Dewormer's Effect on Scratches

If dewormer makes a horse's scratches disappear, there must be a parasitic reason.

Photo: Alexandra Beckstett, The Horse Managing Editor

Q. I have read several articles about scratches this year, and I never see the solution that without fail clears up my horses’ scratches. For my horses, if I deworm a couple of times with ivermectin/praziquantel a week or two apart, their scratches disappear. I don’t have to put any topicals on or scrape them off. I’m wondering why I have never read about this.

Donna Rupp, Orland Park, Illinois


A. First of all, scratches is a skin condition of the lower limbs sometimes caused by a pathogen, be it a virus, bacterium, fungus, or parasite. To answer your question it would help to know more about your horses. What type of environment do they live in, and how do these scratches cases present—is there a seasonal occurrence, or do they appear year-round? Where on your horses are they located? What type of horse do you have? This information would help with identifying the most likely explanation. However, here are a few possibilities for why you saw scratches relief after deworming.

Ivermectin has activity against nematodes, insects, and mites. If the scratches tend to disappear after ivermectin treatment, the explanation could be skin parasites. The most prevalent skin mite infesting horses is Chorioptes equi, which is very common in draft horse breeds with well-developed feathers (long fetlock hair). The mites live in the skin’s superficial layers and cause a chronic itchy dermatitis that often leads to lesions because the horses are scratching themselves. These mites primarily infest the legs, but can also be found elsewhere on the horse’s body. They are sensitive to ivermectin, but oral preparations rarely work because they do not distribute very well to the skin’s superficial layers.

In the insect category we have lice. There are two types of lice that infest horses. The most common is a chewing louse (Bovicola equi), which lives on the skin and eats skin debris. The less common type is a blood-sucking louse, Hematopinus asini. Both of these cause itching, which can lead to scratches. However, ivermectin is not very likely to be effective against the chewing louse, given its superficial location on the skin.

Then there are the worms. Summer sores are common in some areas of the United States and elsewhere. To my knowledge, Illinois is not a hot spot for summer sores, but that doesn’t mean they could not occur there. They have a characteristic granulomatous (chronically inflamed) presentation that rarely resembles simple scratches. They are caused by the Habronema stomach worm’s larvae. This parasite uses different kinds of flies as its intermediate host. These flies are attracted to wounds and deposit larvae in them. The chronic inflammatory condition that develops in the wound does not resolve until the larvae are effectively treated. Summer sores are quite seasonal in occurrence, and many horse owners struggle with lesions returning every year.

Other possible parasite causes are the filarial worms Parafilaria multipapillosa and the neck threadworm Onchocerca cervicalis. Parafilaria cause bleeding lesions in the skin, and larvae can be found in the blood. The neck threadworm causes a localized itching dermatitis along the neck and ventral chest regions. Veterinarians report that ivermectin has good efficacy against both of these.

In summary, there are a few potential explanations for your observations. If ivermectin makes those scratches disappear, there must be a parasitic reason. However, it is very unlikely to find a parasitic cause for every scratches lesion you find on your horses, and I do -encourage you to consider other explanations as well. It’s not likely that a dermatologist would have parasites on the top of his or her list of potential causes of skin scratches. It would be very useful to have some diagnostic work done and possibly find out what is going on with your horses. It always helps to know what you are treating, and depending on what you find there may be additional useful procedures to have in place to help reduce the incidence of these scratches.

Treating with ivermectin certainly would not be the first thing I’d suggest for scratches, but you could try it in cases where other treatments have failed.

About the Author

Martin Krarup Nielsen, DVM, PhD, Dipl. ACVM

Martin Krarup Nielsen, DVM, PhD, Dipl. ACVM, is an associate professor of parasitology and the Schlaikjer professor in equine infectious disease at the University of Kentucky's Maxwell H. Gluck Equine Research Center, in Lexington. His research focus includes parasite diagnostic measures and drug resistance. Known as a foremost expert in the field of equine parasites, Nielsen chaired the American Association of Equine Practitioners’ (AAEP) parasite control task force, which produced the “AAEP Parasite Control Guidelines.”

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