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 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.


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 be certain to find one that offers full consultation services to address questions you may encounter if your F.E.C.R.T. does not showcase the expected 90% reduction in the shedding worm count.


PLEASE NOTE: This article is available for use in its entirety without edit or excerpt, in any media format on condition that credit is given to Horsemen’s Laboratory, and author Nikki Alvin-Smith as a byline at the beginning of the article publication. Horsemen’s Laboratory URL address and Nikki Alvin-Smith URL must be included.  We would appreciate notification of any publication via email to media contact Thank-you for sharing!


This article is brought to you courtesy of Horsemen’s Laboratory Inc., Mahomet, IL. –


About Horsemen’s Laboratory: Established in 1993 by John Byrd D.V.M., an experienced lifelong horseman and a graduate of the University of Illinois College of Veterinary Medicine. As an equine medicine practitioner in California for 13 years, Dr. Byrd served as ex-officio member of the board of directors of the Pacific Coast Quarter Horse Racing Association where he also served as the organization’s official sales veterinarian.  In addition, Dr. Byrd frequently officiated, as veterinarian for horse shows sponsored by the management of Orange County Fairgrounds in Costa Mesa, California.  Dr. Byrd’s extensive experience with horses led him to observe how a horse’s health could impact performance leading to the founding of the specialist lab for equine fecal worm egg counts. Please visit to find out more about F.E.C.T. services available directly to the horse owner including; advice on equine fecal egg count testing; quick and easy purchase of test kits online; reporting and expert consultation services. Dr. Byrd enjoys sharing his wealth of knowledge of equine parasitology with horse owners from all walks of life, and is available to provide lectures/symposiums for your club, organization or event. Please contact Dr. Byrd via his website for rates and further information.


About Nikki Alvin-Smith: International and national published freelance writer and photographer in such world renowned publications such as The Chronicle of the Horse, Horse and Hound, Dressage and CT, Warmbloods Today, The Horseman’s Yankee Pedlar, Reiter, The Equine Journal, Spur, Hoofprints, Horsin’ Around, Horses All, Field & Stream, Western Horse and Gun, Pony Quarterly, Horses All Canada, Catskill Horse to name a few. Ghostwriting, blog services, PR/Marketing copy either direct with manufacturer or for agencies, copy editing and editor services also available. Nikki also produces catalog copy, white papers, e-books, corporate brochures and advertising copy for international corporations and PR/Marketing for celebrities.


As a Brit who has called the America home for the past 34 years, Nikki brings a unique perspective to the equestrian world. Nikki is also an accomplished Grand Prix dressage trainer/competitor, competing at international Grand Prix level to scores over 72% and is a highly sought clinician offering clinics worldwide. She has been a horse breeder/importer of warmblood and Baroque breeds for more than 25 years. Together with her husband Paul who is also a Grand Prix trainer, they run a private dressage breeding operation and training yard in the beautiful Catskill Mountains of New York.


Please visit to learn more about the affordable freelance writing services on offer