Ringworm is a well known fungus that is known to infect dogs, cats and people. There are several different forms of ringworm which can infect both household pets and people alike. The diagnosis and treatment tends to be straightforward for all species, however, some ringworm species that affect dogs can be much more a bit challenging in both aspects. Due to the implications of the consequences of untreated ringworm in their household dogs and cats, as well a possible transmission of ringworm to people, pet owners should be aware of the signs, transmission, and treatment of ringworm.
Several different fungal species found throughout the world can cause ringworm, but the vast majority of cases in dogs are caused by Microsporum canis, Microsporum gypseum, or Trichophyton mentagrophytes; the majority of cases in cats caused by Microsporum cati. The ringworm fungus tends to be most prevalent in hot, humid climates, but oddly, most cases of ringworm occur in the fall and winter. Ringworm is most commonly known to persist on or in the living quarters of infected animals, with spores from infected animals known to shed into the environment and live for up to 18 months or longer. Most healthy dogs and cats do not carry infective fungal spores on their skin or hair. The incidence of ringworm infections in dogs and cats is actually very small, representing approximately only 1%-3% of dogs and cats presenting with active skin problems.
Ringworm is transmitted by direct contact with an infected animal or contact with an item that is contaminated with infective fungal spores. Spores can be on contaminated grooming equipment or brushes, in a contaminated boarding facility or kennel, or in the environment where an infected animal has visited. Cleary, because of the infective spore’s ability to survive for long periods in the environment, dogs and cats can contract ringworm just about any placed where other dogs and/or cats have been. Fortunately, most healthy adult dogs and cats have some innate resistance to the organisms that cause ringworm, and most never develop symptoms from the fungus. Puppies and kittens are most commonly infected, since they have still developing immune systems. Canines and felines with suppressed immune system from other diseases or immune suppression drug therapy, such as steroids, are also more susceptible to contracting ringworm.
The typical ringworm lesion in dogs and cats looks initially like a very round area of hair loss that reveals skin that is red and slightly raised off the normal surrounding skin. Initial lesions range from dime to quarter sized. There is often a moderate degree of itchiness associate with ringworm lesions, but this is sometimes not the
case, especially in cats. As ringworm lesions progress, they can become irregular in shape, as well as grow larger and lead to multiple lesions. In severe cases, ringworm can cover the entire skin and hair coat, an sever and advanced stage of ringworm known as disseminated dermatophytosis.
Ringworm can be diagnosed through several different methods. A popular but not completely accurate way to diagnose the disease is through the use of a specialized black light called a Wood’s lamp. Several species of the ringworm fungus will glow a fluorescent color when exposed to a Wood’s lamp. However, since some species will not fluoresce under the Wood’s lamp and some patient may have spores on their hair coats that will fluoresce but there is in fact no active infection, this technique has limitations.
Another method for identifying ringworm is to pluck and examine hairs on the periphery of the lesion under the microscope using a preparation of KOH (potassium hydroxide solution) to make them more visible. Between 40% and 70% of the infections can be diagnosed this way.
The most consistent and accurate way to identify a ringworm infection is by collecting scales and crust from the affected skin and hair coat and performing a culture. There are special culture mediums designed specifically for identifying ringworm infections. The limitation with culture, however, is that fungus grows slowly, and it can take as long as 2 weeks before fungal infection can be accurately ruled in or ruled out. As such, I typically will perform Wood’s lamp analysis for a quick assessment, as well as submit a fungal culture.
Most small, isolated lesions on healthy dogs and puppies will heal on their own within 4 months. In more advanced cases, there are several different treatment options.. For isolated lesions, the hair adjacent to the affected region should be clipped down as close to the skin as possible, being careful to not to irritate the skin with the clipper, as this may promote spread of the infection. The lesions can then be treated topically twice a day with an antifungal medication. Popular topical creams and sprays have antifungal active ingredients, such as clotrimazole, miconazole, and ketoconazole. My favorite products are ones with clotrimazole, and I prefer spray formulations to creams or ointments, as sprays are less messy and easier to apply on dogs and cats.
In advanced cases where there are multiple or diffuse ringworm lesions, it is necessary to usually combined a good antifungal shampoo with any of the primary antifungal ingredients listed for topical treatment in the previous paragraph, combined with oral antifungal therapy. Traditionally, griseofulvin has been the oral antifungal of choice for advanced ringworm favor, but many vets now favor itrconazole or ketoconazole. I still use griseofulvin, as any minor added benefit of using itraconazole or ketoconazole does not justify their increased cost when compared to griseofulvin.
Roger L. Welton, DVM
Founder and Chief Editor, Web-DVM.net
President, Maybeck Animal Hospital
Article updated 6/3/2014