deworming horses

Q. My senior gelding is out for a lease trial. At home he lives in a drylot, and his fecal egg counts always show him to be a 0 shedder, so I treat him twice a year with a product that includes praziquantel for tapeworms. The barn he is moving to is impeccably clean and has excellent manure management. There, he will get turnout time in shared grass paddocks. However, this farm does not do fecal egg counts and still uses an eight-week rotational program on all the horses. I tried to explain the current recommendations for parasite control to the owner but to no avail.

Our agreement is that I will manage my horse’s parasite program separate from the farm’s. But I’m concerned that he is at risk because of the barn’s practices, although I’m happy there is never manure left on the paddocks. Should I be concerned, and what’s the best way to protect him from internal parasites while he lives there?

Ashley, via e-mail

A. Thank you for your question. Your situation is not unusual at all. We recently published results from a U.S.-wide survey showing that a large proportion of horse owners do not follow the current recommendations. So, this barn manager is definitely not alone in her management techniques. This begs the question: What are we (the researchers) doing wrong in communicating our recommendations? But that is a topic for another day.

With regard to your horse, let’s get a few things straight. If the pasture and paddock hygiene is really as good as you describe, very few, if any, anthelmintic treatments will be necessary. Treating six times a year on a rotational schedule is asking for drug-resistant—probably multi-drug-resistant–parasites. So, the horses are unlikely to benefit much from the six yearly treatments; instead, the pasture hygiene may be effective at keeping parasites in check. And finally, parasite control must be carried out on the herd level.

There are some common misconceptions regarding that last point. As owners, we tend to think about what we can do for our horses. But parasite control requires a coordinated effort for the whole population of horses—we’re aiming to control a parasite population present on the pasture and paddocks shared by all the horses. Therefore, you cannot define one program for one horse and another for the remainder. This is often very challenging, as your example illustrates perfectly well, and I realize you’ve probably already tried discussing these items with the barn manager.

With all that said, my assessment of your situation is that your horse is unlikely to be exposed to a high parasite infection pressure. You didn’t mention anything about stocking density and age distribution of the horses present, so I can’t say for sure, but the cleanliness of the farm suggests a low pressure.

However, the parasites at the facility are likely multi-drug-resistant, and your horse will likely acquire these parasites. We do not know the actual resistance profile without fecal egg counts being done so we can only guess, but resistance to two or all three dewormer classes (benzimidazoles [including fenbendazole], pyrimidines [pyrantel], and macrocyclic lactones [including ivermectin and moxidectin]) is very likely.

You did not say how old your horse is, but some older horses tend to have higher egg counts in general, which suggests they may be more susceptible to infection. A horse that recently moved to a new facility is also likely to have a higher egg count for a while until he is settled into the new environment. All horses have parasites, but parasitic disease is extremely rare. So, your horse will likely get some resistant worms, but the risk of disease is very low.

Regarding your question about what you can do to protect him, unfortunately, there really isn’t much. You can test your horse pre-and post-deworming to ensure good effectiveness but remember that no dewormer completely eliminates all parasites present.

Finally, you can tell the barn owner that she is wasting her money on dewormers that don’t do anything for the horses. At least four out of the six yearly treatments are likely to be ineffective, and even the cheapest dewormer is very expensive if you use it repeatedly and it doesn’t work. So, in reality, those horses also get two treatments a year, just like yours. Why not skip the other four?