Dermatophytosis is a condition of dogs and cats (or people, or other animals) having skin, nails or hair infected by certain fungi.
The condition is commonly called by the ancient term “ringworm,” because, centuries ago, people thought there was a worm living in the circular lesions the fungus sometimes produces. There is no worm associated with ringworm infection.
“Dermato” refers to the skin and its adnexa. “Phyto” means plant, and fungi may be broadly categorized as plant material. “Osis” refers to an abnormal or diseased condition.
Dermatophytes cannot live in healthy skin. They thrive in and feed on dead surface skin cells and hair, which is a fairly inactive structure which shares little or no body defense mechanisms.
Skin fungi fall into a number of categories, but there are three main ones that affect dogs and cats.
Trichophyton mentagrophytes is a soil-based organism, but is right at home on your dog, your cat, or your self. It is likely to affect animals whose skin and/or hair is stressed by disease or lesions. Light abrasion would create good conditions for a skin-infecting fungus to invade. Likewise, if the body’s immune system is compromised the organism could more easily effect entry. If a pet with an abrasion lay in a grassless area of the yard where T. mentag was growing, that pet could easily become infected.
Microsporum gypseum is another fungus that can infect mammals, but it is more geophilic (soil-loving) than T. mentag. Similar conditions would be conducive to infecting your pet as described above.
Microsporum canis, on the other hand, is obligated to live on animals. This is the fungus you or your child is most likely to get from cuddling with a new pet in the household, at the pet shop or in a neighbor’s home.
Fungi can also be transmitted by fomites, such as combs, brushes, collars, harnesses and leashes. Non-porous fomites may be disinfected with a 1:9 dilution of household chlorine bleach and water. Porous items, such as nylon-webbed collars, should be discarded if they are suspected to be carrying dermatophytes.
Veterinarians test for dermatophytes with, what else, dermatophyte test medium, or DTM. DTM is agar infused with nutrients dermatophytes like, so they grow quickly when infected hair or scales of dead skin are applied to the culture. In addition, DTM uses a color indicator, phenol red, to indicate the growth of dermatophytes, as well as some other fungi. When dermatophytic fungi metabolize, they produce an alkaline (elevated) pH. Phenol red, then, responds by changing the color of the culture medium from its natural yellow to pink, then red as these metabolites appear. While this is supposed to be diagnostic of dermatophytes, it’s actually diagnostic only of a pH change. If, for example, a dog had some high-pH product applied to it right before the culture was taken, and the person preparing the sample didn’t clean it first, a false pH change could occur.
Far more accurate for diagnosis is microscopic evaluation of the culture. When a fungus begins to grow on a DTM, a sample can be placed on a microscope slide, then stained, and examined under low to medium power. Having studied the structures of fungi, your pet’s doctor can then determine whether he has grown M. gypseum, M. canis, T. mentagrophytes, some other dermatophyte, or a saprophyte. Saprophytes are non-pathogenic fungi which are simply on the skin, consuming dead skin cells, hair and debris.
Treatment is aimed at removing the fungus from the body. Small, localized lesions may be treated locally, using an ointment. Widespread fungal infection can be treated with special dips, most of which are harmless to the patient but smell perfectly terrible. Systemic treatment is often used when large areas of the body are affected, but have the disadvantage of working from the inside out. As these fungi are zoonotic, the fungus on the outside continues to be able to infect family members, visitors and other pets until the medication reaches the outer layers of the body. This usually takes about 30 days. Therefore, pets administered oral medications for fungal infections of the skin usually have some form of topical treatment also.
Rare is the case that cannot be cured. Treatment may be long and difficult, but it pays off in time.