Equine Neck Threadworms: Signs and Treatment

Adult Onchocerca cervicalis worms, also called equine neck threadworms, live in the large nuchal ligament that runs from the poll to the withers. Adult worms are very thin and may be several inches long. Most of the time, the adult stage causes no problems for the horse. Occasionally, radiographs of the neck can show mineralizations in the nuchal ligament because the adult worms can cause some inflammation and mineralization results. Adult worms reproduce by releasing immature microfilaria, which are spread by biting insects such as no-see-ums and black flies. Microfilaria concentrate along the bottom skin of the horse’s abdomen and chest, where these flies prefer to feed.

The presence of microfilaria can cause intense itching and resultant skin trauma and swelling. Dead microfilaria are often more itchy than live ones, so horses may show signs after they have been dewormed with an effective dewormer. The classic, but not often observed, lesion is a bullseye of hair loss and inflammation on the horse’s forehead. Most affected horses show dermatitis, crustiness, hair loss, itching, swelling, and self trauma to the skin along the bottom of the abdomen and chest. Occasionally, microfilaria may migrate through the eye and cause edema of the cornea (revealed as blueness to the normally clear front of the eye) or moon blindness (anterior uveitis). Flies feeding on wounds may deposit microfilaria, making the wounds itchy and slow to heal.

There are several different and important skin lesions that may resemble Onchocerca spp. infections. These include proud flesh (exuberant granulation tissue), squamous cell carcinoma, pythiosis, and summer sores, which are also caused by parasites (Habronema and Draschiaspp.). Your veterinarian is your best source of advice to examine your horse and recommend additional diagnostics. A skin biopsy may be necessary to determine the exact cause of dermatitis. Onchocerca spp. microfilaria can be commonly found in the skin, even in asymptomatic horses, so their presence alone without inflammation may not confirm actual disease.

Treatment is usually straightforward and involves deworming (ivermectin or moxidectin) to kill microfilaria and symptomatic therapy to relieve itching. Usually, topical steroid creams are sufficient, but some horses may require injectable or oral steroids to control itching, especially after deworming. Dewormers can also be used as a cream directly over the lesions. Rarely, severe lesions may need to be surgically removed. Most horses improve within several days after treatment.