Invermectin Controls Parasites

Ivermectin has been a successful control for internal and external cattle parasites since 1981, but a question about its ecological impact lingers. Ivermectin is a potent, broad-spectrum drug that effectively controls gastrointestinal and respiratory tract nematodes, as well as flies, grubs, ticks, lice, and mites.

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

According to a number of studies, ivermectin use poses no apparent threat to the local ecology. However, a 1987 study at Bristol University in the United Kingdom showed when the drug was given to calves, its insecticidal effect in cowpies killed beneficial dung-degrading insects. Such a situation could slow nutrient cycling and increase cowpie accumulation in pastures.

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.

Kerr Center for Sustainable Agriculture Newsletter. (1987). Considerations in using ivermectin. December, 1987. Poteau, Okla.

Kerr Center for Sustainable Agriculture Newsletter. (1988). Reader’s note on “Ivermectin use” article. March, 1988. Poteau, Okla.

Schmidt, C.D. (1983). Activity of an avermectin against selected insects in aging manure. Environmental Entomology 12(2): 455-457.

Wall, R. and L. Strong. (1987). Environmental consequences of treating cattle with the antiparasitic drug ivermectin. Nature 327: 418-421.

Williams, J.C., J.W. Knox, K.S. Marbury, R.A. Swalley, and R.E. Willis. (1989). Three treatments with ivermectin in year-long control of gastrointestinal nematode parasites of weaner-yearling beef cattle. Veterinary Parasitology 33: 265-281.