Today’s anti-parasitic drugs generally remain effective against cattle parasites in the United States, but occurrence of resistant parasites in other countries and in other livestock species, and signs of emergence of resistant populations here, create a need for prevention measures. A new report from the FDA’s Center for Veterinary Medicine provides tips for preventing and detecting such resistance.

According to the report’s authors, part of the problem could be that veterinarians and producers are too successful in reducing parasites in host populations.

The paper outlines the concept of “refugia,” which is a portion of the parasite population not targeted for treatment. When an animal is treated with an anti-parasitic drug, susceptible parasites are killed but some resistant or tolerant individuals are left behind. Those parasites pass their genetic resistance on to their offspring. If all the parasites affecting a herd of livestock are exposed to the drug, resistant parasites eventually could make up the majority of the population.

The refugia concept creates a “refuge” for some parasites by leaving them 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 anti-parasite drugs.

The authors list the following management practices they believe contribute to anti-parasitic 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 anti-parasitic drug without knowing if it will be effective on the farm.
  • Using anti-parasitic drugs for unapproved uses, such as to increase weight gain.
  • Relying solely on anti-parasitic drugs to control parasites, rather than changing management practices.

The authors offer these tips for managing anti-parasitic resistance:

  • Use clinical signs and diagnostic test results to determine which parasites are present on the farm, level of infection and level of resistance, and use the information for management and treatment decisions.
  • Use only anti-parasitic drugs that are effective based on recent diagnostic test results and approved for the particular parasites present on the farm. Always follow the directions on the drug’s label and don’t use anti-parasitic drugs for unapproved uses.
  • Ideally, identify and cull the animals that are the highest fecal egg shedders. Otherwise, target treatment toward animals at greatest risk of illness from parasitic infections.
  • Treat animals when infective larvae are at the highest number on the pasture to maximize environmental refugia. This typically occurs when temperature and humidity are high.
  • When practical, weigh animals to avoid under-dosing anti-parasitic drugs.
  • Maintain adequate treatment records and egg-count reduction results to use in treatment and culling decisions.

Non-treatment management practices that can help reduce parasite infections in herds include:

  • Quarantining new livestock.
  • Rotating pastures with other livestock species or horses.
  • Dragging or harrowing pastures to break up manure piles.
  • Managing pastures for taller grass during grazing. Most parasite larvae stay within an inch of the ground, so cattle grazing short grass can have more exposure. (Well-managed rotational grazing systems have been shown to reduce parasite loads in cattle.)
  • Reducing stocking density, especially so animals are not forced to graze near manure piles.

The paper also includes a discussion of using fecal egg count reduction tests (FECRT) to monitor treatment efficacy in a herd. This involves examining fecal samples before and after treatment and counting the parasite eggs. Less than a 90 percent reduction could indicate resistance, as can a decline in percent reduction over a series of treatments. The authors note, however, that several biological factors reduce the sensitivity of FECRT in cattle compared with other livestock such as small ruminants.

The paper lists several other available tests to identify resistant parasites, but these tend to be expensive.

The paper is available online from FDA’s Center for Veterinary Medicine.

Editor’s note:

We’re interested to hear the thoughts of veterinarians and parasitologists on the ideas presented in the FDA paper. Are the factors FDA lists as contributing to resistance on target? Is the refugia concept viable? We would welcome your comments, either here or by e-mail to