Dermatophytosis ("ringworm")

Ringworm is a cutaneous infection caused by a variety of different genera of fungi called the "dermatophytes." The name ringworm comes from the characteristic circular red lesions that occur commonly in people.

• Ringworm lesions in animals may be much less circumscribed, even presenting as a diffuse folliculitis; a thickened, gray colored area of alopecia; or, a non-specific ulcerated lesion if the animal has licked and excoriated the site.
  • photo of ringworm on a puppy
  • photo of ringworm on a cat
  • photo of ringworm on a cow

The pathogenic ringworm fungi are divided into 2 major groups. The so-called "anthrophilic" dermatophytes are those for which human beings are the primary reservoir. These fungi are only very rarely isolated from domestic animals. The "zoophilic" dermatophytes are those for which domestic and wild animals are the major reservoirs. However, several of the zoophilic dermatophytes are also zoonotic, most commonly:

• T. equinum (horses)
• T. mentagrophytes-mentagrophytes (horses, dogs, cats)
• T. verrucosum (cattle)
• M. nanum (pigs)
• M. canis (dogs, cats)

In immunocompetent human hosts, infection with these fungi is limited to the keratinized layers of the skin and hairs. However, in immunocompromised hosts, there is mounting evidence for deep tissue, even systemic, infections.

Of particular importance is the fact that dermatophytes can be cultured from cats (and to a lesser degree dogs) without lesions, particularly in multi-animal households, shelters etc. These animals, whether subclinically infected or transient carriers, may be a source of infection for people.

• The risk of infection from cats is greatest from kittens, and especially those from shelters or catteries with a history of dermatophytosis.

Vaccination of animals against zoonotic dermatophytes:

• An inactivated M. canis vaccine is commercially-available for cats. This vaccine has been shown to induce serum antibodies against M. canis and to reduce the severity of clinical lesions following challenge infection, but it does not prevent infection. As such, it is of minimal value in limiting zoonotic transmission of M. canis from cats.
• A live strain (LFT-130) of T. verrucosum has been employed as a vaccine in cattle for many years in Eastern Europe and Scandinavia. This vaccine has been effective in reducing the prevalence of T. verrucosum infection in cattle and, thereby, among in-contact human beings.

Dermatophilosis

Etiologic agent = Dermatophilus congolensis

This Gram (+) actinomycete develops characteristic septate filaments with parallel rows of coccoid cells that form motile flagellated zoospores. Infections occur most commonly in horses, cattle and small ruminants, with transmission between animals occurring by release of zoospores from the lesions when they get wet. Transmission may be either direct or via mechanical insect vectors. The skin lesions appear as exudative, crusted areas in which the hairs are cemented together into characteristic "paint-brush" clumps. Animal diseases related to Dermatophilus congolensis infection have been called "streptotrichosis," "strawberry foot rot," and "mycotic dermatitis."

Dermatophilus congolensis is transmitted to humans by direct contact with lesions on animals. Humans develop an exudative dermatitis similar to that seen in animals.

Sporotrichosis

Etiologic agent = Sporothrix schenkii

This is a dimorphic fungus that can be contracted by inhalation from the environment or, more commonly, through skin wounds ("rose-growers disease" via thorn pricks). Skin lesions in people begin as erythematous nodules. There may be subsequent lymphatic spread proximally to produce tracks of similar lesions. Lesions may also become ulcerated. Extracutaneous sites of infection include joints (osteoarthritis), lungs (granulomatous pneumonia with cavitation) or, less commonly, the CNS (meningitis).

• An association between many human cases and contact with sphagnum moss produced in Wisconsin has been noted.

Cats are unique in the epidemiology of sporothrichosis. This is because the fungus, as it occurs in skin lesions and abscesses in cats, can infect humans through intact skin (i.e., without the need for a pre-existing wound). As such, it poses a direct threat to cat owners and veterinarians. This may be related to the unusually large number of yeast organisms in the subcutaneous tissues of infected cats compared to other animals.

The treatment of choice in dogs, cats and humans appears to be itraconazole.

Malassezia infections

Etiologic agents = Malassezia pachydermatis, Malassezia furfur

Malassezia pachydermatis is a common cause of otitis externa in dogs and, to a lesser degree, cats. This yeast has also been isolated from human infants in intensive care settings with systemic infections and similarly from an immunosuppressed adult human bone marrow transplant patient.

Malassezia furfur is an opportunistic cause of skin disease in otherwise healthy human beings and severe systemic infections in immunocompromised people. This yeast has also recently been isolated from the ear canals of subclinically infected cats.

Rhinosporidium seeberi...no longer considered a fungus!

Historically, R. seeberi has been considered to be a fungal organism. However, recent phylogenetic data indicate that it is a protist parasite most closely related to parasites of fish and amphibians. Rhinosporidium infections are characterized by tumor-like mass lesions of the nasal or conjunctival mucosa.

• Both domestic animals and humans contract the organism from contaminated waters and there is no evidence for direct zoonotic transmission of the organism.
• Treatment requires surgical excision; antimicrobials are ineffective.

References:

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