Dermatophytosis

Chapter 42 Dermatophytosis



Karin Muth Beale


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.



ETIOLOGY



Microsporum Gypseum


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.



Microsporum Canis


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.



Trichophyton Mentagrophytes


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


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.



CLINICAL SIGNS



Canine Dermatophytosis



• Canine dermatophytosis is characterized by alopecia and scaling. Usually there are focal to multifocal, circumscribed, affected patches of skin.

• Hair loss, broken hairs, scaling, pustules, papules, exudation, crusting, and hyperpigmentation may be seen.

• Pruritus is variable.

• The classic lesion is a circular area of alopecia and scaling with central healing; however, the lesions may be irregular in appearance.

• Several factors influence the severity of the lesions. Young and immunocompromised animals tend to develop more extensive lesions that take longer to resolve than do those of healthy adult animals. This is because the ability to mount an effective inflammatory response is necessary to eliminate the infection. In addition, the pathogenicity and the species of dermatophyte affect the degree of inflammatory response.


Kerion


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.


• Kerions are most commonly seen on the limbs and face of dogs. They are usually solitary and mimic histiocytomas.

• The etiology usually is M. gypseum.

• These lesions may be secondarily infected with Staphylococcus intermedius.


Generalized Dermatophytosis


Generalized dermatophytosis is uncommon in the dog.


• Characteristic findings are widespread alopecia and seborrhea with or without pruritus.

• Other lesions, similar to those of focal dermatophytosis, may also be present.

• Generalized dermatophytosis in adult dogs is often associated with immunosuppression or systemic disease.


Feline Dermatophytosis




Key Point


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.




Key Point


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.



Classic Ringworm


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.



Subclinical Infection


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.



Miliary Dermatitis


In cats, dermatophytosis may occur as a miliary dermatitis that may or may not be pruritic (see Chapter 53).



Symmetrical Alopecia


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.



Pseudomycetoma



• Pseudomycetoma (Majocchi granuloma) is a form of granulomatous dermatitis in the cat caused by a dermatophyte, usually M. canis.

• This type of lesion is almost exclusively seen in Persian cats.

• In this lesion the fungus is sequestered in the dermis but does not proliferate; a nodular, ulcerating dermatitis is the result.

• Affected cats usually have generalized dermatophytosis.


Onychomycosis



• This dermatophyte infection of the nails is seen in both dogs and cats (also see Chapter 63).

• The lesions usually are caused by T. mentagrophytes in dogs and M. canis in cats.

• The nails are dry, cracked, brittle, and frequently deformed.

• Onychomycosis very rarely affects all four feet.

• Usually there is a concurrent infection with inflammation of the nail fold, and the pads may be affected as well.


DIAGNOSIS




Key Point


False-positive diagnosis of dermatophytosis in animals with circular lesions of alopecia is common. Follicular infections with Demodex and Staphylococcus organisms produce similar lesions.



• Do not diagnose dermatophytosis solely on the basis of clinical signs. Because of the numerous clinical conditions in dogs and cats that can mimic dermatophytosis, perform specific diagnostic procedures to obtain a definitive diagnosis.


Differential Diagnoses



Canine Dermatophytosis



• In the dog, the most common cause of circular alopecia is staphylococcal folliculitis (see Chapter 38), followed by localized demodicosis (see Chapter 43).

• Follicular infections with different organisms share common reaction patterns of papules, pustules, alopecia, scaling, and crusting.

• Use diagnostic tests such as skin scrapings, surface cytology, bacterial cultures, and skin biopsies to rule out some of the more common causes of focal alopecia in dogs.

• Consider dermatophytosis in the differential diagnosis of a generalized scaling, crusting, alopecic dermatosis. A number of different conditions can cause a generalized dermatitis similar to that seen in generalized dermatophytosis in the dog (Table 42-1).

• The differential diagnosis for kerions and pseudomycetomas includes the various causes of nodular dermatitides, such as cutaneous neoplasms, staphylococcal furunculosis, and acral lick dermatitis (see Table 42-1).


Table 42-1 DIFFERENTIAL DIAGNOSIS FOR DERMATOPHYTOSIS IN DOGS



Focal to Multifocal Dermatophytosis



Demodicosis

Staphylococcal folliculitis

Abrasions

Pemphigus foliaceus/erythematosus

Zinc-responsive dermatosis


Generalized Dermatophytosis



Demodicosis

Staphylococcal folliculitis

Dermatophilosis

Pemphigus foliaceus

Cheyletiellosis

Sebaceous adenitis

Primary cornification defects

Secondary cornification abnormalities (allergic dermatoses, endocrinopathies, nutritional deficiencies, etc.)

Epidermotropic lymphoma


Kerions



Histiocytoma, mast cell tumor, other neoplasms

Staphylococcal furunculosis

Demodicosis and furunculosis

Acral lick dermatitis

Foreign bodies

Subcutaneous mycoses

Actinomycotic infections

Mycobacterial infections


Onychomycosis/Nail Fold Dermatophytosis



Staphylococcal onychitis

Demodicosis

Zinc-responsive dermatosis

Pemphigus vulgaris/foliaceus

Symmetric lupoid onychodystrophy


Feline 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).

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Dermatophytosis
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