Antiparasitic Resistance Invokes FDA Request

By Nikki Alvin-Smith

Resistance to dewormer products currently available on the market has caused the FDA to invoke a request that animal drug companies voluntarily revise their product labels for their approved anthelmintics used in livestock, including horses. This emerging crisis is very real, and it is a smart decision to take notice now rather than later. 

This edict brings the issue of antiparastic resistance front and center and horse owners may wish to heed the advice that the FDA has provided in regard to the need to engage in a targeted, evidence based worm control program for their equines to include fecal equine control testing (F.E.C.T.). The F.D.A. also heralded the importance of retesting (F.E.C.R.T) after administration of an appropriate dewormer treatment. This is necessary to gauge the effectiveness of the treatment.

This an excerpt from the FDA statement regarding their request for voluntary additional labeling: 

New Labeling Information about Antiparasitic Resistance
Cattle, Small Ruminants, and Horses ~

Antiparasitic resistance is particularly concerning in grazing species (cattle, sheep, goats, and horses). Because these animals are continually exposed to worm eggs on the pasture, they can have repeated parasite infections. FDA has requested that animal drug companies add the following statements to the labels of all anthelmintics for cattle, small ruminants, and horses:

• Parasite resistance may develop to any dewormer, and has been reported for most classes of dewormers.

• Do not underdose. Ensure each animal receives a complete dose based on a • current body weight. Underdosing may result in ineffective treatment, and encourage the development of parasite resistance.

• Treatment with a dewormer used in conjunction with parasite management practices appropriate to the geographic area and the animal(s) to be treated may slow the development of parasite resistance.

• Fecal examination {F.E.C.T.} or other diagnostic tests and parasite management history should be used to determine if the product is appropriate for the herd/flock, prior to the use of any dewormer. Following the use of any dewormer, effectiveness of treatment should be monitored (for example, with the use of a fecal egg count reduction test {F.E.C.R.T.} or another appropriate method).

• A decrease in a drug’s effectiveness over time as calculated by fecal egg count reduction tests may indicate the development of resistance to the dewormer administered. Your parasite management plan should be adjusted accordingly based on regular monitoring.”

How has this parasite resistant situation developed you might ask? The overuse of dewormers brings with it increasing likelihood of even larger equine internal parasite populations that have developed resistance to current dewormers on the market. Inappropriate dosing either due to the horse spitting out the dewormer at time of administration or wrong weight estimations for dosage are also causes for dewormer resistance to begin. But the main cause comes down to the survival process part of which is successful reproduction.

Equine Worm Testing Kits for Sale
Round worm egg and strongyle egg

When you administer a dewormer product to your horse it will necessarily be most effective against the adult sexually active worms that are the most sensitive and it will leave behind those worms that are the most resistant. Now you have created a selective breeding situation. These resistant adult worms will now mate together to create more highly dewormer resistant worms. 

As a result, eventually the dewormer will become useless as a method for treatment of worms in that equine population and their environment. As horses move around from place to place, these resistant worms are spread on the pasture to other grazing herds.

There are many options to source for your testing needs but at Horsemen’s Laboratorywe go the extra mile by providing a full service to our customers that includes expert advice on equine parasitology from our own John Byrd, D.V.M, experienced technical support from Ivy Lewis, who is super experienced at reading those worm egg shedding counts accurately after years of experience and training from Dr. Byrd.  We also offer full consultation servicesto address questions you may encounter if your F.E.C.R.T. does not showcase the expected 90% reduction in the shedding worm egg count. 

It may be time to host Dr. Byrd for a symposium/clinicat your barn, or for your equestrian group, so you can learn firsthand how to properly manage your evidence based worm control program, and your horse’s environment.

Please don’t be shy to contact us whatever your need. While our process for testing is quick and simple to use, our follow up care is second to none!