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Antiparasitic Resistance

What is Antiparasitic Resistance?

Antiparasitic resistance is the genetic ability of parasites to survive treatment with an antiparasitic drug that was generally effective against those parasites in the past. After an animal is treated with an antiparasitic drug, the susceptible parasites die and the resistant parasites survive to pass on resistance genes to their offspring.

Although antiparasitic resistance in animals doesn’t directly affect the health of people in the United States, it poses a significant threat to animal health and can result in production losses in food-producing species. Researchers have documented antiparasitic resistance in grazing species, such as cattle, small ruminants (sheep and goats), and horses, both globally and within the United States. There’s also evidence of antiparasitic resistance in swine and poultry.

Many factors contribute to antiparasitic resistance, including:

  • the biology of the parasite;
  • the strength of the infected animal’s immune system;
  • the practices used to treat the parasite;
  • the properties of the particular antiparasitic drug used; and
  • certain livestock management practices.

Science shows that antiparasitic resistance can’t be stopped. Parasites will continue to evolve and develop resistance; however, this natural process may be slowed down.

What is FDA Doing About Antiparasitic Resistance?

To help combat this growing animal health threat, FDA developed the Antiparasitic Resistance Management Strategy. The strategy promotes sustainable use of approved antiparasitic drugs in livestock (cattle, sheep, goats, swine, and poultry) and horses. Sustainable use will help slow the development of antiparasitic resistance in these animals. This, in turn, will help ensure that antiparasitic drugs remain effective for as long as possible.

New Information on Labels of Livestock and Horse Dewormers

In December 2018, FDA requested that animal drug companies voluntarily revise the labels of antiparasitic drugs intended to treat certain types of internal parasites in livestock and horses to add information about antiparasitic resistance. This move comes as a result of the agency’s work with veterinary parasitology experts and the animal health community to find ways to maintain the effectiveness of these drugs. The requested labeling changes are only for approved antiparasitic drugs for animals, and do not relate to antimicrobial drugs or antimicrobial resistance.

The labeling changes specifically affect anthelmintics. Anthelmintics, often called dewormers, are antiparasitic drugs that treat helminths. The term “helminths” refers to several groups of internal parasites, often called worms, that have some similarities. Tapeworms and roundworms are common types of helminths. Helminths are becoming increasingly resistant to dewormers.

Veterinarians, livestock producers, and animal owners who give dewormers to animals may not be aware of the threat of antiparasitic resistance or ways to slow it down. The new labeling information will help them better understand the proper use of dewormers and ways to monitor and slow down the development of antiparasitic resistance at the farm level.

The new labeling information doesn’t replace the need for livestock producers and animal owners to work with their veterinarian to determine the best strategies to manage parasites in individual animals or herds or flocks. Rather, the new language focuses on how to properly incorporate dewormers into an overall parasite control program and how to slow down the development of antiparasitic resistance. Slowing the development of resistance extends the effectiveness of dewormers and better protects animal health in the long term.

The new labeling information emphasizes these important points:

  • Any use of a dewormer can result in the development of antiparasitic resistance. Resistance has been reported for most classes of dewormers.
  • Proper dosing is critical to the safe and effective use of a dewormer. Many dewormers for food-producing animals (cattle, small ruminants, swine, and poultry) are given in feed or water to the entire herd or flock. When a drug is given this way, there’s a greater chance that animals will get too low of a dose, compared to when a drug is given to an individual animal by mouth or injection. Underdosing may result in ineffective treatment and can increase the risk of antiparasitic resistance.
  • Livestock producers and animal owners, together with their veterinarian, should monitor herds and flocks to determine the extent of antiparasitic resistance on a particular farm. Fecal examinations and other diagnostic tests should be used to monitor resistance and determine if a dewormer is effective on a farm.
  • Dewormers should be used as only one part of an overall internal parasite control program. Relying too heavily on dewormers can increase the risk of antiparasitic resistance. Using sustainable non-drug methods (for example, rotating pastures, avoiding over-grazing, and managing manure) along with dewormers to control parasites may slow the development of resistance.

Antiparasitic resistance is particularly concerning in grazing species (cattle, small ruminants, and horses). Because these animals are continually exposed to worm eggs on the pasture, they can have repeated parasite infections. Although most swine and poultry in the United States are not raised on a pasture, a growing number of these animals are being pasture-raised. This increases their chances of being repeatedly exposed to worms which may then become resistant to dewormers. FDA is aware that management practices and production schemes for swine and poultry differ from those for grazing species. The new labeling information reflects these differences.

FDA requested that drug companies add information about antiparasitic resistance to both over-the-counter (OTC) and prescription dewormers. Some approved dewormers for livestock and horses are prescription only, but most are OTC. For a product to be approved as OTC, the label must have adequate directions for use that are written in such a way that a non-veterinarian can use the drug safely and effectively. Including information about resistance on the label of OTC dewormers is important for communicating adequate directions for use to non-veterinarians. More detailed information about the proper use of OTC dewormers and ways to monitor resistance will help producers and owners use these products safely and effectively.

Reporting Problems

FDA encourages you to call your veterinarian if you think one of your animals is having a side effect from a dewormer or if you think it’s not effective, potentially because of antiparasitic resistance. A side effect associated with a drug and a lack of effectiveness are called adverse events. Adverse events also include reactions in people who handle the drug. Call your healthcare provider if you have a reaction to a dewormer. 

FDA also encourages you to work with your veterinarian to report any adverse event—in either animals or people—associated with a dewormer or any product defect, such as the drug has a bad odor or is off-color. How to Report Animal Drug and Device Side Effects and Product Problems.

Additional Information

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