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K. A. Moriello - D. J. DeBoer

Dermatophytosis

Dermatophytosis is a superficial fungal infection of the skin. It is caused by keratinophilic fungi of the genera Microsporum, Trichophyton and Epidermophyton. These fungi invade, inhabit and multiply in the stratum corneum and the keratin of hair and nails. Dermatophytosis (ringworm) is the most common infectious skin disease of catsIt is clinically very variable, difficult to eradicate, and carries a substantial zoonotic risk. It is very important for veterinary surgeons to be familiar with this condition.

Aetiopathogenesis

Causal organisms

Microsporum canis is the most common cause of feline dermatophytosis¹². It is not part of the normal fungal flora of cats and its isolation indicates either an active infection or mechanical carriage of the organism on the hair coat ²-³. In rare instances, other organisms such as Trichophyton spp. or Microsporum gypseum may be isolated. Trichophyton infections are rare and occur following contact with rodents (usually asymptomatic carriers), horses, cattle or a contaminated environment. Trichophyton rubrum is a common cause of human dermatophytosis (e.g. tinea pedis) and exposure of cats to infected people (e.g. while playing with the owneris feet) or contaminated surfaces may lead to dermatophytosis in the cat. Microsporum gypseum is a geophilic organism and infection is most likely to result ffom exposure to fungal spores in the soil.

Modę of infection

The naturally infective stage is the arthrospore, formed by segmentation and fragmentation of fungal hyphae. Cats become infected either by direct contact with an infected animal or by contact with a contaminated environment. The latter is an under-recognised reservoir of infection for both people and other animals. The number of spores needed to trigger dermatophytosis in a susceptible cat is unknown. Large amounts of infective materiał are present in homes and the environment of infected cats. Airbome spread of spores occurs readily and heating and ventilation materiał can easily be contaminated. Fomite transmission by contaminated collars, brushes, toys, hide boxes, transport cages, etc. may be considerable. Depending on ambient conditions, spores can remain viable for many months.

Predisposing factors

When infective arthrospores contact a cat’s coat, many factors influence whether or not successful infection occurs⁴.

Grooming behaviour of cats is an important natural defence against infection; it can be difficult to establish experimental infection in cats which lick the inoculation sites. Interestingly, the first signs of dermatophytosis in kittens often occur at, or around, the time they are separated from the queen. Additionally, lesions commonly occur on the face, an area that is difficult for young kittens to groom. The importance of grooming in mechanically removing spores may explain the common clinical obsemtion that long-haired cats are morę susceptible to dermatophytosis. It is obviously morę difficult for a long-haired cat to groom its coat as thoroughly as a short-haired cat. Under optimal conditions, infective spores may germinate within 6 hours of adherence to keratinocytes ⁵. Trauma to the skin may promote germination. Arthrospores cannot penetrate healthy intact skin and some type of trauma, even if only very slight, is necessary to facilitate infection. Other factors to consider that may predispose cats and kittens living in colonies to infection include microtrauma from other skin diseases, including those provoked by fleas, lice, and mite infestations.

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