Is There a Parasite Resistance Crisis Brewing?
Small strongyles, the worms most resistant to parasites. Photo courtesy of Dr. M. Nielsen.
By: Nikki Alvin-Smith for Horsemen’s Laboratory Inc., Mahomet, IL.
The short answer is yes. 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.
You’ve heard of Herbert Spencer’s ‘survival of the fittest’ and Darwin’s theories on evolution. Part of the survival process 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. Whoops!
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.
Dr. John Byrd of Horsemen’s Laboratory, a leading laboratory in testing equine fecal samples for worm egg counts (F.E.C.T), has this to share:
“There are only 3 classes of drugs available in the USA, and there is already evidence that there is great deal of resistance to 2 classes in small strongyles. There is also some evidence of resistance in roundworms, and pinworms to the third class of drugs. Therefore, we must make every effort to slow or stop further development of resistance.”
“Fecal egg counts are the most important tool to slow the development of resistance in horse parasites. Doing fecal egg counts and using the knowledge that the result provides to owners can determine when and which horses need to be dewormed. This will greatly reduce the selective breeding of parasites for resistance.”
A blind man on a galloping horse can see the logic of this situation. You cannot tell if your horse is harboring a resistant worm population by looking at the gloss of his coat or the number of times he has succumbed to a colic attack. The only method to know the level of infection with worms, whether those worms are dewormer resistant, and whether any given equine individual is exposing another to infection, is by F.E.C.T.
Evidence based targeted dewormer protocols are more important than ever to the health of our horses. You can be part of the problem or part of the solution to slowing down the spread of dewormer resistance.
