Anthelmintic resistance (AR) status in Madras Red sheep from selected field flocks of a government funded scheme, covered by regular, sustained
anthelmintic treatment for more than 10 years was determined. Parameters such as fecal egg count reduction test (FECRT), larval
paralysis assay (LPA), and allele-specific-PCR (AS-PCR) were used to test the efficacy of
fenbendazole,
tetramisole, and
ivermectin at recommended doses, in two seasons. Sheep belonging to non-beneficiary farmers were used as controls. Mean FECRT values of beneficiary group during winter and summer seasons were 77.77 and 76.04, 93.65 and 92.12, and 95.37 and 98.06 %, respectively, for
fenbendazole,
tetramisole, and
ivermectin. In the non-beneficiary groups, the corresponding values were 74.82 and 81.09 %, 96.05 and 97.40 %, and 97.26 and 98.23 %, respectively. The results revealed resistance to
fenbendazole, suspect resistance to
tetramisole and susceptibility to
ivermectin in beneficiary flock. In non-beneficiary flock, while resistance was noticed against
fenbendazole, both
tetramisole and
ivermectin were effective. FECR values were found to be significantly different between beneficiary and non-beneficiary groups against
tetramisole. The results of LPA confirmed this finding, as 50 % of the Haemonchus contortus larvae were paralyzed at the concentration of 0.0156 μg/ml in the beneficiary group, while those of non-beneficiary groups required lower concentrations of 0.0078 μg/ml. AS-PCR revealed the predominance of heterozygous susceptible population of H. contortus in the beneficiary group. In this study, resistance to
fenbendazole was confirmed in both the beneficiary and non-beneficiary groups and this could be attributed to frequent use of
benzimidazoles as seen from the deworming records. Emergence of
tetramisole resistance was detected in the beneficiary group, where the
drug was used continuously for 4 years.
Ivermectin was found to be effective in all the flocks. It is recommended that the practice of routine deworming of three to four times a year should be avoided, as it can lead to emergence of
anthelmintic resistance.