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Replacing drug-resistant worms

Worms that are resistant to multiple deworming compounds are becoming more and more prevalent. But what if you could replace the resistant population of worms with a susceptible population, in order to re-establish the efficacy of dewormers.

At the University of Georgia, a population of Haemonchus contortus that was highly resistant to benzimidazoles and avermectin/milbemycins with a subpopulation that was resistant to levamisole, was replaced with a susceptible laboratory isolate of H. contortus in a flock of sheep. The anthelmintic susceptibility and population genetics of the newly established population were evaluated for 3.5 years using in vivo, in vitro, and molecular methods. Successful replacement of the resistant population with a susceptible population was confirmed using phenotypic and genotypic measurements. Albendazole treatment yielded a 98.7% fecal egg count reduction.

To sustain the susceptibility of the new population, targeted selective treatment was implemented using albendazole. Surprisingly, within 1.5 years post-replacement, the population reverted to a resistant phenotype, and resistance to albendazole, ivermectin, and moxidectin was confirmed. Targeted selective treatment was then carried out using levamisole. However, within one year, resistance was detected to levamisole. In the study, successful replacement of a resistant population of H. contortus with a susceptible population did not result in long term maintenance of drug efficacy. However, the researchers felt that a flaw in the methodology, rather than a flawed strategy was probably responsible. The researchers believe that parasite replacement likely remains a viable strategy for enhanced parasite control on farms, but that great care is needed when selecting the methodology to use. Parasite populations must have sufficient fitness to survive under local conditions to be effectively used in replacement strategies.

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