Dewormer’s Effect on Scratches

Find out what kinds of skin parasites can cause scratches and how these pests respond to ivermectin treatment.
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Dewormer
If dewormer makes a horse's scratches disappear, there must be a parasitic reason. | Photo: Alexandra Beckstett/The Horse
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

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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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