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What are Onchocerciasis?

Onchocerciasis is an infestation of the Onchocerca genus of roundworm, also referred to as neck threadworms. These worms can be around 2.5 to almost 12 inches long in adulthood, and they generally live in the ligament that runs down the nape of the neck, although they have occasionally been found residing in other ligaments as well. When the adult worms release their larvae into the system, they travel to the skin where they cause small, itchy bumps. On rare occasions they may also travel to the eye, causing swelling and pain.

Onchocerciasis is an infestation by a roundworm in the Onchocerca family that lives in the nuchal ligament. Its larvae travel to the skin and cause severe itching.

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Symptoms of Onchocerciasis in Horses

The symptoms of onchocerciasis don’t appear until the larvae, also referred to as microfilariae, migrate to the skin. Once this journey has taken place, the larvae form small, itchy bumps, which the horse generally rubs against fence posts or other hard surfaces, in order to relieve the itching, causing ulcerated sores, crustiness, and hair loss. In some cases, the worms will travel to the eye instead. When this occurs, inflammation of the eye and discoloration can occur.

  • Itchy bumps
  • Ulcerated crusty sores
  • Hair loss
  • Eye inflammation (in some cases)
  • Eye discoloration (in some cases)


There are several parasitic worms in the onchocerca family, although not all of them infest horses. Onchocerca raillieti affects horses in Africa and onchocerca cervicalis and onchocerca reticulata affect horses worldwide. The other worms in this family include: 

  • Onchocerca dukei infests the subcutaneous and muscular tissues of cattle in Africa
  • Onchocerca ochengi is an African worm that creates nodules in the udders, flanks, and scrotum of cattle
  • Onchocerca armillata lives in the middle layer of the aorta of African cattle
  • Onchocerca gutturosa in the nuchal ligament and in the connective tissue around the spleen and the rumen of cattle in Australia, North Africa, and Europe
  • Onchocerca gibsoni is an causes subcutaneous nodules in the chest and the hind legs of cattle in Australia, Asia, Africa, and North America
  • Onchocerca lupi is a worldwide parasite that affects the the eyes of dogs

Causes of Onchocerciasis in Horses

Onchocerciasis is caused by a roundworm in the Onchocerca genus. The larval form of the worm is transmitted to the horse by a flying insect of some sort. This larva, also known as a microfilaria, travel throughout the body until they mature and settle in the nuchal ligament, the ligament that runs down the nape of the neck. They are an incredibly thin worm that grows to reach between 2.5 and just under 12 inches long. Once they mature, the females release thousands of larvae which migrate back up to the skin and instigate the dermal symptoms.

Diagnosis of Onchocerciasis in Horses

Adult onchocerca frequently live in the equine body symptom-free, although sometimes the body may develop mineralization around the worm. An infestation by onchocerca is similar in appearance to many other disorders including infections by other parasites, allergies to bug bites and environmental components, and some molds and mildews. The examining veterinarian will most likely take a sample of the affected skin to examine it microscopically. 

Regular microscopes will be able to use this to rule out conditions like mites and other parasites, however, an electron microscope may be able to also identify if viral particles are present. A blood test usually is not suitable for making a diagnosis, so the definitive diagnosis will be made based off of a  full-thickness biopsy of the dermal tissue, which is minced and soaked in isotonic saline before evaluation. The biopsied tissue will be evaluated to rule out cancers and other types of tumors.

Treatment of Onchocerciasis in Horses

There is no effective treatment for eliminating the adult worms from the ligaments at this time, and they can live there for up to a decade or longer, with no negative symptoms. Dewormers such as ivermectin and moxidectin are, however, quite successful at killing the microfilariae and eventually providing relief from the itching and swelling. Because this treatment has little to no effect on the adult Onchocerca worms, the deworming medication should be repeated every four months or so, to prevent new larvae from taking hold and reinfesting the skin and eyes. 

In the majority of cases, a topical paste is used to apply the deworming medication, but it can also be administered orally or by injection. Many horses will experience temporarily increased dermal symptoms shortly after deworming, and additional steroids may be needed to reduce the initial itching and swelling. In very rare situations, lesions created by the microfilariae will require surgical removal.

Recovery of Onchocerciasis in Horses

It should be understood that neck threadworms, like the kind that induce onchocerciasis, are a very different parasite than threadworms. Threadworms, as opposed to neck threadworms, are in the Strongyloides family of roundworms. Strongyloides are also thin like Onchocerca, but much shorter at only 1 to 6 millimeters long, and they affect mainly foals. These parasites are not spread by insect, but live in the soil after completing their journey through the former host’s digestive tract. In adult horses, infestations of threadworms may have very little effect, however, it can cause diarrhea, respiratory conditions, and even blood loss in the younger animals.