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Treatment for Neck Threadworms in Horses

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Table of Contents

Treatment for Neck Threadworms in Horses

neck threadworm in horses

Table of Contents

What the heck is a neck threadworm?

Neck threadworms are the common name for Onchocerca cervicalis, a filarial parasite. While the worm can be found across the globe, it is more prevalent in hot, humid climates from mid-summer through fall. The adult worm lives in or around the nuchal ligament, which is the ligament that runs from the withers to the base of the skull. The adult worm is thin and can grow from 3-60 cm long and can live approximately 10-12 years. Most clinical signs are caused by the adult worm’s larvae, microfilaria.

The growth stages of the neck threadworms

neck threadworm life cycle

Onchocerca cervicalis, commonly referred to as neck threadworm, is a parasite transmitted by the culicoides fly. In the adult stage, the worm lives in the nuchal ligament. Michrofilaria, the larvae of the adult worm, cause these symptoms:

  • extreme itching
  • a scaly appearance of affected skin
  • lumps
  • open sores

Extreme cases can also affect the horses’ eyes. There is no treatment for adult worms, but the larvae can be treated with ivermectin. Prevention should be prioritized. Neck threadworms should not be confused with threadworms, but are related to the parasite that causes river blindness in humans.

How do Horses get Neck Threadworms? 

Neck threadworms are transmitted by the female culicoides fly, commonly known as midges, sand gnats, and no-see-ums. As is typical with parasites, the neck threadworm goes through several stages, best described in an article by Jane Clothier in www.thehorsesback.com, June 20134.

In the first stage, the microfilariae live on the skin of the horse. As they die, they can cause intense itching in which may present as patches of scaly, dry skin. The more the horse itches, the more likely they are to break the skin, further attracting the cuclicoid fly.

As the fly bites the horse, it ingests the microfilariae along with blood. The larvae then develop into a second stage inside the fly. The next time the fly bites the horse, the larvae are transmitted back to the horse, this time in the bloodstream. As Onchocerca cervicalis prefers connective tissue, the larvae make their way to the nuchal ligament. This is the ligament that runs along the crest, or mane, of the horse. The larvae then molts and enters its adult stage, which can last 10-12 years. Each year, the adult female releases thousands of microfilariae, continuing the cycle.

Neck Threadworm Symptoms in horses:

Sudden, severe itching caused by dead or dying larvae

  • A scaly crest in the mane
  • Lumps or swelling, especially on the underside of the horse’s neck, face, and belly
  • Open, weeping sores, especially near the base of the mane
  • Weeping eyes, persistent white or yellow mucous, and even blindness
  • While rare, neck threadworm larvae can travel to the eye, which makes for some pretty graphic photographs.

Don't confuse with Sweet Itch

Symptoms of neck threadworms are commonly confused with sweet itch, also referred to as summer itch. Summer seasonal recurrent dermatitis (Sweet itch or summer itch) is caused by an allergic reaction to the saliva of the biting gnat discussed earlier—the cuclicoides fly.

How to diagnose neck threadworm

Neck threadworms can be difficult to diagnose, but not impossible. One test that has been successful in some cases is a skin biopsy. Once processed, this test shows the presence of microfilariae. The fastest way to tell if your horse has neck threadworms is to dose them with ivermectin wormer. If the adult worm’s larvae are present, the horse may respond with intense itching 1-3 days after worming. Sweet itch, commonly confused with neck threadworms, does not respond to ivermectin treatment. An injectable dose of ivermectin may be more effective than the oral version. Always consult a licensed equine veterinarian before treating your horse.

How to Treat Neck Threadworm in Horses

There is no effective treatment effective against adult worms. Calcification can occur around the adult worm as an immune response to the presence of a foreign body. This can feel like pea-sized bumps in and around the nuchal ligament. In older horses, mineralized nodules can be common. Fortunately, the microfilaria, or larvae of the adult worm, can be treated.

Ivemectin and moxidectin are both effective in killing these larvae. Some horses experience skin reactions from deworming as the microfilaria die off. These reactions can look very similar to summer eczema, or sweet itch.

Neck Threadworm Prevention is the Best Cure

Since there is no cure, a focus on prevention is key. Many horse diseases are transmitted by insects, therefore efforts to control insect activity will benefit the horse in many ways. Below are three tips to preventing insect bites:

Manure management

Regularly cleaning stalls and pastures of manure will eliminate the breeding ground of certain flies. Manure should be removed from the property at least once per week, or properly composted.

Eliminate standing water

Mosquitoes and flies both can breed in stagnant water.

Utilize fly protection

horse sheets

Fly spray, fly sheets, and fly masks can all help keep insects from landing on and biting your horse.

horse fly guard

Additional Information

Strongyloides westeri, an intestinal parasite that are commonly referred to as threadworms.  They are primarily passed to foals through the mare’s milk.

These worms are very small and unique in that they only develop female parasitic stages. Eggs are passed in feces and can infect other horses through the environment, such as a pasture. Similar to neck threadworms, one treatment for Strongyloides westeri is ivermectin.

Fenbendazole can also be effective against this parasite. Environmental contamination can be reduced by ensuring the mare’s udder, the barn, and pastures are kept clean.

A similar parasite can also be found in humans. River blindness, or Onchocerciasis, is caused by the worm Onchocerca volvulus. A tropical disease, it is transmitted by repeated black fly bites. Symptoms also include severe itching, skin conditions, visual impairment, and permanent blindness. River blindness is found in 31 African countries, Latin America, and Yemen. Similar to equine parasites, treatment uses ivemerctin. The pharmaceutical company Merck has been donating MECTIZAN® to affected areas for more than 30 years.


Onchocerciasis has low awareness, even in regions where it is more common. If your horse exhibits symptoms such as severe itching, lumps or swelling, and weeping sores especially near the mane, consult your veterinarian to discuss further diagnosis and treatment. While there is no cure, it is possible to manage the symptoms and improve the comfort of your horse.


University of Kentucky: https://equine.ca.uky.edu/news-story/what-neck-threadworm-and-can-it-hurt-my-horse

Merck Vet Manual: https://www.merckvetmanual.com/integumentary-system/helminths-of-the-skin/onchocerciasis-in-animals

Horse Nation: https://www.horsenation.com/2014/09/19/what-the-muck-is-that-neck-threadworms/

The Horses Back: https://thehorsesback.com/neck-threadworms/

European Scientific Counsel: https://www.esccap.org/uploads/docs/rtjqmu6t_0796_ESCCAP_Guideline_GL8_v7_1p.pdf

World Health Organization: https://www.who.int/news-room/fact-sheets/detail/onchocerciasis

Merck: https://investors.merck.com/news/press-release-details/2017/Merck-Commemorates-30-Years-of-MECTIZAN-Donation-Program-Progress/default.aspx



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