The U.S. Food & Drug Administration announced Dec. 6 that it has requested that animal drug companies voluntarily revise the labels of drugs intended to treat certain internal parasites in livestock and horses to add information about antiparasitic resistance.

FDA said 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.

FDA emphasized that the requested labeling changes are for approved antiparasitic animal drug products only and do not relate to antimicrobial drug products or antimicrobial resistance.

The requested labeling changes specifically affect anthelmintics for livestock (cattle, sheep, goats, poultry and swine) and horses. Anthelmintics, often called dewormers, are animal drugs that treat helminths. Helminth 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 drugs that were generally effective against them in the past, FDA said. In these situations, after an animal is treated with a dewormer, the susceptible worms die, and the resistant worms survive to pass on resistance genes to their offspring, the agency explained.

Although antiparasitic resistance in livestock and horses does not directly affect human health in the U.S., it is a growing animal health threat in this country. Heavy worm infections can cause diarrhea, weight loss, anemia and death. Antiparasitic resistance is particularly concerning in grazing species (cattle, sheep, goats and horses) but is also a problem in swine and poultry, FDA reported.

The new labeling information emphasizes these important points:

* Any use of a dewormer can result in the development of antiparasitic resistance.

* Proper dosing is critical to the safe and effective use of a dewormer.

* End users should work with their veterinarian to monitor herds and flocks to determine the extent of antiparasitic resistance on a particular farm.

* Dewormers should be used as only one part of an overall internal parasite control program.

FDA also reminds veterinarians, livestock producers and animal owners about antiparasitic resistance and the importance of developing an overall parasite control program to slow resistance to dewormers.

Veterinarians, livestock producers and animal owners may not be aware of the threat of antiparasitic resistance or ways to slow it down, FDA said, noting that 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 does not replace the need to work with a veterinarian to determine appropriate parasite management strategies for individual animals or herds or flocks, FDA said. 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, the agency added.

Because grazing animals are continually exposed to worm eggs on the pasture, they can have repeated parasite infections. Although most swine and poultry in the U.S. are not raised on pasture, FDA said the number of these animals being pasture raised is growing, and this increases their chances of being repeatedly exposed to worms that may then become resistant to dewormers.

FDA said it is aware that management practices and production schemes for swine and poultry differ from those in grazing species, and the new labeling information will reflect these differences.

FDA is requesting 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.

FDA is requesting that drug companies include this information on the labels of their currently marketed FDA-approved dewormers for livestock and horses within the next 12 months and on labels for any new dewormer that FDA approves for use in these animals.