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Sheep producers will be familiar with drench products containing ivermectin, but there are also ivermectin-based products registered for the control of sheep blowfly and lice.
Ivermectin belongs to the macrocyclic lactone class of insecticides, which is unrelated to any of the other registered flystrike or lice control insecticides. Aqueous solutions of ivermectin may be applied by hand jetting to treat or prevent flystrike, treat lice in long wool (up to 6 weeks before shearing), or be applied as a flystrike wound dressing. It is extremely potent against blowfly larvae by contact or ingestion, with no resistance known to occur in sheep blowfly populations including IGR-resistant strains.
Ivermectin is a nerve poison with relatively rapid effect. Large sheep blowfly larvae are immobilized within 6-8 hours of contact with lethal concentrations of ivermectin. Effectiveness of hand jetting depends on the thoroughness of application. The jetting solution must contact blowfly larvae or lice to kill any present at treatment. As a flystrike preventative thorough application is essential to ensure sufficient insecticide is deposited to skin level of flystrike prone sites to achieve the label claim of up to12 weeks protection.
Ivermectin is an excellent choice as a flystrike dressing. Effective flystrike treatment is achieved by the elimination of maggots from strike lesions and protection of the wound site for sufficient time to allow healing and the return of the skin to a non-susceptible state. To reduce fly breeding there is an advantage in killing larvae and sheep producers certainly expect treatments to kill larvae quickly. Ivermectin is extremely effective in killing organophosphate or IGR-resistant third instar maggots.
It is important when treating struck sheep that larvae remain in contact with the insecticide for as long as possible to achieve maximum kill. In addition to the high larval kill that can be achieved by the application of ivermectin, sheep producers can expect treated wounds to retain enough insecticide to provide sufficient protection from restrike at least until flystrike wounds heal. Ivermectin has relatively low toxicity to mammals. Sheep jetted with ivermectin are subject to a 7-day export slaughter interval (ESI), a 7-day meat withholding period (WHP) and a 42-day WHP for wool.