Some parasitic worms, particularly nematodes (roundworms), can cause diseases of the skin in horses and other animals. The most common are discussed below.
Cutaneous Habronemiasis (Summer Sores)
Cutaneous habronemiasis is a skin disease of horses caused by the larvae of spirurid stomach worms (see Digestive Disorders of Horses: Gastrointestinal Parasites of Horses). The larvae move from flies feeding on preexisting wounds or on moisture of the genitalia or eyes, into the skin of a horse. When in the skin, they cause irritation and wounds. These wounds generally occur during the summer and are commonly called summer sores. The wounds are reddish brown, greasy areas of skin that contain yellow, calcified material the size of rice grains. Healing is slow. Diagnosis is by skin biopsy. Larvae, recognized by spiny knobs on their tails, can sometimes be found in scrapings of the boils.
Many different treatments have been used, most with poor results. Use of insect repellents may help, and organophosphates rubbed onto the area of the wound may kill the larvae. Some products (such as those containing ivermectin or moxidectin) with broad-spectrum activity against parasites have been shown to be effective. Surgical removal or cauterization of the excessive granulation tissue may be necessary. Control of the fly hosts and regular collection and stacking of manure may reduce the incidence of the disease. As with many other diseases, good sanitation practices significantly reduce the number of cases of cutaneous habronemiasis and can go a long way toward protecting your horse.
Equine onchocerciasis is a disease caused by a parasitic worm and transmitted by biting midges (see Skin Disorders of Horses: Biting Midges (Gnats, No-see-ums)). It causes skin lumps and crusty dry patches that are irritating. The life cycle of the worm begins when a midge takes a blood meal containing the larvae of the worm. Larvae develop to the infective stage in the fly and then pass into the horse host when the flies feed on other horses. The larvae migrate to the connective tissues of the neck, where they mature into adult worms over a period of 1 to 3 months. After the worms have matured they mate. Female worms produce eggs, which hatch to produce new larvae. The larvae are then sucked up by a biting midge and the life cycle continues. Adult worms are very thin and 1.2 to 23.5 inches (3 to 60 centimeters) long. The larvae are tiny, only 0.008 inches (0.2 millimeters) long.
Adult worms live in the nuchal ligament (the large, powerful ligament in the neck that helps support the head). They cause inflammation which can lead to hardened lumps. These lumps are more common in older horses. Large numbers of larvae can cause skin inflammation of the face, neck, chest, withers, forelegs, and abdomen. Signs often include areas of scaling, crusts, ulcers, hair loss, and color loss. There may also be itching. Larvae can also accumulate in the eyes of horses.
The most effective method of diagnosis is by skin biopsy. Allergic reactions to the bites of flies can cause similar signs. Therefore, diagnosis of onchocerciasis may be based on laboratory tests and a positive response to antiworm treatment.
No treatment is effective against the adult worms. Drugs with a broad spectrum of activity against worms and other parasites (such as those containing ivermectin or moxidectin) are very effective against larvae. A small portion of horses infected react to the treatment with a noticeable swelling of affected areas 1 to 3 days after treatment. These conditions may resolve on their own; however, treatment of the signs may be necessary for the comfort of your horse.
Parafilaria multipapillosa Infestation (Summer Bleeding)
Parasitic worms known as Parafilaria multipapillosa infest the tissue just beneath the skin of horses in various parts of the world. They are especially common in the Russian steppes and Eastern Europe. In the spring and summer, the worms cause skin nodules primarily on the head and upper forequarters. Bleeding from the nodules is seen periodically and may be heavy. This condition is sometimes called summer bleeding. As the parasite moves under the skin, new nodules develop. Occasionally, the nodules will accumulate or discharge pus. The nodules are unique in formation and their presence is a clear sign of this disease. Both the nodules and the bleeding are unsightly and may interfere with the harnessing of working horses. Otherwise, they do not seem to bother the animal. No satisfactory treatment is available, but fly control may reduce the incidence.
Pelodera dermatitis is a rare skin worm infestation that causes a sudden, serious skin infection. The condition is caused when larvae of the roundworm Pelodera strongyloides invade the skin. These larvae are widespread in decaying organic matter (such as damp hay) and on or near the surface of moist soil. They are only occasionally parasitic. In most cases, animals are exposed to the larvae through direct contact with infested materials, such as damp, filthy bedding. Animals with healthy skin are not usually at risk of infection.
The sores usually only appear on parts of the body that contact the infested material, such as the legs, groin, abdomen, and chest. The affected skin is red and partially or completely hairless. In addition, there may be bumps in the skin, or lumps filled with pus, crusts, or ulcers. Often—though not always—there is severe itching, causing the animal to scratch, bite, or rub the infected area.
Veterinarians can usually make a definitive diagnosis by examining a skin scraping under a microscope to check for worm larvae. Animals with Pelodera dermatitis can be treated in the same manner as those with other skin worm infestations. In many cases, simply moving the animal to a dry area with clean bedding will lead to recovery.
Last full review/revision July 2011 by Karen A. Moriello, DVM, DACVD; Carol S. Foil, DVM, MS, DACVD; John E. Lloyd, BS, PhD; Bertrand J. Losson, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice Villalobos, DVM, DPNAP; Patricia D. White, DVM, MS, DACVD; Thomas R. Klei, PhD; David Stiller, MS, PhD; Stephen D. White, DVM, DACVD