Horse owners are able to easily access information on the common parasites that affect horse and ponies. Anthelmintics (drugs that expel parasitic worms ‘helminths’ and other internal parasites from the body by either stunning or killing them and without causing significant damage to the host.) are readily available. When used correctly, anti-parasitic preparations can help get rid of the most common parasites found in horses. If you look closely at the packet of any Ivermectin or Moxidectin dewormer, you’ll see a parasite listed right near the end of the list: Onchocerca Microfilariae, otherwise known as neck threadworms.
The filarial parasite, Onchocerca cervicalis is commonly known as the neck threadworm. This worm lives in or around the large nuchal ligament of the horse. This is the ligament that runs the full length of the neck, from poll to withers, with a flat ligament part connecting with the cervical vertebrae. The neck threadworm can vary in length from 6cm to 30cm, can live for many years but the horse generally does not show any clinical signs with the presence of the adult worms. However, the adult threadworm releases microfilarie which can be the cause of discomfort for the horse. Microfilarie are the microscope developmental stage of the parasite. They shed from the adult worm and migrate into the loose connective tissue under the skin. They remain there until the intermediate host, the Culicoides midge, bites the horse and carries the parasite outside of the horse’s body. When the microfilarie migrate, this can cause a reaction in the horse whereby they become itchy, mostly around the head, neck, chest, shoulders and underside of the abdomen. Not all horses will react to the microfilarie, there is no known reason why some horses react while others do not, but when reaction does occur it can present in a similar way to sweet itch.
The neck threadworm is often confused with sweet itch because skin reactions are found in the same area. Clinical signs of neck threadworm include:
Skin reactions can actually be exacerbated by deworming, a mass of dying microfilariae can cause a localized reaction and may appear as large lumps on the skin in this areas. It may be difficult to determine whether the condition is sweet itch or neck threadworm given that the clinical signs are remarkably similar. Should the signs resolve completely after deworming with Ivermectin and Moxidectin, it is likely to the threadworm. If the signs remain, the horse is likely to be suffering with sweet itch. The neck threadworm is not to be confused with the threadworm. This parasite, Strongyloides Westeri, primarily infects foals and has an intestinal lifecycle that involves eggs being passed in the horse's feces.
Monitoring for a reaction after deworming was once the only definitive way to diagnose the neck threadworm parasite, given the difficulty veterinarians faced in locating the microfilariae. A newer diagnostic method involves the veterinarian taking a skin biopsy, placing it in warm saline and viewing it under a microscope to see if swimming microfilariae are visible
Due to the problems inherent in diagnosing neck threadworm, there has not been significant research into this parasite. A comprehensive deworming program that includes either Ivermectin or Moxidectin will help get rid of the adult parasites and therefore prevent the shedding stage. By following the manufacturer’s recommendations for periods between deworming, it is likely the horse will be kept free from neck threadworm.
Image courtesy of Dollar Photo Club
© Spalding Laboratories. All Rights Reserved.