While there is some disagreement on strategies, experts agree on a need to preserve the efficacy of antiparasitic drugs to protect the health of cattle. Back in April 2013, FDA’s Center for Veterinary Medicine (CVM) released a paper titled “Antiparasitic Resistance in Cattle and Small Ruminants in the United States: How to Detect It and What to Do About It.” We posted a summary of the paper to our website, BovineVetOnline.com, and invited veterinarians and parasitologists to offer their thoughts on drug-resistant parasites and the FDA’s recommendations.
The paper outlines the concept of “refugia,” which involves intentionally leaving a portion of the parasite population untreated. The idea is to maintain a population of drug-susceptible parasites to dilute the population of resistant ones. Refuges can include untreated animals, parasite eggs and larvae on pastures when animals are treated and life stages of the parasite not affected by treatment. The authors suggest, for example, that treating just half the herd at one time could reduce parasite loads while helping retain the population’s susceptibility to antiparasite drugs.
The authors list the following management practices they believe contribute to antiparasitic resistance:
• Treating every animal in the herd.
• Frequent routine deworming without performing diagnostic tests or determining if treatment is necessary.
• Deworming when environmental refugia is low.
• Giving an antiparasitic drug without knowing if it will be effective on the farm.
• Using antiparasitic drugs for unapproved uses, such as to increase weight gain.
• Relying solely on antiparasitic drugs to control parasites, rather than changing management practices.
The report also provides treatment tips for managing antiparasitic resistance. These include using clinical signs and diagnostic tests in management decisions, treating animals when infective larvae are at the highest number on the pasture to maximize environmental refugia, using animal weights to determine dosage, and maintaining treatment records and egg-count-reduction results to use in treatment and culling decisions.
Among non-treatment options, the authors recommend quarantining new livestock, rotating pastures with other livestock species, dragging or harrowing pastures to break up manure piles, managing pastures for taller grass during grazing and reducing stocking density.
Response from Lou Gasbarre
Veterinary parasitologist Lou Gasbarre, PhD, sent a detailed response to the article, applauding the efforts of FDA/CVM for bringing attention to the issue but pointing out what he believes are misconceptions in the paper.