Commentary: Act on Drug-resistant Parasites

Antibiotic resistance has captured headlines lately, while the issue of drug-resistant parasites has mostly flown under the radar. The time has arrived though, for cattle producers, with guidance from their veterinarians, to shift toward practices that will delay or prevent the emergence of resistance among cattle parasites.

At the recent Academy of Veterinary Consultants spring conference, University of Georgia veterinarian Ray Kaplan, DVM, PhD, outlined resistance trends and options for addressing the emerging problem.

Kaplan points out the drugs currently in use for controlling worms in U.S. cattle fall into three chemical classes: benzimidazoles, imidazothiazoles, and macrocyclic lactones. Resistance to one drug within a class can result in varying degrees of resistance across that class, and in some locations, nematode parasites have developed multi-drug resistance resulting in "total anthelmintic failure," Kaplan says.

In New Zealand, researchers conducted a prevalence study in 2006 and found multi-drug resistant worms, primarily Cooperia species, on 74% of the operations surveyed. Cooperia, Kaplan points out, cause production losses in U.S. cattle herds, but generally do not cause clinical disease.

However, Kaplan says macrocyclic lactone resistance appears to be emerging in

Ostertagia ostertagi, a more damaging nematode parasite in cattle. In a recent Australian study, three of 20 farms were found to harbor O. ostertagi populations with resistance to all three classes of anthelmintics. "Widespread resistance in O. ostertagi would be a big game changer," Kaplan says.

Resistance is likely to outpace introduction of new anthelmintics, Kaplan says, so the best strategy for long-term parasite control is to preserve the efficacy of existing products through management. He outlined several steps toward that goal:

Manage refugia: In short, refugia involves leaving some animals in a herd untreated, to maintain a population of drug-susceptible parasites. For more detailed information on the refugia concept, search online for an FDA article titled "Antiparasitic Resistance in Cattle and Small Ruminants in the United States: How to Detect It and What to Do about It."

Develop drug combinations: There currently are no approved combination dewormers available in the United States, but combinations have been used successfully in other parts of the world.

Use proven products that provide a consistently efficacious dose.

Non-drug strategies: Pasture and grazing management can reduce worm populations. Cull the most heavily infested cattle, as susceptibility to worms is a heritable trait.

Conduct fecal egg count reduction tests: Periodic testing to monitor drug efficacy can help determine whether parasite populations are developing resistance.

In 2007, Iowa State University economist John Lawrence, PhD, led a review titled "Economic Analysis of Pharmaceutical Technologies in Modern Beef Production." He estimated the impact from eliminating dewormers across the beef-production chain at nearly $190 per head. Investments in preventing drug resistance today could provide big savings down the road.

More information on AVC conferences, and member access to the full proceedings, are available on the AVC website.