What is a Neck Threadworm, and Can it Hurt My Horse?

An equine parasitologist answers a horse owner question about these mysterious worms.
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Q. Have you any knowledge of and advice for dealing with equine neck threadworms? I suspect my mare is experiencing all the symptoms of having neck threadworms. I have always thought she had sweet itch, but now the open, oozing wounds on her belly—some as big as silver dollars—are very bad, and I can’t get them to heal. My veterinarians both said they have never heard of neck threadworms.–Deborah, via Facebook

A. The neck threadworm is a common name for the filarial parasite Onchocerca cervicalis. The adult worm lives in or around the large nuchal ligament that runs from the withers to the base of the skull. No clinical signs have been associated with presence of the adult worms. However, the worms release microfilariae, which are microscopic parasite stages that can be found in the loose connective tissue under the skin.

The adult worms live for many years, but the majority of clinical signs associated with this parasite are due to the microfilaria. These are typically present in areas where the intermediate host, the Culicoides midges (no-see-ums), have free access to exposed skin, such as on the ventral midline or along the neck.

Clinical signs typically involve dermatitis with itching and swelling. Skin reactions are sometimes exacerbated by deworming, as there seems to be a tissue reaction to dead or dying microfilariae. These signs can look a lot like summer eczema (sweet itch), which ironically is caused by an allergic reaction to Culicoides midges. In other words, the same insect can cause two different diseases that look very much alike

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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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