A parasite is any living thing that lives in, on, or with another living thing (known as a host) and that depends on the host for its food and shelter. Some parasites depend on a host for their entire life while other parasites depend on a host only during a part of their life. Many worms are parasites that infect dogs, cats, horses, and other animals. Some of these worms may also infect people. The following section describes parasitic worms that affect the skin of pets.

Dracunculus Infection

Dracunculus insignis is a species of roundworm found mainly in the connective tissue beneath the skin of the legs. They are known to infect raccoons, minks, and other animals, including dogs, in North America. Female worms can reach 10 feet (3 meters) in length. Male worms are tiny in comparison, around 0.6 inches (20 millimeters) long. These worms can produce skin ulcers on their hosts. When the ulcers touch water, the worms stick their heads out of the wounds in order to lay their long, thin-tailed larvae. The larvae then develop inside of another host, the water flea. Dogs can also become infected when they drink contaminated water or eat another host, such as a frog.

Signs of D. insignis worm infestation include snake-like, swollen tracks under the skin and crater-like red ulcers on the skin's surface. These infections are rare, but have occasionally been found in dogs that have been around small lakes and shallow, stagnant water.

Veterinarians treat the infection by carefully and slowly extracting the parasites. Antiparasitic drugs of the miridazole or benzimidazole classes can also be useful (see Drugs and Vaccines: Anthelmintics).

In parts of Africa, Asia, and the Middle East, a guinea worm (Dracunculus medinensis) is a well-known parasite of humans that may also infest dogs and other animals.

Pelodera Dermatitis

Pelodera dermatitis is a rare skin worm infestation that causes a short-term skin infection. The condition is caused when larvae of roundworms known as 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, 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 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; Patricia D. White, DVM, MS, DACVD; Michael W. Dryden, DVM, PhD; Carol S. Foil, DVM, MS, DACVD; William W. Hawkins, BS, DVM; Thomas R. Klei, PhD; John E. Lloyd, BS, PhD; Bernard Mignon, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; David Stiller, MS, PhD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice Villalobos, DVM, DPNAP; Stephen D. White, DVM, DACVD