Why Rotational Horse Deworming is Irrational


Nikki Alvin-Smith

Administering paste dewormers such as Ivermectin every 6-8 weeks, seemed like a good idea at the time, but that was nearly 40 years ago.

Paste dewormers provided a quick and easy solution to control internal horse parasites and off we all went down that road. Along the way the scientific community began to notice equine worms were developing a resistance to some of the paste dewormers and the alarm was sounded. Thus the rotational rationale was adopted.

So with due diligence we all changed direction, trotted down the new path and utilized a variety of active chemical dewormer ingredients. We were nervous to hear that dewormers were losing their efficacy and that with no new parasite treatments on the horizon, we might have a real equine parasite problem re-emerging. Good for us. According to academic experts there are still no new treatments in the pipeline, so yes, let’s worry.

Has rotational deworming worked? The short answer is an emphatic, “No!”

So does your horse farm have a dewormer resistant horse worm population? Do you know how to find out? { www.horsemenslab.com} Do you practice a sensible selective deworming program that targets the worms present in your horse population? Or are you still operating your horse worm control program in 1970’s style. Perhaps you are still wearing bell-bottom jeans and listening to The Ramones, Jackson 5 and The Eagles too. Well actually, I confess to the latter. But you get the idea, things move along and we need to keep up to date, especially where something as important as your horses’ health is concerned.

Perfectly healthy looking horses can harbor high worm egg counts with no apparent negative health symptoms.

Here are a few things you should know to bring you up to date (without boring you with a Chemistry 101 lesson!):

The only way to establish whether your horse population harbors worms resistant to dewormers (and to know which dewormers! ) is to do a FECRT. That is Fecal Egg Count Reduction Test. You actually need to do two tests. The first one will establish a baseline for your horses’ equine worm egg count. Based on the results you will selectively administer the correct dewormer paste to target that particular type of worm. You will then retest several weeks later. If worm egg counts are still present in the horse’s stool sample then you have a resistant worm population. To treat this population you should consult a vet experienced in equine parasitology, like our own Dr. John Byrd here at Horsemen’s Laboratory, who will guide you through the process of treatment and answer all your questions.

The equine worm life cycle matters and there are a few basics you should understand. While there are variances in life cycles between different worm types, they all begin life as a worm egg. For example the small strongyle egg hatches into several stages of larvae (L1). The larvae grow (L2), become infective (L3), grow and molt (L4), and finally become sexually mature (L5).  It is stage L3, the infective stage, which occurs outside the horse on the pasture that sets the stage for horses to ingest these larvae and be exposed to the parasite. You cannot tell from looking at a horse which horse has been exposed. Perfectly healthy looking horses can harbor high worm egg counts with no apparent negative health symptoms. Different types of equine internal parasites obviously have different life cycles, some migrate through lung tissue, some encyst in the lining of the gut etc. and most worm eggs /larvae you cannot see with the naked eye in manure.

You may diligently deworm your horses and take a sample but certain worms may still be harbored in your horse, because dewormers cannot select or ‘hit’ every larval stage when worms are encysted in the horse’s body. This is because the worms are in a refuge location and/or in a stage of development where the dewormer cannot penetrate or select them. With encysted small strongyles for example, the only dewormer product currently available that can penetrate the encysted stage is Quest, but when should you use it?

Eventually, these worms will emerge from their encysted state and location (even as much as two years later though usually within a matter of two or three months depending on environmental factors), and enter the gut where the dewormers can attack them.

What does this mean to you as the horse owner? It means regular fecal worm egg count testing is important for the health of your horse herd to know what is going on.

At Horsemen’s Laboratory 35% of the samples we currently receive test positive for worm egg count. Of those counts 95% are strongyle eggs, and as there is no method to differentiate at the egg stage of development between large and small strongyles we cannot say which type of strongyles these are. Treatment for both types however, is the same.

You don’t necessarily want zero worms in your horse worm population even if that was possible.

All horses are not created equal. Certain horses may be higher worm egg shedders than others and this is quite normal. In fact the majority of the horses that carry and shed worms are usually in the minority of your horse herd population. So you need to know which horse needs treatment and which horse doesn’t, so that you can stop wasting money and overmedicating your horses with dewormers. Not only is this a waste of time it is not a sustainable practice for your farm as it will likely result in dewormer resistant worms on your farm pastures.

Worm eggs that hatch or are ready to hatch on the pasture, are generally more prone to die off during hot, dry spells of weather than cold, wet spells. So timing of both fecal worm egg count testing/retesting and paste administration is important and will depend to some extent on your geographic location.

Worms such as tape worms and bots are not counted in fecal worm egg count tests, so all horses should be dewormed once or twice a year in late Spring/early Summer and late Fall/early Winter, with a horse dewormer targeted at these types of worms. If there is no incidence of tape worms or exposure to tape worms in your horse herd, the late Fall/early Winter dewormer administration may suffice.

Foals and young horses (under 3 years old) require a different protocol than adult mature horses, as worm egg count testing may not accurately reflect their exposure or host population.

Lastly, in horse breeding circles you’ve heard the expression, “ Breed the best to the best and hope for the best.” Well its true that if you have a dewormer resistant equine parasite, its best mating partner is another resistant equine parasite. Survival of the fittest. So you don’t necessarily want zero worms in your horse worm population even if that was possible, because by leaving some more dewormer sensitive worms behind, you are providing the resistant strain with sexual partners who will diminish their genetic mutational power to avoid dewormer destruction. This will slow down the resistance of the worm population overall.

Take Home Message

We need to adjust our strategy and target our horse worm control program. Testing of equine fecal samples should be the method we choose.  { https://horsemenslab.com/shop }  It’s smart. It’s simple. And it is a rational approach.