Scientific name: Onchocerca sp cervicalis
Common name: Neck Threadworms

Description: Neck threadworms occur in the ligaments of the neck and forelegs. Adult worms are long and coiled. Females reach up to 12 in. (30 cm) in length and males are generally 2 to 3 in. (7 to 8 cm) long. Infection produces soreness and swelling in the neck, as well as the suspensory ligaments and flexor tendons of the forelegs, restricting a horse’s freedom of movement. Dead worms calcify and create bumps that can disable the horse. Microfilariae may infiltrate the lens of the eye and cause swelling of the cornea and, in some cases, blindness.

Life cycle: Unlike most parasitic worms, neck threadworms have an indirect life cycle and require an intermediate host. The biting midge, because of its aggressive feeding habits, serves as the host and perpetuates the transfer of neck threadworms. When an infected midge bites a horse, threadworm larvae are deposited under the skin and migrate to ligaments in the horse’s neck and legs. Larvae that remain in the skin can be picked up by other midges and carried to other horses.

Symptoms: Signs of neck threadworm infestation can appear in fly season and in winter. Clinical signs include skin lesions, hair loss, itching and other indications common with dermatitis. Swelling of the fetlocks, lameness and bumps under the skin covering ligaments and tendons can all mark the presence of neck threadworms. When microfilariae invade the eye, small white nodules (less than 1mm) may appear near the corner of the eye.