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April 11, 2017

Shortfalls Of Traditional Deworming For Your Horse

by DePaolo Equine Concepts
As with many medications, a rising concern with dewormers lies in drug resistance. Over the past thirty years, the ease and availability of paste dewormers has vastly increased the number of horses that receive treatment at regular intervals. During this time, resistance to various treatments has become widespread. The continuous dewormer approach is also problematic. Preventative treatment through continuous daily dewormers, though appealing, involves daily exposure to medications and significantly increases the risk of drug resistance. Each year more and more farms are diagnosed with “superworms” resistant to many of the standard deworming products. Continued use of paste purge dewormers on a set schedule and continuous daily dewormers, regardless of contamination levels, will lead to more resistance and a lack of effective products.
The use of paste dewormers on an established schedule also fails to effectively tailor treatment for each individual horse. Inevitably, some horses are “wormier” than others. Issues such as genetics, immune deficiency and overall health can affect a horse’s susceptibility to internal parasites. Indeed, studies indicate that twenty percent of the equine population yields eighty percent of the parasite eggs in any given setting. Typically a small number of horses in a given population have a high parasite burden, a similarly small number have a very low burden, while the rest fall somewhere in between. The one-size-fits–all approach of purge deworming with paste products and continuous dewormers ignores these individual differences. This is especially troublesome as a complete and total purge of parasite populations is not beneficial. In small numbers, the symbiotic horse/parasite relationship is healthy. The horse manufactures antibodies to the parasite to keep levels low and the worms have a place to live and reproduce. For horses with low parasite burdens, scheduled treatment with purge paste dewormers can actually reduce parasite levels to unhealthy lows.
In addition to the risk of resistance, such a broad-based approach unnecessarily exposes horses with low parasite burdens to harsh chemical treatments. As implied by their name, purge dewormers use chemicals to kill off parasite populations. This means every six to eight weeks horses are exposed to low-grade poisons – whether or not they carry a high parasite burden.
Unnecessary exposure to poisons, no matter how small the level, is less than ideal. It is not uncommon for horses to have small reactions to synthetic paste dewormers varying in severity from swollen glands and skin reactions to colic and laminitis. Anecdotal evidence has even linked the most powerful purge dewormers, like Quest, to severe health issues and even death as some horses struggle to eliminate massive dead worm populations at once.
Even a drug like Ivermectin, which has demonstrated the greatest longevity and effectiveness against parasites, can have adverse effects when used too frequently or in large doses. A 2009 study Journal of the American Veterinary Medical Association study detailed severe neurological reactions in three Quarter Horses after receiving an ivermectin paste. The horses experienced depression, lack of coordination, impaired reflexes and muscle twitch. Despite treatment, one animal had to be euthanized. After all considerations, dewormers are still toxic chemicals and should be used only when needed.
Overuse of paste dewormers has increased the risk of drug resistance and ignores the fact that 20% of horses in a given population are responsible for 80% of parasite eggs shed in pastures and stalls.
Natural treatments are available, but must be approached carefully in terms of effectiveness and safety.  For information on alternative treatments, please visit our blog post next week on the Merits of Alternatives To The Traditional Approach To Deworming.

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