Chapter 42 Dermatophytosis
Dermatophytosis is an infection of keratinized tissues usually caused by dermatophytes of the genera Microsporum, Trichophyton, and Epidermophyton. These organisms are keratinophilic and invade and live within the keratinized hair, nail, or skin. The majority of infections in dogs and cats are caused by three species of dermatophytes: Microsporum canis, Microsporum gypseum, and Trichophyton mentagrophytes. Other fungi are uncommon causes of dermatophytosis in pets. Dermatophytes are classified into groups based on their natural habitat as geophilic, zoophilic, or anthropophilic. Geophilic dermatophytes naturally inhabit the soil, zoophilic species are adapted to animals, and humans are the hosts for anthropophilic species. As a general rule, geophilic and anthropophilic dermatophytes tend to produce many more inflammatory lesions in animals than do the more host-adapted species.
M. gypseum is a geophilic dermatophyte that normally inhabits the soil and decomposes keratinaceous debris. However, this organism is the second most common cause of dermatophytosis in dogs in the United States, and it occasionally infects cats. M. gypseum is most commonly isolated from animals that spend much time outdoors. Because the organism is not specifically adapted to living on animals, it tends to incite inflammation. Lesions are commonly seen in areas with significant soil contact, such as the feet and muzzle.
This zoophilic dermatophyte is responsible for the majority of the clinical cases of dermatophytosis in dogs. M. canis is the cause of approximately 98% of the cases of feline dermatophytosis. It was previously thought that cats served as the reservoir for M. canis; however, studies show that it is rarely isolated from healthy pet cats. The isolation of M. canis from a dog or cat is a significant finding, and the condition requires treatment.
This zoophilic dermatophyte is the third most common cause of dermatophytosis in dogs and less commonly affects cats. T. mentagrophytes is the most common cause of dermatophytosis in rodents and rabbits. Pet rodents or rabbits should be considered possible reservoirs of infection. Wild rodents are commonly infected, and the infections may be clinically inapparent. Cats that hunt may be pre-disposed to acquiring this dermatophyte infection, as are dogs that dig and root in soil.
Anthropophilic dermatophytes such as Microsporum audouinii rarely affect pets but can cause intensely inflamed lesions in animals. Zoophilic species of dermatophytes other than M. canis and T. mentagrophytes, such as Trichophyton equinum, Trichophyton verrucosum, and Microsporum nanum, may cause dermatophytosis in dogs or cats; however, such animals usually are in contact with livestock that are natural reservoirs or hosts for those organisms.
A kerion is a round, raised, well-circumscribed, erythematous, alopecic, nodular lesion that results when follicular rupture, furunculosis, and pyogranulomatous inflammation occur with a dermatophyte infection. The lesions have a spongy feel on palpation and will sometimes exude purulent to hemorrhagic material that may result in crust formation.
Generalized dermatophytosis is uncommon in the dog.
Feline dermatophytosis can occur in many different clinical forms. Consider dermatophytosis in the differential diagnosis of most feline dermatoses. Feline dermatophytosis is the most common infectious skin disease in cats.
Cats of any age may be infected, but younger, older, and long-haired cats seem to be affected more frequently. Systemic disease may increase susceptibility to infection. Cats with feline immunodeficiency virus (FIV) have an increased risk of dermatophytosis.
Ringworm lesions may be present in cats; however, the circular lesions of alopecia and scaling with central healing are less common in cats than in dogs. Crusting, scaling, and hair loss are most commonly seen. Cats may have what appears to be a localized infection when in reality the infection is generalized. This is especially true of long-haired cats.
Adult cats may have subclinical dermatophyte infections. These cats may have minimal (e.g., a minor degree of scaling or a few broken hairs) or no apparent clinical lesions. These cats serve as an important reservoir in the spread of dermatophytosis, and culture is necessary to identify affected cats.
In cats, dermatophytosis may occur as a miliary dermatitis that may or may not be pruritic (see Chapter 53).
Symmetrical alopecia may be caused by dermatophytosis in cats. Excessive grooming as a result of pruritus, combined with follicular inflammation, can lead to excessive hair loss in some cats.
False-positive diagnosis of dermatophytosis in animals with circular lesions of alopecia is common. Follicular infections with Demodex and Staphylococcus organisms produce similar lesions.
Table 42-1 DIFFERENTIAL DIAGNOSIS FOR DERMATOPHYTOSIS IN DOGS
Focal to Multifocal Dermatophytosis
The list of disorders to be considered in the differential diagnosis of feline dermatophytosis is extensive and includes all causes of miliary dermatitis and symmetrical alopecia (see Chapters 52 and 53).