Ringworm

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Ringworm on the forelegs of a dog

Ringworm in a common zoonotic skin disease of dogs caused by dermatophytes.

Pathogenic species of fungi include:

Clinically affected dogs are usually young, and a history of litter mates being affected is common. This disease is zoonotic and cross-species infections with cats is common. Asymptomatic M. canis carriers are considered to be a critical factor in the epidemiology of dermatophytosis in humans[1].

Skin lesions classically present as circular regions of alopecia. In young dogs, lesions often occur on the face and forelegs, but may be present over the entire skin[2]. Concurrent infections with mites such as Demodex spp have also been reported.

More rare dermatophytes such as Fusarium spp can occur, but are usually confined to the nails where they cause paronychia and onychomycosis[3].

Diagnosis is based on visualization of mycelia under light microscopy, UV examination of skin, and culture of dermatophytes. The diagnostic value of Wood's lamp fluorescence and microscopic examination are not as effective as fungal cultures[4].

Infected hair shafts are fragile and dislodged hair fragments containing infectious arthrospores are the most efficient means of transmission to other hosts. Such material, when kept at room temperature, may remain infectious in the environment for between 1 - 2 years. The prolonged survival time of dermatophytes in clinical material is of considerable epidemiological importance. Although direct contact may be a more efficient means of transmission, indirect transmission through fomites and environmental contamination can occur readily.

Treatment usually requires systemic therapy with oral antifungal therapy.

Griseofulvin (25mg/kg PO daily/4 weeks) and terbinafine (20mg/kg PO daily/4 weeks) are the most common medications used, although ketoconazole (10 mg/kg PO daily/4 weeks) is often prescribed in cases resistant to the other antifungal agents.

Antifungal shampoos and topical antifungal creams appear to be less effective in altering the course of dermatophyte infection.

The critical role of environmental disinfection in eradication of M canis from an endemic cattery or household cannot be overemphasised.

References

  1. Cafarchia C et al (2006) Isolation of Microsporum canis from the hair coat of pet dogs and cats belonging to owners diagnosed with M. canis tinea corporis. Vet Dermatol 17(5):327-331
  2. Hermoso de Mendoza M et al (2010) A zoonotic ringworm outbreak caused by a dysgonic strain of Microsporum canis from stray cats. Rev Iberoam Micol 27(2):62-65
  3. Namitome K et al (2011) Isolation of Fusarium sp. from a claw of a dog with onychomycosis. J Vet Med Sci 73(7):965-969
  4. Cafarchia C et al (2004) The epidemiology of canine and feline dermatophytoses in southern Italy. Mycoses 47(11-12):508-513