The myth that internal parasites cause weight loss in horses

Controlling internal parasites in horses is as important as ever, but not for the reasons you might think.

Question: How likely is it that parasites are keeping a horse from gaining weight? I have a 9-year-old gelding who is a very hard keeper. I’ve dewormed him regularly and had his teeth checked, but he’s still not picking up weight. Will a fecal egg count reveal if he has parasites that are robbing him of calories?

Answer: The short answer to your question is “extremely unlikely.” Parasites rarely exert any sort of negative effects in well-managed horses. It may be an uncomfortable thought, but we have to come to terms with the fact that parasitism is a normal state for our horses. Every horse has parasites. We can never eliminate them, nor is it wise to try.

Click here to read an in-depth article on the best deworming strategies for horses.

On the very rare occasions when we do see parasitic disease, it is usually in horses that have other health issues. They may be battling bacterial or viral infections ,or they may have underlying metabolic disease. They may be living under stressful conditions with high stocking density, poor pasture quality and suboptimal feeding regimens. Parasites are opportunistic troublemakers. Most of the time they just hang out and do their thing. I have specialized in equine parasites for the past 20 years, and most of the parasite disease cases that I have come across have been in rescued horses with histories of neglect and poor management.

Likewise, it is also extremely unlikely that parasites are the cause of weight loss in a horse that has no other apparent disease. Tapeworms and strongyle parasites really don’t compete with the horse for nutrients. They are all part of the same intestinal fauna that also contains lots of bacteria, fungi and other micro-organisms that multiply and reproduce constantly. These organisms all consume nutrients, but they produce just as many. They help horses digest the feed, and they provide nutrients for each other. The parasites are part of this complex ecosystem, and you cannot accurately view parasites as cunning thieves stealing the horse’s food. Think of it this way: Intestinal bacteria probably consume even more nutrients than the parasites, but we don’t keep our horses on regular antibiotic treatments, do we?

You asked about having a fecal egg count done from your horse. While there are good reasons for getting parasite egg counts done, this is not one of them. Parasite fecal egg counts are useful for three things:

• To check that a dewormer has worked and there is no resistance to the specific chemical compound among the parasite population on a farm. We do that with the fecal egg count reduction test, comparing the number of parasite eggs in a horse’s manure before and after deworming treatment. If you have never had this done before, now is a really good time to start.

• To detect the presence of strongyles and ascarids in young foals and yearlings, and decide which dewormer to use in these horses (different parasites require different deworming agents).

• To identify the high strongyle shedders among the adult horses in a herd. These horses need more frequent deworming than the low shedders.

There are lots of good reasons to do egg counts, and implementing a parasite control program without utilizing them is like driving blindfolded—you simply will not know what you are doing. That said, the one thing that egg counts are not useful for is the situation you describe: Where a horse is having health issues and you are wondering if it could be parasites.

In short, egg counts will not be helpful in your situation for two reasons: First, it is the larvae that cause parasitic disease, not adult worms. Larvae do not make the eggs you’d be counting. Secondly, there is no correlation between egg counts and worm counts. More eggs does not mean more worms. Whatever the count is, it will not reveal anything about how many worms the horse has, or whether the worms are causing him any sort of harm.

Having said all this, I do recommend that you check the efficacy of the dewormers that you have been using. You want to make sure that your regular deworming has actually been working as intended. If you have been using products that no longer work because of drug resistance, you have just wasted money and effort.

Martin Nielsen, DVM, PhD,


Gluck Equine Research Center

University of Kentucky

Lexington, Kentucky


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