Onchocerciasis in Horses
This is an itchy skin condition, caused by an allergic reaction to larval forms of Onchocerca or Neck Threadworm. The condition is most prevalent in Northern Australia, during wetter summer seasons, however its distribution is spreading south. Incidence is highest in late summer to winter.
Lifecycle of the neck threadworm
Adult females worms up to 30cm long live in the major ligaments of the neck and, in rare cases, the flexor tendons and suspensory ligaments of the fetlock. Tiny juvenile forms (microfilariae) are produced by the female worms, and these migrate out of the ligaments towards the skin surface to form small, itchy lumps under the skin.
The horse rubs and abrades the lumps, which may seep serum. Biting insects, particularly sand flies, and possibly mosquitoes, are attracted by the abraded lump and digest microfilariae as they feed. These insects are capable of spreading the infestation to other horses within 20 – 25 days after ingesting microfilariae.
The lifecycle is complete in approximately 4 – 5 months. The adult worms have no recognised detrimental effect on the neck ligaments, however occasionally they may cause swelling of the flexor tendons and lameness in the front limbs.
The larvae can also invade the eye and cause blindness, particularly if the horse rubs and lacerates the eye.
Signs of Onchocerciasis
Small lumps, from pea to marble size, develop in the skin on the underside of the belly, chest, withers, neck and face
Itching and rubbing causes thinning and loss of hair, and scaly skin, particularly along the mane
Surrounding weepy and scabby areas develop in severe cases
Often white tufts of hair regrow on healing
Biopsy of the lumps by your vet to identify microfilariae can confirm the diagnosis of onchocerciasis
Treatment & control of Onchocerciasis
Control of biting insects is essential to prevent the condition
Stable and rug the horse day and night
Install insect screens in stables during summer months
Control of the microfilariae that cause skin itching can be achieved by worming with Equimax, Equiminth, Equimec, or Equest
After treatment, horses often become intensely itchy within 24 – 72 hours, causing severe skin abrasions and mutilations, and damaging feeders, walls and fences on which they rub
The itch may be controlled by prednisilone granules added to the feed for 3 – 5 days (consult your vet for advice).
By Virbac Animal Health - Last updated 16 November 2012
