Ringworm (Dermatophytosis)

Ringworm is caused by infection with a fungus know as a Dermatophyte.   Dermatophyes are fungi with affinity for cornified epidermis and adnexal structures (keratin) which include hair, nails, flaky skin etc. Most dermatophytes have to live on a host as a parasite; some however can be found in keratin-rich soil (such as around animal burrows) or in other organically rich soils. Some dermatophytes live mostly on animals (zoophilic, others live mostly in the soil (geophilic) while others prefer humans (anthrophilic) .

Zoophilic (attracted to animals) species important in small animal dermatology include:

Anthropophilic (those attracted to people) species (e.g., T. rubrum) uncommonly infect dogs and cats. "Reverse zoonosis" is possible. The geophilic (live in soil) species of importance is Microsporum gypseum. In dogs: 70% of infections are caused by M. canis, 20% by M. gypseum, and 10% by T. mentag. In cats, 90% or more of clinical infections are due to M. canis, depending on geography.

Microsporum canis is an important zoonotic (able to spread from animal to human) agent. It is shown that in a household with an infected cat, more than 60% of household members are likely to develop lesions. Young and immunosuppressed people are at greatest risk for infection and at risk for acquiring more serious cases. It is a cause of scalp ringworm in children and causes very itchy and uncomfortable lesions in infected persons. For HIV-infected people, M. canis can cause a very widespread and difficult to treat infection. Allergic persons are at greater risk. Along with T. verrucosum, M. canis is the cause of most occupational dermatomycoses amongst animal health workers.

How does Ringworm infection occur?

Young, immunosuppressed, debilitated, or previously unexposed animals are more susceptible to ringworm infection. This implies the development of protective immunity can occur after infection or exposure. Natural infection is acquired by contact of susceptible skin by infective spores. Damaged or abraded skin is more susceptible. Grooming in adult cats confers a natural protection after exposure; this is one reason young kittens are more susceptible after weaning.

Ringworm fungi reproduce on the hair and skin scale. They form infective spores called “arthrospores” which remain infective for several months after shedding from the host. Fomites (blankets, brushes, clippers, cages, air filters) are important as sources of infection. Also animal caretakers can spread it in multiple animal facilities.

Chronic inapparent carrier states often develop in longhaired cats, especially those from multi-cat households. Some of these only carry the organism transiently as a contaminant and may not be truly infected.

Geophilic infections are acquired by rooting or digging in contaminated soil.  In these infections,the feet and muzzle are most often affected.

Course Of Infection

In animals, dermatophytes invade hair follicles and hair shafts. After contact with the skin, spores germinate and hyphae grow into hair follicles. Affected telogen hairs (hairs at the end stage of development) break up and fall out. When the fungus invades the shaft of a growing hair (anagen) it grows down to the level of active hair formation. Thereafter, the fungus continues to grow in the hair as the hair grows. This is one reason why kittens are more often widely infected and persistently infected.

Depending on the species of fungus and the host, inflammation can be severe or subclinical (hardly noticeable). The primary lesion is a red swelling around the infected hairshaft.  Inflammation or termination of anagen eventually eliminates the fungus from individual follicles. Meanwhile adjacent follicles are invaded, starting the cycle again. This pattern produces the classical circular spreading lesion with central scaling.

Secondary changes due to follicle inflammation and hair shaft invasion include breakage of infected hairs, hyperkeratosis (thickening of the skin), and itchiness. Most lesions will exhibit loss of hair and scaling. .

In most healthy hosts, dermatophyte infections are self-limiting, as inflammation and specific immunity will limit the establishment of infection in new hair follicles. Patients that do not eliminate ringworm infections readily include: Long-haired cats, Immunosuppressed animals, Those with Trichophyton infections.

Symptoms of dermatophytosis  include:

How do we Diagnosis Dermatophytosis?

Clinical signs or history cannot diagnose dermatophytosis. In order to make a firm diagnosis, the fungus must be shown to be present on the pet.  This is done by any combination of the following tests:

Direct exam of hairs and scales: Examination of carefully plucked hair and scale from suspect lesions for the presence of fungal hyphae and arthrospores. If the fungal arthrospores are seen, the diagnosis is made, however, negative direct exams are inconclusive.

Wood's Lamp: Ultraviolet light will cause some fungi to emit fluorescent light.. M. canis is the only common animal dermatophyte that will produce fluorescence on infected hairs. Only about half of culture-proven M. canis infections cause fluorescence; thus a negative exam proves nothing. False positive exams may be made with build-up of medication or sebum on hair shafts..

Fungal culture:In a fungal culture, samples of the affected hair are placed on a fungal growth media in an attempt to grow the fungus. If the fungus grows, it can be identified under a microscope. This test not only allows us to tell if the fungus is present, but also allows us to identify which type of fungus it is .

How do we treat dermatophyte infection?

Our goals in therapy are to eliminate the infection from patient and to prevent further spread of the infection to other animals or people. 

Topical Therapy: Generally, we will use a topical agent on the pet to help reduce the spread of infective hair and scale.  Usually an anti-fungal shampoo or dip will be prescribed.  Twice weekly bathing and/or dipping is generally indicated for a period of 6-8 weeks. Clipping of the fur from affected areas or even whole body clipping is recommended to help prevent spread of the disease.

Lime Sulfa Dips are recommended twice a week and can be performed either by the hospital or at home. If you attempt this kind of dipping at home, you should expect:

·        Lime sulfur will stain clothing and jewelry

• Lime sulfur will cause temporary yellowing of white fur
• Lime sulfur smells strongly of rotten eggs.

The dip is mixed according the label and is not rinsed off at the end of the bath. The pet should be towel dried. Shampooing is not necessary.

Systemic therapy: Oral anti-fungal medication is often prescribed to kill the growing fungus from within the body.  Oral drugs are usually given for a period of 6 to 8 weeks. Like any drug, anti-fungals can have side effects that may require monitoring.  Grieseofulvin (fulvicin) and Intraconazole are the most common drugs prescribed.

Environmental Management:   Since the infected hairs and scale remain a threat in the environment, we must do everything possible to eliminate them from the environment.  The following methods are recommended: