The three dermatophytes that are the most common causes of feline ringworm (dermatophytosis) can be divided to two groups depending on where they normally grow. Microsporum gypseum is a geophilic species because it normally grows in soil. Under the appropriate conditions it can cause disease in cats, other animals, and humans. The fungus can be transmitted from one animal to another. Microsporum canis is a zoophilic dermatophyte because it is typically associated with cats and dogs. It may cause disease in humans, especially children who have an infected pet. Trichophyton mentagrophytes may be either zoophilic or anthropophilic. In animals such as cats, T. mentagrophytes is zoophilic. Like the two other dermatophytes, it can be transmitted between animals. Unlike the other two dermatophytes, it will not grow in soil. It can survive in skin flakes for extended periods of time. Some strains of this fungus prefer to grow on humans and are referred to as anthropophilic, that is, people loving.

The diagnosis of dermatophyte infection is based upon examination of the cats looking for clinical symptoms, direct examination of hair and skin using a microscope, and if necessary isolation of the dermatophyte on a culture medium. As a component of the clinical examination, examination of the animal's hair with a Woods Light looking for fluorscening hair is appropriate. It is important to allow your eyes to adjust to the darkness prior to examining the hair with a Woods Light. Older Woods lights may also require a short period of time for the output of the proper wavelength of light. Not all dermatophytes florescence in hair, and not all dermatophytes invade hair. The identification to genus and species of the particular dermatophyte causing disease is done by isolating the fungus on culture media and examining the fungal growth with a microscope.

In the cattery, one is confronted with several problems. The first is the health of the cats. The above-mentioned three species of dermatophytes are easily spread from one animal to the other. The second problem deals with the ability of these three dermatophytes to infect people. People infections traced to a cattery will result in a public health system response and bad press. Management of the problem and protective steps to eliminate the fungi so that no additional infections occur involves three steps.

First, the animals must be treated with the appropriate antifungal agent. Consultation with a veterinarian is a must! All people working with the cats should also be checked for ringworm and treated if they have an infection.

The next step involves cleaning the facility to insure that the dermatophytes are gone so that they can no longer infect cats. When dermatophytes like M. canis grow in the dead outer layers of the skin, they also invade hair and produce large numbers of highly infective conidia (spores) in and around the hair shaft. These fungi may remain viable in hair, and in skin flakes originating from the infected animals. The areas where the animals have been kept should be cleaned with a solution of bleach like Chlorox. This will kill fungi that it contacts.

The final step involves a program to monitor the facility and the animal's health. This would probably include periodic careful examination of the cats to insure that they remain dermatophyte free.

American Veterinary Medical Association
Vetinfo.com's discussion of animal ringworm