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February 15, 2020

Survey: Veterinarians see risk from drug-resistant parasites

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Veterinarians and parasitologists tend to perceive antiparasitic drug resistance as at least a moderate risk to ruminants and horses, according to survey results.

Sheep standing in a pasture
Parasites resistant to antiparasitic drugs threaten ruminants and horses, and Food and Drug Administration officials are trying to teach animal owners how to avoid selecting for resistance. Survey results indicate veterinarians tend to recognize that resistance is a risk.

Respondents saw high risk among goats and sheep, moderate to high risk among horses, and low to moderate risk among cattle.

Food and Drug Administration officials published in December 2019 the results of a fall 2015 survey on antiparasitic drug use, perceived prevalence of resistance, and preferred responses to treatment failures. About three-quarters of respondents said they had seen antiparasitic resistance within the prior three years.

FDA officials also noted in a December 2019 announcement that, in the prior year, one-third of drug companies selling dewormers for livestock and horses changed their product labels to add information about antiparasitic drug resistance. In December 2018, agency officials had asked drug companies to note that any dewormer use can select for drug resistance, proper dosing is vital for safe and effective use, veterinarians should monitor for resistance on farms, and dewormers are only part of an internal parasite control program.

FDA officials also recently published two educational videos for animal producers and owners on antiparasitic drug resistance: one about how to detect resistance and respond and the other on use of refugia, or the practice of leaving some animals untreated to maintain a population of anthelmintic-susceptible parasites. The announcement, with links to the videos, is available at the FDA website.

The survey results published late last year indicate many animal owners make decisions on antiparasitic drug use without consulting veterinarians. About 58% of private practitioners who work with cattle said most of their clients involve a veterinarian in such decisions, and that was true for just less than half of the private practitioners who work with horses or small ruminants.

The responses suggest veterinarians most often check the results of treatment through fecal egg reduction counts and reduction of clinical signs, with practitioners who work with horses and small ruminants more likely to choose reduction counts and cattle veterinarians more likely to watch for clinical signs. The most common responses to treatment failures differed by species.

“Fifty-six percent of respondents with cattle experience selected the use of another antiparasitic drug as their first choice,” the report states. “Forty-four percent of respondents with horse experience selected a change to a selective treatment program as their first choice.

“For respondents with small ruminant experience, 32% selected animal management changes and 34% selected treatment with another antiparasitic drug as their first choice.”

Veterinarians who work with cattle and small ruminants were more likely than those who work with horses to recommend pasture management changes such as adding multispecies grazing, controlling forage height, and adding or modifying rotational grazing.