Neck Threadworm (Onchocerca species)
The adult worms live for many years, potentially reaching up to 12 years, and inhabit around the nuchal ligament (the bit that holds up the horse’s neck and head), but the majority of clinical signs associated with this parasite are due to the microfilaria.
The larvae (which are called microfilariae) live in the tissue under the skin and are ingested by midges as they feed, they can also congregate in the eye tissue causing infections. The adult worms live in the tendons and ligaments. They are long and coiled in shape with the males being around 6cm in length and females being around 30cm in length. Neck Threadworms have to depend on an intermediate host, the biting midge - Culicoides spp. (Insecta: Diptera: Ceratopogonidae) to get to the horse. The neck threadworm microfilariae live just under the horse's skin and wait to be ingested up by a visiting midge. Once inside the midge they develop to the infective larvae L3 stage within 24-25 days. When the midge bites another horse the neck threadworms the larvae migrate to the ligaments in the neck and also to the flexor tendons and suspensory ligaments, particularly the forelegs.
Symptoms include: Sores along the topline and the stomach, and irritation and swelling around the
eyes.
Uveitis - This occurs when there are large quantities of dead microfilariae in
the eye. The dead give off large amounts
of antigens which cause inflammation in the eye.
A constant water stream out of the eye or eyes often along with a white or
yellow mucous in the eye on a regular basis.
Hair loss around the head and neck area.
Swelling around ligaments.
Swelling around tendons.
Lameness.
Lumps under the horse’s skin on the ligaments.
Blindness can occur if the infection around the eye is severe or if treatment
is delayed.
Diagnostic options are not great, but one approach has been found successful in some cases. A simple skin biopsy collected from an affected area is left in warm saline; the microfilariae can then be observed swimming under the microscope. Dead microfilaria are often more itchy than live ones, so horses may show quite radical discomfort signs after they have been wormed with effective products, which in a way can be deemed as confirmation that neck threadworm was indeed the issue.
Treatment is only available for the larvae; products containing Ivermectin are the wormers to use. Be sure to take care with dosing amounts, dosing intervals should be observed and consultation with your SQP or vet is advised.
Neck threadworm is one of the few parasites of horses that involve an intermediate host; in this case that host is the midge. In theory this should not be such an issue in the colder months or in general in colder climates such as the UK. I personally have yet to hear of a confirmed diagnosis in the UK but climates change and issues can go unnoticed or unconfirmed.