Internal parasites. A study of gastrointestinal parasites in Louisiana weaner-yearling beef cattle showed that three ivermectin treatments during grazing in fall, winter, and spring were more effective than other treatments in controlling the increase in winter-spring parasitism when grazing on contaminated pasture after weaning. Other treatments included one dose at weaning, or two treatments with ivermectin or fenbendazole in the fall and early summer.

External parasites. Because of the systemic approach of ivermectin, it is also effective against some external parasites such as lice. However, it has had a limited effect on ticks. Also, residues excreted in the feces function as an insecticide that kills manure-breeding livestock pests, i.e. horn flies, stable flies, face flies, and house flies.

Ecological Impact

The Bristol University study differed from others in the method of ivermectin application. A sustained release bolus was used, which gave a constant supply of the drug to the animal and its feces. But ivermectin is generally injected subcutaneously, which limits animal exposure to the drug. No dung decomposition problems were observed in studies using this method.

The Bristol University study established the potential of ivermectin to kill nontarget beneficial insects in manure and perhaps will lead to other studies about the possible environmental impacts of ivermectin use in cattle.