Each week we investigate one equestrian conundrum in “What the muck is that?” This week we take a look at neck threadworms.
Onchocerca (neck threadworms) is a parasitic filarial worm that never live in the intestines. The microscopic larvae live in the horse’s skin. The adult worm can be found in the nuchal ligament. That’s right — in the neck. The trouble starts with the larvae, though, which can cause severe, obsessive itching.
Source: Sustainable Dressage
The worm can be found globally, though it prefers warm, humid climates. Its primary carrier are biting midges (no-see-ums). This insect is also a carrier for sweet itch, which is very difficult to differentiate from a neck threadworm infestation.
Source: Horse Hints
What are the symptoms?
Sudden and severe itching → some horses scratch to the point of bleeding
Small lumps → underside of neck, face and ventral midline (belly) especially
Scaly crest in mane
Constant water stream out of one eye or both eyes → Yep, the little nasties can travel to the eye!
White or yellow mucous in the eye on a regular basis
Source: Merck Manual
How do we test for it?
You’ll need a skin biopsy. Merck Manual tells us, “The most effective method of diagnosis is by skin biopsy, preferably a full-thickness biopsy ≥6 mm. The tissue is minced and macerated in isotonic saline for several hours. Microfilariae are concentrated and stained with new methylene blue after removal of skin pieces. The microfilariae can be differentiated microscopically from Setaria spp, found in the blood of cattle, and Equidae by the presence of a sheath around Setaria.”
Source: Merck Manual
What’s the treatment plan?
No treatment is effective against adults. Grossly, they can live in the nuchal ligament for 10 to 12 years. The horse’s immune system will often encase the worm with calcification.
The microfilariae (larvae) can be effectively treated with ivermectin and moxidectin. The massive die off of larvae can cause reactions in some horses. They may have swelling 1–3 days after treatment and eye lesions have been reported. Reactions can be mild to severe so be sure to keep your vet updated after deworming.
Beyond that, the only thing to do is keep flies away. Fly sheets, masks, boots, and spray all thankfully deter the carrier midges. Proper manure management and clean up of stagnant water is also key.
Source: SS Tack