FDA wants label warnings about antiparasitic resistance

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Food and Drug Administration officials want labels for anthelmintics used to deworm horses and livestock to include a warning that these drugs can select for resistant parasites.

The proposal is a response to growing resistance to those drugs among parasites. The drug labels also would state that proper dosing is important, animal owners should work with veterinarians to monitor antiparasitic resistance, and dewormers should be considered just one part of a comprehensive parasite control program.

Sheep standing in a field
Parasites, especially nematodes, that are resistant to antiparasitic drugs are threats to the health of grazing animals. A Food and Drug Administration proposal calls for adding information on resistance to the labels of anthelmintics administered to cattle, sheep, goats, poultry, swine, and horses.

In December 2018, agency officials announced they were asking that drug companies agree to revise in 2019 labels for over-the-counter and prescription anthelmintics given to horses, cattle, sheep, goats, swine, and poultry. They also said new anthelmintics administered to those species should include the same information.

"Although antiparasitic resistance in livestock and horses does not directly affect human health in the U.S., it is a growing animal health threat in this country," the announcement states.

Helminths that survive anthelmintic treatment can pass resistance to offspring, selecting for resistant populations, the announcement states.

In a letter to veterinarians also published in December, FDA officials wrote that antiparasitic resistance in general—and resistance among nematodes in particular—threatens animal health, animal welfare, and agriculture.

"In the U.S., many veterinarians and livestock producers acknowledge that antiparasitic resistance is a severe problem in small ruminants and a growing problem in horses," the letter states. "However, awareness of antiparasitic resistance in cattle is relatively low, even while reports of antiparasitic resistance in this species are increasing."

Relying on antiparasitic drugs alone to control parasites, without also altering management practices, increases the risk that parasites soon will develop widespread resistance to all major families of broad-spectrum anthelmintics.

The FDA wants labels to say that those who are considering administering anthelmintics should assess parasite management history and perform fecal examinations or other diagnostic tests, the letter to veterinarians states. Those who administer the drugs should monitor treatment effectiveness.

Extended-release macrocyclic lactones should have an additional warning that prolonged drug exposure can increase selection pressure for resistance, the letter states.

Visit the FDA's web page on antiparasitic resistance or read the letter to veterinarians at jav.ma/FDAletter for more information.