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Antiparasitic Resistance

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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. (Parasites are often called “worms” and antiparasitic drugs are often called “dewormers.”) 

Parasitic diseases must be managed to maintain animal health and welfare and prevent production losses in food-producing animals. Unfortunately, however, researchers have documented an increasing level of antiparasitic resistance in grazing species, such as cattle, small ruminants (sheep and goats), and horses, both globally and within the United States.

Grazing species become infected with parasites after eating the eggs or immature larvae from the pasture. The eggs or larvae of the parasites continue to develop within the grazing animals, eventually causing disease. Because these animals are continually exposed to worms from the pasture, they can have repeated parasitic infections and also be repeatedly treated with dewormers. Antiparasitic resistance becomes a problem when an increasing percentage of a parasite population on a given farm carries resistance genes, allowing more resistant worms to survive and re-infect animals in the herd or flock.

There’s also evidence of antiparasitic resistance in swine and poultry worldwide, including in the United States. 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.

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 dewormers 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 dewormers remain effective for as long as possible.

International Partnerships to Address Responsible and Prudent Use of Dewormers

FDA is an active member of the World Organisation for Animal Health (WOAH, originally founded as the Office International des Epizooties (OIE)), which includes five geographic regions of the world encompassing 52 countries. FDA is part of a WOAH group that developed a document discussing the best practices for managing antiparasitic resistance worldwide. In December 2021, the group released the publication, Responsible and prudent use of anthelmintic chemicals to help control anthelmintic resistance in grazing livestock species. The term “anthelmintic chemicals” refers to dewormers that specifically treat helminth parasites, which are several groups of internal parasites that have some similarities. Tapeworms and roundworms are common types of helminths. With only a few classes of dewormers available to treat and control helminths, the paper outlines several practical measures—both drug and non-drug methods—for managing parasitic infections while also preventing or reducing the development and spread of antiparasitic resistance.

Information about Antiparasitic Resistance on Labels of Livestock and Horse Dewormers

At FDA’s request, animal drug companies that make dewormers for livestock and horses have voluntarily added information about antiparasitic resistance to the drugs’ labels. The goals of the labeling information are to increase awareness among veterinarians, livestock producers, and animal owners about the threat of antiparasitic resistance, help them better understand how to properly incorporate dewormers into an overall parasite control program, and how to monitor for and slow down the development of antiparasitic resistance at the farm level. Slowing the development of resistance extends the effectiveness of dewormers and better protects animal health in the long term.

The 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.

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 think you're having 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. See Report a Problem.

Additional Information

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