Last Updated on February 21, 2022 by Allison Price
Adult Onchocerca cerca worms are also known as equine neck threadworms. They live in the large nuchal, which runs from the poll to where the withers. Adult worms can be as thin as a few inches in length. The adult stage is usually not a problem for horses. Sometimes, radiographs of a horse’s neck may show mineralizations in its nuchal ligament. This is because adult worms can cause inflammation and mineralization. The immature microfilaria of adult worms are released by biting insects like no-see-ums or black flies. Microfilaria are found along the bottom of the horse’s chest and abdomen, which is where they prefer to feed.
Microfilaria can cause severe itching, as well as skin trauma and swelling. Horses may experience itching after being dewormed. Dead microfilaria can be more itchy than the live ones. A bullseye is a visible lesion that causes hair loss and inflammation to the forehead of horses. The most common symptoms are dermatitis, crustiness and hair loss. There is also itching and swelling along the chest and bottom. Sometimes, microfilaria can migrate through the eye and cause edema (a condition that causes blueness to the otherwise clear front of an eye) or lunar blindness (anterior Uveitis). Microfilaria can be deposited by flies when they feed on wounds, making them itchy and slow-healing.
Many skin lesions can look very similar to Onchocerca. infections. These include proud flesh, squamous cells carcinoma, pythiosis and summer sores. Parasites can also cause them ( Habronema, and Draschiaspp. Your veterinarian can provide expert advice and perform additional diagnostics on your horse. To determine the exact cause, a skin biopsy may be required. Onchocerca spp. Microfilaria can often be found in the skin even in horses that are not symptomatic.
It is easy to treat horses with deworming (ivermectin, moxidectin), to kill microfilaria. Symptomatic therapy is used to relieve itching. Although most horses can be treated with topical steroids, some horses may need injectable or oral steroids to manage itching after deworming. You can also apply dewormers directly to the lesions. Sometimes, severe lesions will need to be removed surgically. Most horses recover within a few days.
