Q.Many articles are about deworming horses and how worms are getting immune to the dewormers we’re giving them. But how about an old dewormer: diatomaceous earth? Worms will never get “used” to this dewormer because it slices the worms into small pieces. Every morning for about two months in spring and fall I add one heaped spoon of it to my horses’ feed.
The larvae-counting manure test does not tell the whole story, as the test cannot count what is left behind in the intestines. In concert with my vet, I had one of my horse’s blood tested before the diatomaceous earth and after. Before he tested high on worms. After two months his blood count was back to normal. Mind that I remove the manure from the paddocks every day.
—Irene Louw, The Netherlands
A.Thank you for your comments about parasite control remedies. I do agree that we need reliable alternatives to existing dewormers, as equine parasites have developed some degree of resistance to all of them.
As an equine parasite research scientist, I get many questions from horse owners and veterinarians every year, and diatomaceous earth (a silica-rich powder that’s supposedly abrasive to worms) is a clear top scorer. Many people are using it, and lots believe they experience good effects. Unfortunately, this is not backed up by scientific evidence. Several studies have been performed to evaluate this alleged antiparasitic effect, and none of them have shown any effect. My qualified opinion is that the suggested mechanical disruption of parasites is unlikely to occur within the horse’s intestine. It might be that the product contains sharp edges, but ingesta will dilute it to a degree where it would be quite easy for the worms to avoid being cut. These worms and larvae are very small, often too small to see with the naked eye, so they would have plenty of chances to avoid any potentially sharp edges. Besides, if the particles are really that sharp, one would expect them to cause lesions in the mucosal membranes of the horse, and that does not appear to be the case. Furthermore, all horses ingest soil and sand, which also can have edges, but we don’t see any reduction of parasite loads in response.
Some have suggested that diatomaceous earth would actively disrupt developing parasitic larvae within the fecal pile on pasture. Again, a number of my colleagues evaluated this in controlled studies and found no such effect. All in all, there are no sound biological reasons to expect an antiparasitic effect of diatomaceous earth, and this is supported by research.
I commend you for removing manure from your horses’ paddocks every day. That is quite a commitment and takes some perseverance. Removing feces dramatically reduces parasite burdens, as it constantly removes the source of infection. This is backed up by science, and it is likely the reason you are experiencing low parasite counts. I know of examples where people were able to skip deworming their horses completely as the manure removal effectively kept the parasites at an absolute minimum.
You mention that worms are unlikely to become resistant to a remedy like diatomaceous earth. I often hear people claim this incorrectly. Resistance can develop to any type of treatment remedy, provided it works to begin with, and it does not matter how the remedy works. This is a true example of Darwin’s Law of survival of the fittest. Mechanisms for resistance are countless. If a treatment remedy would indeed effectively reduce parasite burdens by “cutting worms,” parasites with different movement patterns or more durable cuticles (flexible exoskeletons) may have an advantage and survive the treatment. Then, these traits will be passed on to the next generation of parasites, and we have selected for a resistant population. Resistance will always develop, no matter the treatment. It is just a matter of how quickly it happens. The only thing parasites will not be able to develop resistance to is pasture and paddock hygiene. So keep up the good work!