Dog Ringworm Infection
"Ringworm" disease has nothing to do with worms. It is the common name for a fungal infection of the superficial layers of the skin, nails and hair fibers. Also called dermatophytosis, ringworm is highly contagious among animals and can infect humans as well.
Causes of Canine Ringworm
Ringworm is caused by fungal microorganisms, not by worms. The most common species of ringworm fungi in domestic dogs and cats are Microsporum canis, Microsporum gypseum and/or Trichopyton mentagrophytes. All of these fungi thrive in non-living keratinized tissues – outer skin layers, hair follicles and nails. Microsporum gypseum also lives in soil and is more common in the summer and fall. Ringworm is spread by direct physical contact and is extremely contagious. Ringworm infections are more prevalent in hot, humid climates.
Prevention of Ringworm Infection
The best way to prevent dermatophytosis, or ringworm, is to prevent contact between affected and unaffected animals.
While ringworm often resolves on its own (is "self-limiting"), prompt treatment can prevent the spread of infection to other pets, and to people. Veterinary personnel and owners should be cautious when handling dogs with ringworm and preferably should wear gloves when applying topical medication.
Ringworm, also called dermatophytosis, is caused by exposure to one of several forms of environmental fungi. Once the fungi become established on a dog's hair, nails or outer layers of skin, characteristic ringworm signs begin to appear.
Symptoms of Ringworm
This superficial fungal infection causes few if any troublesome symptoms in affected animals. However, ringworm must be managed carefully, because it is highly contagious. When they do appear, signs of ringworm infection in dogs include:
Circular regions of unexplained hair loss (alopecia), with varying degrees of crusting and scaling but usually without itchiness or scratching
Small patchy areas of hair loss
Crusty skin patches
Scaly skin patches
Cutaneous bumps (on the skin)
Irritated, inflamed hair follicles
Pustules on the skin
Skin abscesses (uncommon)
The most characteristic signs of ringworm are areas of elevated skin, caused by inflamed hair follicles, which take a circular or oval shape. Ringworm can occur anywhere on a dog's body, including the face and legs. The infection typically stays in one main area. The patches may grow, but they do not routinely spread to other locations on the body. However, if a dog has an impaired immune system or another underlying medical condition, ringworm is more likely to spread across its skin. Ringworm does not normally cause itchiness or scratching, but it can.
Dogs at Increased Risk
Young puppies have an increased chance of contracting ringworm due to their underdeveloped immune systems. Dogs whose immune status is compromised, such as those in high-density housing facilities and those with cancer, poor nutritional status, parasites or other systemic diseases and those undergoing chemotherapy, are also predisposed to ringworm infection. Ringworm is highly contagious between dogs and can infect people as well.
Ringworm in domestic dogs is probably over-diagnosed, especially when a presumptive diagnosis is made based only on observable signs. However, using one or more of several readily available in-house tests, most veterinarians will come up with an accurate assessment of the dog's condition.
How Ringworm is Diagnosed
When presented with a dog showing symptoms of circular, patchy hair loss, a veterinarian will first take a thorough history and perform a complete physical examination. There are several more advance diagnostic tools available to confirm a diagnosis of ringworm. One of those tools is a trichogram, which involves plucking hairs from the edge of one of the patchy lesions and immersing them in oil or potassium hydroxide mixed with Indian ink to look for evidence of fungal spores. Alternatively, the area of hair loss can be examined under an instrument called a Wood's lamp, which emits a certain wavelength of ultraviolet light that causes roughly 50% of the strains of Microsporum canis to fluoresce an apple-green color. Unfortunately, a number of other substances also fluoresce under a Wood's lamp, which can cause false positive test results.
Fungi can also be cultured (grown in a laboratory) using specific dermatophyte (fungal) test media that change color in the presence of ringworm growth. Samples can also be examined microscopically, using special stains to identify the presence of fungal organisms.
Veterinary and laboratory personnel should use caution when taking and assessing samples from suspected ringworm lesions, to prevent the chance of contagion. Owners should also use extreme care if they suspect ringworm and should avoid touching circular or oval areas of patchy hair loss on their dogs without wearing rubber or latex gloves.
Ringworm can be treated once the causative organism is identified. The goals of treatment are to eliminate the fungal organisms and restore the dog's skin and haircoat to their normal condition.
Treatment options for ringworm include application of topical medications, administration of oral drugs or a combination of both. Affected areas should be clipped to remove any remaining hair. In severe cases, the entire dog may need to be shaved. Isolated lesions can be spot-treated with anti-fungal creams prescribed by a veterinarian. It usually is also recommended to treat the animal with twice weekly disinfectant shampoos, followed by an antifungal rinse. Oral antifungal drugs are widely available, including griseofulvin, ketoconazole, itraconazole, lufenuron and terbinafine, among others. Each of these medications has its own risk of adverse side effects, some of which can be quite severe. Treatment normally must continue for several months, until the dog has two negative fungal cultures.
The dog's living environment should be thoroughly cleaned, vacuumed and disinfected. Bedding, rugs and other salvageable items should be washed in a bleach solution. A Microsporum canis vaccination may be available but is rarely effective in preventing ringworm outbreaks. However, it may be useful as an adjunct to oral and topical treatment in severe cases.
The prognosis for dogs with dermatophytosis is generally quite good. Unfortunately, dogs with compromised immune function have a more guarded outlook. People with immunodeficiency disorders should be extremely careful around animals with ringworm infection.