What is Parasitic Worms of the Skin?
Two of the main parasitic worms affecting the skin of the horse are Onchocerca cervicalis worms and the larvae of the spirurid stomach worms. These worms can cause trauma to the skin, itching, and non-healing lesions to develop. Management of these parasites is essential to reduce damage caused by their life cycle and improve your horse’s quality of life. If your horse is showing poor coat or skin health and you are concerned parasitic worms may be the cause it is essential you contact your veterinarian.
There are many parasites that affect horses, in fact, horses are hosts for over 150 parasites including both external and internal parasites. These include skin worms such as the spirurid stomach worm larvae or adult Onchocerca cervicalis worm which often cause no symptoms to develop in horses. Parasites may be managed through regular deworming treatments, provision of good nutrition, pasture and environmental management. In cases of poor parasite control symptoms such as itching, inflammation, and damage to the skin may occur.
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Symptoms of Parasitic Worms of the Skin in Horses
Often horses with parasitic worms of the skin have no symptoms. However, in severe infestations symptoms may develop.
Onchocerca cervicalis worms
In horses affected by these worms, the microfilariae on the chest and abdomen may cause dermatitis on the face, neck, chest, withers, forelegs, and abdomen. This is often due to dead or dying microfilariae and may worsen following treatment. In some cases, these lesions may become infected and pruritic leading to crusting, ulcers, hair loss and depigmentation.
- Hair loss
The larvae of the spirurid stomach worms cause non-healing lesions to develop. If your horse is suffering from this condition you may notice reddish brown, skin granulomas. These are often very distinctive and may contain calcified material that has the appearance of yellow rice grains.
- Skin granulomas
Causes of Parasitic Worms of the Skin in Horses
There are many parasites that affect horses, however the main skin worms seen are onchocerca cervicalis worms and spirurid stomach worms which cause cutaneous habronemiasis.
Onchocerca cervicalis worms live in the large nuchal ligament in the neck of the horse. These worms may be several inches long, however despite their length often cause no symptoms in horses. In some cases, mineralization in the nuchal ligament may be seen in radiograph investigations.
Once the worms are present in the horse, the adult worms release immature microfilariae. These concentrate along the bottom of the horse, on the chest and abdomen where black flies are drawn. These flies then spread the microfilariae.
Cutaneous habronemiasis is a skin disease caused by spirurid stomach worms. This occurs when the larvae emerge from flies, and feed on the skin, open wounds, or the secretions of the genitalia or eyes of the horse. The larvae cause damage as they migrate into the tissues.
Diagnosis of Parasitic Worms of the Skin in Horses
Your veterinarian will perform a full clinical examination on your horse, and discuss recent behavior and medical history with you. As many horses have infestations of equine neck threadworms with no symptoms, a diagnosis may be difficult for your veterinarian to make. Although infection may be confirmed, your veterinarian may be unable to diagnose the infection as the cause for symptoms until possible response to microfilaricidal treatment and resulting reduction of symptoms is seen.
For horses suffering from cutaneous habronemiasis the diagnosis is often made due to distinctive, greasy skin granulomas that are often reddish-brown and contain small, yellow calcified material the size of rice grains. These are often non-healing. Your vet may also perform a skin scraping of the lesion, where larvae may be seen under microscopic investigation to further confirm diagnosis.
Treatment of Parasitic Worms of the Skin in Horses
Unfortunately, there is no effective treatment for the adult worms. However, ivermectin and moxidectin have shown to be effective against microfilariae and reduce symptoms in horses who are suffering from onchocercal dermatitis. In some cases, worsening of symptoms or ventral midline swelling may be seen 1–3 days following treatment.
Symptomatic treatment may be beneficial for horses suffering from either worm infection, such as insect repellents and organophosphates applied topically to the lesions.
Your veterinarian will likely advise treatment with ivermectin. Although this has shown to be effective there may be a temporary worsening of symptoms while the larvae die. Moxidectin may also be used to kill the adult Habronema spp in the stomach. Further surgical treatment may also be required to remove excess granulated tissue.
Recovery of Parasitic Worms of the Skin in Horses
Unfortunately, the adult worms are extremely difficult to remove; however, with proper parasite management the population should be controlled to prevent symptoms from developing in your horse. A thorough worming schedule, supportive diet, and environmental management program should be discussed with your veterinarian.
Environmental precautions should be taken to control fly presence and reduce incidences, such as regular mucking out, and collection and removal of manure from the property.
Parasitic Worms of the Skin Questions and Advice from Veterinary Professionals
I have two horses living in Spain and both itch terribly and one, Lucia is very underweight even though she eats plenty and gets as much hay as she will eat. Their coats are poor. Neither of them go off the land or mix with other horses as they are retired and I have not followed a regular worming routine. One year ago I had a worm count done and my vet told me that they had 0 worm count but were infected with Coccidia. He gave me an antibiotic - but with absolutely no visible change in the horses' condition. Lucia also suffers from pain in the gut and her flank trembles regularly. Once she had a kind of fit where she lay on the floor shaking violently. The vet didn't know what the cause was. I now want to worm them and I want to be sure to choose the right wormer to cover all possibilities that could be causing such mal-condition (maybe the fit had nothing to do with it but their general poor state...). They rub continually and have staring coats. Could you please advise which worm could be causing this and which de-wormer I should use? They haven't been wormed since the last worm count. BTW Is it really possible to have 0 worm count? And could the Coccidia still be present and causing this? Thank you very much for your answer, Soraya
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my horse has lesions on the pastern and some going up the front legs. Has been treated with cream for scratches but has not helped. Also had some hairless spots on the nose. Could this be from stomach worms?
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My horse has 2 Fairly large hairless patches on either side of her spine in the center of her back that are very painful to the touch! I was treating it with fungus cream and it hasn't seemed to help!Could this be some kind of worm?
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