The U.S. Food and Drug Administration (FDA) has recently asked animal drug companies to voluntarily revise the labels of some livestock and horse dewormers to add information about antiparasitic 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.
The FDA has requested that drug companies add information about antiparasitic resistance to both over-the-counter (OTC) and prescription dewormers (e.g., ivermectin). 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.
What is Antiparasitic Resistance?
Antiparasitic resistance is the genetic ability of parasites to survive treatment with an antiparasitic drug (dewormer) that was generally effective against those parasites in the past.
Many factors contribute to antiparasitic resistance.
- The type of the parasite
- Your horse’s immune system
- Your deworming practices
- The properties of the anthelmintic (dewormer) used
- Your pasture and manure management practices.
Resistance Can Not Be Reversed
Resistance can’t be stopped. However, the process can be slowed down.
What is FDA Doing About Antiparasitic Resistance?
To help combat this growing animal health threat, the FDA developed an Antiparasitic Resistance Management Strategy. The strategy promotes the sustainable use of approved dewormers horses. Sustainable use will help slow the development of antiparasitic resistance in these animals. Which will help ensure that the current dewormers remain effective for as long as possible.
New Label Information for Horse Dewormers
The FDA has 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 anthelmintics.
Anthelmintics or dewormers are used to treat several types of common equine parasites Small Strongyles (redworms), Ascarids (roundworms) and Tapeworms which are becoming increasingly resistant to dewormers.
The new labeling information will help veterinarians, livestock producers, and animal owners better understand the proper use of dewormers and ways to monitor and slow down the development of antiparasitic resistance at the farm level such as fecal egg count testing (hyperlink here).
The new labeling information does not replace the need for horse owners to work with their veterinarian to determine the best strategies to manage parasites at the farm level. 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. Weighing your horse and calculating the correct dosage is critical to avoid underdosing. It is also important to ensure your horse receives the entire dose (including what your horse spits out). Underdosing may result in ineffective treatment and increases the risk of antiparasitic resistance.
- Horse owners, together with their veterinarian, should monitor their animals to determine the extent of antiparasitic resistance on their farms. Fecal egg count (FEC) tests and fecal egg count reduction tests (FERT) 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 increases 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.
Read the FDA’s December 6, 2018 Press Release here: https://www.fda.gov/AnimalVeterinary/NewsEvents/CVMUpdates/ucm626959.htm.