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

Last Updated on February 21, 2022 by Allison Price

Q.Do you have any advice or knowledge about equine neckworms? My mare may be suffering from neck threadworms. Although she has always had sweet itching, her open, oozing stomach wounds, some as large as $1000, are very serious and difficult to heal. Both of my veterinarians said that they had never heard of neck threadworms.


A The common name for the filarial parasite Onchocercacerca cervicalis is “neck threadworm”. The adult worm lives within or around the large nuchal, which runs from the base of your skull to the withers. The presence of adult worms has not been linked to any clinical symptoms. The microfilariae are microscopic parasite stages released by the worms and can be found under the skin’s connective tissue.

Adult worms can live for many years. However, most clinical signs of this parasite are caused by microfilaria. These can be found in areas where the intermediate host, Culicoides miges (no-seems), has free access to the skin.

Common signs include dermatitis, which can cause itching and swelling. Deworming can sometimes make skin reactions worse, because there is a tissue reaction due to microfilariae that are dead or dying. These symptoms can look similar to summer eczema, which is actually caused by an allergic reaction Culicoides mitges. The same insect can also cause two diseases that may look similar.

It can be difficult to distinguish the two conditions. However, your horse’s clinical signs should improve after deworming with ivermectin or moxidectin. Sweet itch is more likely to occur if they don’t respond to treatment.

Neck Threadworm Horse

Although there are many options for diagnosing the problem, one method has been proven to be effective in certain cases. Warm saline is used to take a simple skin biopsy from the affected area. The microscope can then be used to observe microfilariae swimming under the microscope.

There is very little information available due to the lack of practical and reliable diagnostic methods. The parasite could depend on the availability of suitable habitats due to its insect vector. However, this is not known. Midges prefer to lay their larvae in areas where there are running creeks. They are also less common in windy places.

There is a lot of discussion about neck threadworms in horse communities. The parasites are often accused of causing many other problems in horses. Most often, the infection is not confirmed. The issues that people report might not be due to the parasite.

Skin lesions can also be caused by other parasites. The summer sores that are caused by the larvae of the stomachworm Habronema are the most well-known. The larvae are then left open to develop until they are eliminated by the fly. Although owners might notice a bloody liquid oozing from these wounds, the lesions are generally not as severe as with Onchocerca. Some veterinarians have reported that Habronema can be resistant to antihelmintic treatment, which is not the case with Onchocerca.

It is easy to confuse the term neck threadworm with another parasite called “threadworm”. Horses can get infected by Strongyloides. This parasite infects foals and has an intestinal cycle with eggs passing through horses’ feces.

Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!