Dermatophilosis in Cats

Cats develop dermatophilosis primarily from exposure to M. Canis fungus (ringworm). D. congolensis can also cause skin disease to form, although it is seen far more often on larger domestic animals, such as horses. While the infection generally affects the outer layers of the skin, it is possible for abscesses to exist deep in the muscle tissue of the cat. Cats are not the primary hosts of these fungi, but are still very susceptible to infection from them. The strength of the cat's immune system, its grooming habits, age, skin condition, and overall health seem to have an impact on whether or not dermatophilosis will develop. In cases of ringworm, kittens are far more vulnerable to infection than older cats, and the disease is zoonotic (can spread from cats to humans).

Various forms of fungi including Microsporum, Trichophyton and Epidermophyton can infiltrate the skin of mammals and cause an infection to develop. The fungi need a wound or patch of keratinized (hardened and dead skin) tissue to enter into the cutaneous layers. The spores may exist and reproduce on the surface of a healthy animal in wait of optimal conditions for infection to occur. Once inside the skin, these spores will begin to attack the hair follicle and hair shaft until the area becomes inflamed and the hair is lost. This type of fungal infection is called “dermatophilosis”.

While most infected cats will exhibit symptoms, some cases may be hard to notice in their early stages, especially if the cat has long hair. Whether or not symptoms have manifested, the condition is very contagious to other cats and sometimes also to humans, depending on the type of fungal infection that exists. All signs to watch for include: 

• Scaling
• Crusting
• Lesions on the face, ears and in the mouth
• Alopecia (hair loss)
• Oral fistulas draining pus
• Dermatophyte granulomas (growths)
• Fever
• Anorexia
• Enlarged lymph nodes 

The overall health of the cat can greatly affect whether or not a fungal infection of the skin will develop. Exposure to fungal spores does not guarantee that dermatophilosis will form in the cat. All possible causes are listed as follows:

• Contact with an infected animal
• Exposure to infected soil
• Contact with broken-off hairs of an infected animal
• Insect bites from an insect carrying spores on its body

It should also be noted that a weakened immune system, open wounds, and moisture increase the chance of infection. Long-haired breeds may develop more severe infections than those with short hair.

A physical and microscopic examination of the cat will need to be performed by a veterinarian for a diagnosis to be reached. Be sure to provide the cat's full medical history to assist in differentiating a case of dermatophilosis from other skin diseases. The vet may ask about the environment that the cat is allowed in to determine if fungal exposure is likely. Regular blood work, including a complete blood count and a biochemical profile, should be run to get a better idea of the general health of the cat. 

Various microscopic evaluations can be performed to determine which type of fungus is causing skin disease in the cat. Hair samples of at least 0.3 cm in length or skin scrapings of affected areas can be used for these tests. A Wood’s Lamp examination is very useful in identifying cases of M. canis infection in cats. Ultraviolet light exposure will cause infected hairs to turn florescent chartreuse in color. Fungal cultures can be used to identify which fungus is present. Clean samples must be kept in moist, unsealed containers for up to seven days to allow growth to occur. Stains such as a methylene blue or Giemsa stain may be used to find clusters of immune cells called “neutrophils” and thread-like stands of fungi organisms. 

Eradicating fungal skin infections tends to be easier if the cat has short hair. Shaving a long-haired cat may contribute to a faster healing process and also reduces the risk of the fungus spreading to other hosts via broken hairs.

Topical Therapy 

Ointments, creams and other solutions that can be applied directly to the affected area can help remove harmful fungi. Application may need to be done using gloves, as in the case of lime sulfur dip administration. Chlorhexidine, miconazole or other forms of antibiotic shampoo can also effectively cleanse the infected lesions of dermatophilosis.

Antifungal Medication 

An oral prescription of antifungal medication will help treat the infestation from the inside of the body. Low-dose prescriptions of itraconazole, fluconazole or terbinafine given over a long duration of time are generally the most effective. 

Antibiotics 

A prescription of oral antibiotics can also be helpful with the eradication of certain fungal infections. Penicillin is often administered as treatment of dermatophilosis.

In the weeks and months that follow, check-up appointments will be needed to adjust medication dosages if negative side effects have begun to develop. Sometimes administering treatment every other week can be enough of a reduction to stop reactions from occurring. Side effects tend to be worse in cats who are FIV positive, or who suffer from other immune deficiencies. More cultures will need to be collected starting one month after treatment has begun to determine if the cat has been cured or not. Several negative tests are needed for this diagnosis.

To prevent reinfection or the spread of infection to humans or other pets, all areas that the cat has been in should be disinfected. Use of bleach or an enilconazole solution to clean dishes, bedding and any litter boxes should suffice in removing the fungal spores. While a cat is receiving treatment for dermatophilosis, limit its interaction with other pets and family members. After a full course of treatment has been administered, most cats recover completely from the skin disease.