Equine Deworming Resistance

Jessamyn Kennedy DVM

Years ago, intestinal parasites killed horses on a regular basis due to complications. With the advent of equine dewormers, the number of colics (and other symptoms) caused by these worms has reduced to the point that we almost don't think of it anymore. Dewormers were made safe, easy to use and readily available to the public, and have done wonders for horse health.

Scary News!  Resistance to the dewormers we all know and love is becoming more and more of a problem. The worst part is, there are no new medications on the market or being developed at all! What does that mean? It means that if we do not correctly handle how we deworm our horses, we could be in big trouble- back where we started years ago. The days of dewormer rotation are at their end- we need to outsmart the parasites, before they outsmart us.

What is resistance?  Resistance occurs when a population of (parasite, bacteria, etc) changes or mutates in some way, allowing it to survive the medication's effects. What happens next is that the parasite breeds, and passes this trait along to the next generation- in other words, the population becomes selected for this survival trait. Pretty soon, the ones that cannot survive the medication die out, and are replaced with ones that can. That is bad, bad news for us.

How does resistance happen?  It happens when a drug is used exclusively, excessively and frequently- giving parasites a chance to select for survival traits. This happens with wormers when they are given daily, or even monthly; when used several times a row; or when used incorrectly, such as not dosing properly or using the medication at the right time.

What does this mean for my barn, or my horse?  Well, put simply- you may be wasting your money, if you're using a dewormer that doesn't work! You may also be putting your horse or herd at risk.

What do we do now?  We at the Visiting Vet are now endorsing a program of performing fecal examinations on horses to determine: *Which dewormers used on the farm are still effective *How frequently an individual horse needs to be dewormed After we determine these facts, we can develop a deworming protocol for the farm and for the individual horses.

Not all horses need to be dewormed the same way!  Although it is easier to simply walk down the aisle and treat everyone at once, every two or three months, we know now that this is not necessary and in fact detrimental to the continued efficacy of the deworming drugs. Horses can fall into three categories, based on how they respond to dewormers: mild worm load, moderate, and heavy. They are categorized based on something called a Fecal Egg Count; then they are treated accordingly- mild, twice yearly; moderate, three times; and heavy, four times- in the summer. It is no longer recommended to deworm in the winter, as worms hibernate in the pasture below a certain temperature and do not pose a threat.

What is a Fecal Egg Count (FEC)?  It is a test that which looks at manure and counts the egg load. This helps us to more accurately determine if a dewormer is working properly. Samples are taken just before deworming, then 14 days later (exactly) in order to see if the dewormer is doing anything at all. On any given farm, this should be done for each product used. Although it may seem like an expense up front, it could end up saving you a lot of money- by not wasting it on dewormers that no longer work, or by over treating horses that do not need to be dewormed so frequently. To figure out what category a horse falls into, the FEC is performed at the end of winter (before the first deworming).

What we recommend now: a Deworming Program tailored to your horse's and herd's needs!  *Determine if there is resistance on the farm: do FEC testing on 10% of the herd (or, no less than 6 horses) immediately before using a certain product, and 14 days later exactly. This should be done for every product used, with the same animals every time. *Determine the status of individual horses by performing FEC during late winter, before spring deworming starts. The count then will help determine if the horse is a mild, moderate or heavy parasite egg shedder, helping us figure out how often to treat the horse.*Pasture management: worms are mostly spread on pasture. Drag pastures when unoccupied, on hot days- temperatures over 85 degrees kill larvae! At other times, pick paddocks to keep the load down. *NEVER use daily dewormers... nothing breeds resistance more quickly! *Proper use of the medication: some are better used at certain times of year, to target parasites more effectively. Avoid the risk of parasite dewormer resistance on your farm; protect your horses the smart way- let us help you find the deworming program that works effectively for your herd.