Laboratory selection for ivermectin resistance in Haemonchus contortus

Area of Science:

• Veterinary Parasitology
• Anthelmintic Resistance
• Molecular Biology

Background:

• Haemonchus contortus is a major gastrointestinal nematode parasite of small ruminants.
• Ivermectin (IVM) is a widely used broad-spectrum anthelmintic.
• The emergence of anthelmintic resistance threatens livestock health and productivity.

Purpose of the Study:

• To investigate the development of ivermectin resistance in Haemonchus contortus.
• To assess the efficacy of ivermectin against a selected strain of Haemonchus contortus.

Main Methods:

• A susceptible strain of Haemonchus contortus was subjected to repeated selection with ivermectin (0.02 mg kg-1) over nine passages in lambs.
• Worm egg counts (epg) were monitored throughout the selection process.
• The efficacy of ivermectin against the selected strain was compared to the original susceptible strain post-treatment and at necropsy.

Main Results:

• Ivermectin treatment achieved >99% reduction in epg for the first seven passages.
• No significant reduction in epg was observed after the seventh passage.
• The ninth selected generation of Haemonchus contortus showed no significant reduction in epg after ivermectin treatment, unlike the susceptible strain which showed a 99.9% reduction.

Conclusions:

• The selection procedure successfully induced resistance to ivermectin in Haemonchus contortus.
• This study demonstrates the rapid development of ivermectin resistance under selection pressure.
• Findings underscore the importance of judicious anthelmintic use to mitigate resistance development.