FDA wants help fighting drug-resistant parasites

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Dr. Michelle Kornele said antiparasitic drug resistance, long a problem managed by owners of small ruminants, is increasing in horses and cattle.

“It’s no longer somebody else’s problem,” she said.

Drs. Kornele and Anna O’Brien, veterinary medical officers with the Food and Drug Administration’s Center for Veterinary Medicine, said in presentations this past fall to the AVMA Animal Agriculture Liaison Committee that the CVM is seeking the veterinary profession’s help in encouraging use of management practices that preserve anthelmintic effectiveness. Dr. O’Brien cited parasite control guidelines published by the American Association of Equine Practitioners as progress in recognizing the problem in horses, and she would like to see the cattle industry similarly acknowledge the problems caused by anthelmintic resistance in livestock.

(Photo by Greg Cima​)

“A lot of vets, it seems, are really just not aware of the emergence of antiparasitic resistance in the United States, primarily in cattle,” she said.

Ivermectin had been a “magic bullet” in cattle, allowing less attention to parasite management and leading to dependence on the drug, Dr. Kornele said. She said veterinary journals are increasingly publishing scientific reports that describe cases in which ivermectin, other macrocyclic lactones, and antiparasitic drugs in general are becoming less effective against parasites of cattle, small ruminants, and horses.

Kornele said the CVM wants veterinarian involvement and use of parasite management practices to maintain the usefulness of anthelmintics.

Management instead of treatment

Dr. O’Brien said refugia, the practice of leaving some animals untreated to maintain a population of anthelmintic-susceptible parasites, is among the management practices the CVM is promoting under the center’s Antiparasitic Resistance Management Strategy. The practice is intended to dilute resistance in a herd’s overall parasite population.

Through ARMS, the CVM plans to collaborate with the FDA, universities, and veterinary organizations to promote sustainable antiparasitic drug use, improve understanding of antiparasitic resistance, and develop policies for approving sustainable antiparasitic products, according to the CVM. Such products could include anthelmintic combinations, none of which currently is approved.

Those combinations would contain drugs with overlapping antiparasite indications.

Dr. O’Brien said the CVM previously had concerns that such products could hasten development of multidrug resistance. But, at a CVM-hosted meeting in March on antiparasitic drug use and resistance, some veterinary parasitiologists presented information indicating resistance development could slow following use of certain combinations of drugs with overlapping claims.

Almost all FDA-approved anthelmintics used in livestock are available over the counter, with the exception of eprinomectin, an extended-release injectable dewormer. Dr. O’Brien expressed concern that, when veterinarians are not involved in decisions on anthelmintic use, the drugs are less likely to be used in accordance with the label, and farms are less likely to benefit from a veterinarian’s knowledge on other parasite management practices.

The CVM wants to collaborate with veterinary educators and practitioners to increase awareness of the problem, Dr. O’Brien said.

Dr. Steven D. Vaughn, director of the FDA Office of New Animal Drug Evaluation, said anthelmintic resistance is not yet causing a crisis—which the agency is trying to avert—and the CVM alone cannot solve the problem of resistance.

The agency is considering the benefits of combinations of anthelmintics from various classes as a means to delay resistance development, particularly resistance to new classes of antiparasitic drugs, Dr. Vaughn said. But if those drugs are used in the same manner as ivermectin has been used in livestock in recent years, they likely will lose effectiveness within 10 years.

Cattle veterinarians respond

Dr. Daniel Grooms, president of the American Association of Bovine Practitioners, said the AABP is concerned about prudent use of all pharmaceuticals.

“We as the veterinary profession have to be really cognizant that, anytime we administer some type of a product to a food-producing animal, there’s a potential for a food safety and public health issue there,” he said.

“So, being veterinarians and being trained in pharmacology, parasitology, and microbiology, we are the people that are positioned best to make sure that any product is used safely and also used so that it’s effective.”

Dr. William S. Swafford, executive secretary of the Academy of Veterinary Consultants, said the AVC has provided education for members on drug resistance.

“Ultimately, the AVC effort and action in this area is to provide the most current information so our members will have the knowledge to make the best possible judgement in clinical practice for their clients’ parasite management and control programs and proper selection and timely use of anthelmintics,” he said.