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Selection for anthelmintic resistance by macrocyclic lactones in Haemonchus contortus
L F Le Jambre1, R J Dobson, I J Lenane
1CSIRO Division of Animal Production, Pastoral Research Laboratory, Armidale, NSW, Australia. llejambr@ram.chiswick.anprod.csiro.au
International Journal for Parasitology
|September 29, 1999
Summary
Investigating anthelmintic resistance in Haemonchus contortus, this study found moxidectin (MOX) and ivermectin (IVM) treatments significantly impact parasite populations. MOX generally reduced worm burdens, but simulations suggest IVM-capsule may accelerate resistance development most rapidly.
Area of Science:
- Veterinary Parasitology
- Anthelmintic Resistance
- Sheep Nematodes
Background:
- Haemonchus contortus is a major gastrointestinal nematode affecting livestock globally.
- Anthelmintic resistance, particularly to macrocyclic lactones (MLs), is a growing concern in H. contortus populations.
- Understanding selection pressures from different anthelmintics and treatment strategies is crucial for resistance management.
Purpose of the Study:
- To investigate the selection for anthelmintic resistance in Haemonchus contortus using ivermectin (IVM) and moxidectin (MOX).
- To compare the effects of different selection types (Head, Tail, Head + Tail) on resistance development.
- To simulate the field-level rate of resistance development under strategic parasite control programs.
Main Methods:
- Used two morphologically marked H. contortus strains: CAVRS (ML-resistant) and McMaster (ML-susceptible).
- Exposed adult sheep and lambs to IVM (non-persistent) and MOX (persistent) under three selection scenarios.
- Employed computer simulations to model resistance selection under various strategic parasite control programs.
Main Results:
- Moxidectin significantly reduced total worm numbers in adult sheep compared to controls.
- Moxidectin treatments resulted in a higher proportion of resistant worms in adult sheep, while selection type had no significant effect.
- In lambs, IVM Tail, MOX Head + Tail, and MOX Tail selections resulted in more resistant worms; MOX selected for higher proportions of resistant worms than IVM.
Conclusions:
- Anthelmintic type significantly influences resistance selection, with MOX potentially selecting for higher proportions of resistant worms in certain scenarios.
- Simulations indicated that Closantel (CLS) resistance accelerates macrocyclic lactone (ML) resistance selection.
- IVM-capsule is predicted to select most rapidly for ML resistance, while oral IVM is least selective; MOX is intermediate.