance by the Houghton strain of Eimeria tenella to the anticoccidial drugs amprolium, clopidol and methyl benzoquate has been studied. Resistance to amprolium and clopidol developed more readily in experiments where a large number of coccidia were exposed to the drug, either by increasing the number of oocysts in the inoculum or by increasing the number of birds in the group. When 45 birds were given 2·0 × 106 oocysts, resistance to amprolium and clopidol appeared after 6 and 7 passages respectively. In previous experiments, under similar conditions, resistance to robenidine developed after 6 passages, suggesting little difference between these three drugs. Resistance to amprolium and clopidol arose gradually as the concentration of drug was increased, but resistance to methyl benzoquate appeared in a single step from sensitivity to high-level resistance. Both amprolium and clopidol-resistant lines showed an 8-fold reduction in drug sensitivity. Attempts to measure the degree of resistance by calculation of the ED50 were unsuccessful.

Eimeria tenella, E. acervulina and E. maxima: studies on the development of resistance to diclazuril and other anticoccidial drugs in the chicken

“…In E. tenella the development of resistance to diclazuril was compared with that to two other drugs. We have previously shown that resistance to methyl benzoquate and amprolium can be induced rapidly or slowly respectively in the laboratory (Chapman, 1978) and resistance is known to have developed either rapidly (methyl benzoquate) or slowly (amprolium) in the field (Chapman, 1984a). In this study resistance to methyl benzoquate and partial resistance to diclazuril occurred after 6 and 10 passages respectively.…”

“…Oocysts of E. tenella were recovered from the caeca on day 7 after inoculation of the birds and those of E. acervulina and E. maxima from the faeces of infected chickens between days 4-6 and 6-8 after inoculation respectively. Lines of E. tenella resistant to amprolium, arprinocid, clopidol, halofuginone, methyl benzoquate, monensin and robenidine have been developed from the Houghton strain of E. tenella by serial passage of this parasite in the presence of the drugs (see Chapman, 1976Chapman, , 1978Chapman, , 19846, 1985Chapman, , 1986a. A line of E. tenella resistant to dinitolmide was obtained from the field (Chapman, 1989).…”

“…The parasites were passaged in medicated or unmedicated birds using methods based on those previously described (Chapman, 1978). Each group consisted of 10 birds (housed in 2 cages of 5 birds) and each bird was given a standard dose of 2 x 10 5 oocysts for each passage.…”