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Changes in Deworming Horses

In my senior year at vet school we were told that 90% of all that we were being taught would be outdated within 10 years. I didn’t believe it, but it has proved to be true. In parasite control it is more like 100%.

We were taught how to administer wormers through a stomach tube that we slid through a horse’s nose and down into the stomach. Understandably, it was a rare horse that didn’t need to be twitched for the procedure. (Tranquilizers at the time were not as safe or effective as today’s). The liquid wormers that we pumped and poured in did the job, but they were harsh. 12 hours of diarrhea or mild colic was quite common after deworming. Not one of those liquid wormers is still on the market, and tube worming is essentially a thing of the past. There were a couple of wormers that could be sprinkled on the feed, but most horses took one smell and said, “No thanks.”

As you know, most of today’s wormers are in paste form and available anywhere. Pastes made deworming safe and easy. It seemed like the equine parasite problem had been solved. Not true. Worms have co-existed with their hosts for thousands of generations, and weren’t fooled that easily. All available paste wormers fall under 3 or 4 general types based on their ingredients and how they act. Each of those types or classes of wormers is becoming less effective in killing equine parasites. Too frequent worming has meant that the resistance has grown to the point where some of the wormers are just not very effective anymore.

In the past 2 or 3 years new thoughts on the parasite problem have been evolving. Here is the current thinking. Most horses are able to co-exist with parasites. It has proved to be unrealistic to make our animals parasite free. So all horses carry some worms, but seem to have their own resistance to them. In other words, they have a natural immunity to the worms. However, there are some horses that don’t. Parasites in those horses thrive. The adult worms living in their intestines produce thousands of eggs daily, and these pass out with the manure, making the pasture or paddock a sea of microscopic worm eggs. Several days in the warm sun and the eggs hatch into larvae which are picked up by grazing horses. The horses that carry this big burden of parasites are called “shedders”, and are largely responsible for contaminating the ground with worm eggs. Parasitologists have shown us that in a herd of 10 horses there is often one or two that are the culprits. It is those shedder horses that revised worming programs are targeting.

You can’t tell by looking which of your horses is a shedder. It could be the healthiest looking horse in the herd. Fecal exams have to be done on all of your horses to identify who are the shedders. The exams are performed either by veterinarians in their office or they might send the sample to a commercial lab. Shedders are identified by a very large number of eggs per gram of manure. Once identified, the shedders are the ones who get the intensive deworming attention, while others in the barn may need only once or twice a year worming. This system is called targeted worming. It is a program best guided by your veterinarian.

The object of targeted worming is to cut way back on the amount of worm medicine being used and so lessen the ability of the parasites to develop immunity to it. It’s similar to human medicine where your doctor doesn’t want to give you antibiotics for every little infection, knowing that it doesn’t take bacteria many generations to develop immunity to drugs. At the moment there are no new wormers being developed, so resistance will become more and more of a problem.

It is tempting to throw the baby out with the bath water and say, “well, if immunity to paste wormers is a problem, I’ll just stay away from manufactured paste warmers and use ‘natural substances’ for parasite control.” The problem is that there have been no quality controlled studies showing effectiveness for any of these substances. One of the more controversial natural wormers is DE (diatomaceous earth) which is a fine powder made of exoskeletons of long dead tiny organisms. DE has lots of uses, but I am not convinced that it kills worms. The absolute highest number of worm eggs on any fecal exam I have ever done was on a horse that had been on DE for over a year.

Here’s the bottom line. If you are currently worming all of your horses on an every other or every 3rd month schedule, talk to your veterinarian about doing a targeted worming program. Even with the cost of the fecal lab tests, it should save money in the long run, and will definitely slow down the resistance problem.


David A. Jefferson, DVM

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