Natural Habitat

The geophilic (soil-loving) dermatophytes inhabit the soil and can exist there as free-living saprophytes. Microsporum gypseum and M. nanum are examples of geophilic dermatophytes that can also cause lesions in animals and in man. The zoophilic dermatophytes are obligate pathogens, primarily parasitizing animals but also capable of infecting humans. Humans are the main host for the anthropophilic dermatophytes and these very rarely cause ringworm in animals and are not considered in this chapter. Some dermatophytes have become adapted for survival in the skin of specific host animals, for example:

These dermatophytes generally give rise to subclinical or inapparent infections in the host animal, although they can also produce clinical lesions. Zoophilic species tend to be associated with a particular animal species. However, infection can also occur from reservoirs such as rodents (T. mentagrophytes), hedgehogs (T. erinacei; Fig. 38.6), soil (M. gypseum) and fomites such as bedding, grooming gear and harness containing hairs with infective arthrospores. Whilst growing on keratinized structures these fungi rarely produce macroconidia and rely instead on the production of arthrospores for transmission from host to host. Arthrospores can remain viable on shed hairs and skin particles for at least six to 12 months. The reservoir, transmission and site of the lesions can often be related to the actual dermatophyte involved. This is especially evident in the horse and the dog and is an argument in favour of attempting to isolate and identify the dermatophyte in an infection. Table 38.1 lists the main dermatophytes affecting animals, the main hosts of each and their geographical distribution.

Pathogenesis

Infective arthrospores germinate within six hours of adhering to keratinized structures. Minor trauma of the skin and dampness may facilitate infection. The ability of the dermatophytes to hydrolyse keratin causes damage to the epidermis, hair shafts, hair follicles and feathers. The nature of the lesions is affected by the virulence of the fungus and the immunological response of the host. Very young and very old animals as well as debilitated or immunosuppressed individuals are most susceptible to infection. The host mounts an inflammatory response to the fungal metabolic products that is harmful to the fungus, so the dermatophyte moves away peripherally towards normal skin. The result is the commonly seen circular lesions (ringworm) of alopecia with healing at the centre and inflammation at the edge (Fig. 38.7). There appears to be a balanced host–parasite relationship where a dermatophyte has become adapted to a specific host animal. While these animals may not show lesions they can act as a reservoir of infection. The manifestations of dermatophyte infections can vary and may be summarized as:

Laboratory Diagnosis