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 parasiteOnchocerca 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.

Telling the two conditions apart can be a big challenge, but your horse’s clinical signs should resolve following deworming with ivermectin and moxidectin. If they appear unaffected by treatment, then they are more likely to be caused by sweet itch.

Diagnostic options are not great, but one approach has been found successful in some cases. A simple skin biopsy collected from an affected area is left in warm saline. Microfilariae can then be observed swimming under the microscope.

Because of the lack of reliable and practical diagnostic methods, very little prevalent information is available. Given its insect vector, the parasite might depend on suitable habitats for the midges, but that is unknown. Midges generally prefer areas with running creeks to hatch their larvae, and they are rarely found in windy regions.

I hear a lot of talk about neck threadworms in various horse communities, and the parasites often get accused of causing all sorts of unlikely problems in horses. But most often the infection is not confirmed and the issues people are reporting might not have to do with the parasite at all.

Other parasites can cause skin lesions as well. Most widely known are probably the summer sores caused by larvae of the stomach worm Habronema. Flies deposit the larvae in open wounds, which then remain open until the larvae are removed. Owners might observe a bloody fluid oozing out of these wounds, but the lesions generally look much different than with Onchocerca. Some veterinarians report thatHabronema, unlike Onchocerca, can appear resistant to anthelmintic treatment.

The term neck threadworm can be confusing as horses can also get infected with another parasite called “threadworm.” This parasite, Strongyloides westeri, primarily infects foals and has an intestinal lifecycle with eggs being passed in the horse's feces.