Q: I have five mares ranging in age from 5 years to 29. For almost 30 years, I’ve been conscientious about parasite control, relying on rotational dewormers. As attitudes about deworming evolved, I began collecting samples for fecal egg count analysis. Sometimes, in spite of careful environment management and correct rotation of chemical dewormers, I found resistance was mounting. I was confused. 

I was also stymied by what was causing an unthrifty appearance in my mares. Dental floatings, physical exams, disease prevention, regular exercise, feed supplements, high-quality hay, alfalfa—nothing I tried helped them gain weight or improved their spirited quality.

Then I tried a mechanical dewormer, specifically food-grade diatomaceous earth. The difference was almost immediate. All five mares have put on weight and are alert and comfortable. Now I can’t say that the mechanical deworming program here at my farm will work for everyone. But with some chemical dewormers losing efficacy, shouldn’t we be considering them? Could you explain the positive and/or negative aspects of products such as diatomaceous earth for equine parasite control?

A: We very frequently get questions about diatomaceous earth (DE), and with good reason. As you indicate correctly, resistance to commercially available dewormers has become common in equine parasites, and levels seem to be increasing worldwide. The pharmaceutical industry has not introduced new drug classes with new modes of action for equine usage since the early 1980s, and it remains uncertain when such new products will be marketed. We need effective and reliable alternatives to the traditional drugs.

In the meantime, some people are turning to feed supplement-type products for horses. I have seen numerous garlic products, mineral supplements, homeopathic medicine, herbal and plant extracts all purported to work as dewormers. Because these are marketed as feed supplements rather than medicine, they need to prove only that they are safe—they do not have to show that they actually work. And, in fact, very few of them offer supporting data documenting any antiparasitic effect. When parasitologists have tested some of these products, they have rarely found convincing results.

DE is one such example. A naturally occurring soft, talc-like powder, DE consists of the fossilized remains of diatoms, single-celled phytoplankton with hard cell walls made of silica. DE has many industrial and medical applications—as an abrasive agent or for filtration, for example. Products that contain DE are used against bedbugs, fleas, cockroaches, ticks and many other pest species; they are generally sprinkled on surfaces, including animals’ skin, where the insects will come into contact with DE.

A number of people strongly believe that DE also has an antiparasitic effect when administered orally. However, I know of several studies that have evaluated the effects of DE on intestinal worms. Unfortunately, none of these have found that DE has any impact on internal parasites.

My qualified opinion is that physical damage to the parasites is unlikely to occur within the intestine of the horse. The worms and their larvae are very small, often too small to be seen with the naked eye. Infective larvae are very hardy and survive the grinding teeth as well as the stomach acids before reaching the intestine. By the time an oral dose of DE reaches the gut, it will be so diluted in the ingesta that the chances of individual particles encountering a parasite would be very small.

If the sharp particles really were present in numbers sufficient to cut the parasites, one would also expect them to cause lesions in the mucosal membranes of the horse. Furthermore, all horses ingest soil and sand, which also can have sharp edges, but we don’t see any reduction of parasite loads in response. All in all, there are no sound biological reasons to expect DE to have an antiparasitic effect—and this is supported by research.

Some have suggested that DE might be effective at disrupting parasitic larvae within the fecal pile on pasture. But, again, a number of my colleagues evaluated this hypothesis in controlled studies, and they found no such effect.

As scientists we take every opportunity to evaluate new possible remedies that can help reduce parasite loads in our horses. A few of these have some potential, but the majority have not worked well. At University of Kentucky, we are currently doing research with a couple of new treatment methods, and there is some promise. Many plants contain antiparasitic substances, but it has proven difficult to extract these and feed them in sufficiently high concentrations to the horses for them to have an effect. Many fungi also produce antiparasitic compounds. In fact, the most widely used dewormer, ivermectin, is actually produced by a fungus. So chances are good that there is an effective dewormer out there that just has not yet been discovered. Unfortunately, diatomaceous earth is not in this category, and we discourage horse owners from relying on it for parasite control.

Martin K. Nielsen, DVM, PhD,  DEVPC, DACVM
M.H. Gluck Equine Research Center
University of Kentucky
Lexington, Kentucky

This article first appeared in EQUUS issue #439.

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