The Horse

OCT 2017

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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49 October 2017 THE HORSE TheHorse.com FARMCALL TheHorse.com/Farm-Call This column features readers' questions on specific illnesses, diseases, and conditions, with answers from American Association of Equine Practitioners members or their designates. Send questions to FarmCall@TheHorse.com, or The Horse, 3101 Beaumont Centre Circle, Suite 100, Lexington, KY 40513. Questions will be edited for publication and must include the author's name, address, and daytime telephone number. Farm Call is compiled by Managing Editor Alexandra Beckstett. 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 de- wormer, 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 diatoma- ceous 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 pad- docks every day. Irene Louw The Netherlands A Thank you for your comments about parasite control remedies. I do agree that we need reliable alter- natives 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 own- ers 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 lar- vae 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 diato- maceous 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 diatoma- ceous 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 infec- tion. 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 deworm- ing 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 diato- maceous 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 mat- ter 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 differ- ent 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 para- sites will not be able to develop resistance to is pasture and paddock hygiene. So keep up the good work! Martin Nielsen, DVM, PhD, Dipl. EVPC, ACVM Associate Professor University of Kentucky Gluck Equine Research Center Diatomaceous Earth's Deficiency Unlike diatomacous earth administration, removing feces from paddocks has been scientifically proven to reduce parasite burdens significantly. THE HORSE STAFF

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