The development of drug resistance in Eimeria is common because of extensive use of anticoccidial drugs for the control of avian coccidiosis. The significance of chemotherapy is evident from the fact that, in spite of advancement in the field of immunological, biotechnological and genetic methods, prophylactic chemotherapy with anticoccidial drugs is still widely used for the control of coccidiosis. In such situations, new drugs should be available to replace the older ones against which resistance has developed, however it takes a long time to develop any new compounds. It is therefore currently necessary to develop strategies to minimise the emergence of resistance in Eimeria strains and to prolong the effect of available anticoccidial drugs. This paper summarises the resistance status of Eimeria species in different parts of the world and reviews different types of resistance, mechanism of resistance development, factors involved in the development and spread of resistance, management of resistant strains and strategies to preserve the efficacy of the available anticoccidial drugs. Use of vaccines, synthetic and botanical anticoccidials and educating farmers about recommended coccidiosis control practices are discussed in this review, along with the integration of currently available options for the management of drug resistance and, ultimately, the control of coccidiosis.

Resistance to diclazuril was induced by 10 passages of the Houghton strains of Eimeria acervulina and E. tenella in chickens given progressively greater concentrations of the drug. This resistance was, however, not complete since the drug retained some efficacy against the drug-passaged lines. Attempts to passage the Houghton strain of E. maxima in birds medicated with concentrations of diclazuril greater than 0·016 parts per million (ppm) were unsuccessful and after 10 passages at this concentration resistance had not developed. Resistance to methyl benzoquate developed after 6 passages of E. tenella in medicated chickens but resistance to amprolium was only partial after 10 passages. The pathogenicity of diclazuril- and amprolium-resistant lines of E. tenella was lower than that of the parental line. Diclazuril was effective against lines of E. tenella resistant to amprolium, arprinocid, clopidol, dinitolmide, halofuginone, methyl benzoquate, monensin and robenidine.