IN ANIMALS
AETIOLOGY AND EPIDEMIOLOGY • Cats and dogs are recognised as the natural hosts of M. canis , the most frequently isolated dermatophyte from companion animals. 6 Microsporum canis also infects humans and other animal species including cattle, horses, pigs, goats, rabbits and guinea pigs. 3 • Microsporum canis infections may be acquired through direct or indirect contact with an infected animal. Arthrospores shed from animal hair and scales can remain infective for 12-24 months. 4,5 Contaminated fomites, including bedding, collars, brushes and toys, may be a source of infection. • Studies on the fungal flora of healthy cats and dogs have shown M. canis is not part of the normal skin microbiome. 5 PREVALENCE AND RISK FACTORS • Dogs and cats may show clinical signs or be asymptomatic (subclinical) carriers. Asymptomatic carrier states are common in cats, particularly long-haired breeds. Asymptomatic infection is rare in dogs but has been reported in Yorkshire Terriers. Persian cats and Yorkshire Terriers are noted in the literature as predisposed to dermatophytosis. 5-9 • The highest incidence of dermatophytosis is reported in kittens, puppies, immunocompromised animals and long-haired cats. In immunocompromised animals, the outcome may be a multifocal or generalised skin disease. 5,8 Photomicrograph of Microsporum canis macroconidium. Microsporum canis macroconidia are spindle-shaped, have a thick wall and roughened surface, and are divided into six or more internal compartments (Public Health Image Library, CDC)
Patchy alopecia in a kitten with ringworm (Courtesy of Prof. Jacqui Norris)
• Dermatophytosis is more common in warm, humid climates and may be seasonal in temperate areas. 4,8 • Animals in group housing (kennel environments, catteries, animal shelters) and stray animals may be at greater risk of exposure to dermatophytes. Companion animals living with other cats and dogs are also at greater risk of acquiring dermatophytes. 4,8
CLINICAL DISEASE • Dermatophytosis is an infection of the keratinised layer of the epidermis, hair shafts and stratum
corneum. Patchy alopecia is the most common clinical sign, with or without associated skin lesions. Skin involvement can be localised, multifocal or generalised, and skin lesions may include any combination of papules, crusts, scaling, erythema, seborrhoea, and alopecia (patchy alopecia is more common than circular alopecia). Affected animals are frequently non- pruritic, or if pruritus is present it is generally mild. 5,8 • In some cats, dermatophytosis can present as a papulocrustous dermatitis (‘miliary dermatitis’) affecting mainly the dorsal trunk. 7,8 • The incubation period of M. canis ranges from 1-4 weeks. 3,5 Lesions caused by dermatophytes can be mild to severe depending on several factors including the infecting species, infective dose, virulence factors, location of infection, presence of secondary infections, physiological stress and
CONTENTS
Companion Animal Zoonoses Guidelines 68
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