Neck Threadworm



On any ivermectin or moxidectin wormer you’ll see a list of parasites. At the end you will see the words Onchocerca Microfilariae, commonly known as neck threadworm. Like something out of a sci-fi movie, they live in your horse’s nuchal ligament. The nuchal ligament runs the entire length of the neck, from poll to withers and connects to the vertebrae. The majority of horses have neck threadworm. For most it does not present a problem, but some have a violent reaction to the resultant larvae or microfilariae. This is known as Onchocerciasis. The horse becomes itchy around the mane and tail, chest, shoulders and the midline of the belly.

Many owners mistake neck threadworm for Queensland itch or sweet itch. Neck threadworm is a parasitic filarial worm that releases thousands of larvae during the course of its ten year life cycle. The larval form live in the horse’s skin, primarily around the mane and tail, head, shoulders, chest and mid-line of the belly, while the adult worm sets up shop in the nuchal ligament. The issue is found worldwide and horses frequently have this parasite although it is more prevalent in hot, humid climates. The biting insect that serves as the larval carrier is the female culicoides fly, commonly known as no-see ums, sand gnats and midges. These insects are also the cause of insect bite hypersensitivity; Queensland Itch, Sweet Itch and Summer Itch. Many cases of neck threadworm are misdiagnosed because they are assumed to simply be a result of itching due to the culicoides bite. Truly the two are tied together in a three way vector between horse, culicoides and threadworm.

Sweet Itch or Neck Threadworm?

Is your horse simply itchy, or is something else going on? There are two options: your horse has sweet itch and is simply reacting to the culicoides saliva, or your horse is having a reaction to neck threadworm. Is your horse itching/rubbing the mane primarily at the withers, or around the neck and face, chest and the mid line and the belly and tail head?
nIdentifying Neck Threadwormn

Neck ThreadwormThe most telltale sign is more intense itching at withers after worming with ivermectin.  This incites the parasite and a flare up is common.
Ventral line neck threadworm – Photo Courtesy Elizabeth McIvor


The female culicoides fly feeds off your horse and ingests microfilariae with blood. Inside the insect larvae develops, and when the fly bites again, this larvae is injected back into your horse or another. The deposited larvae enters the bloodstream and comes to rest most commonly in the nuchal ligament. It is here they mature and become adult threadworms.
Midline of the belly treated with Equiderma characteristic of Neck Threadworm – Photo Courtesy of Ann McBain Ezzell


Wherever the adult worms land, the microfilariae they produce will cause intense itching. The issue becomes a viscous cycle; the horse rubs and causes the skin to break open producing serum; the serum draws in more culiciodes where the larvae are located; the culiciodes ingest the exposed larvae and reinfect the horse, or carry it to another horse where the cycle begins again.

As if this weren’t bad enough, the larvae can migrate to the horse’s eyes with the potential for serious damage: keratitis, uveitis, parapapillary choroidal sclerosis, and vitiligo of the bulbar conjunctiva of the lateral limbus.

Definitively the microfilariae is identified through biopsy of the nuchal ligament, however within 30-35 days of worming with ivermectin there won’t be any conclusive results, so if you choose to do a biopsy timing is critical.

Ivermectin wormer is the best way to determine if your horse has Neck Threadworm. If microfilariae are present, the horse will respond with intense itching within 48 to 72 hours after worming.  This too is a double edged sword.  Ivermectin is necessary to stop the microfilariae from coming to maturity and reproducing.


Equiderma Neem Shampoo and Equiderma Skin Lotion have proven highly effective against the symptoms of Neck Threadworm. Equiderma Skin Lotion will halt the cycle by stopping the itching and subsequent damage, render the area unappetizing to the culicoides, and allow the skin to heal.

It is also recommended to combat the issue internally by using the advised dose of ivermectin wormer. Consult your vet for amounts and frequency. Often it is recommended to use injectable ivermectin. Again this is something you should discuss with your vet to establish safe dosage limits. The adult neck threadworm cannot be killed, but their offspring can.  With effective management with Equiderma, you can help to ensure your horse’s comfort.


Equiderma Neem Horse Shampoo and Skin Lotion Combo
Equiderma Neem Horse Shampoo and Skin Lotion


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