Ringworm in cats: diagnostic difficulties of dermatophytosis
General characteristics of dermatophytosis in cats
The prevalence of dermatophytosis in cats varies greatly depending on the geographical area, with a worldwide level estimated to range from 5–50%. It is a zoonotic disease of public health concern, as owners can catch it from their cats.
Dermatophytosis in cats is both highly contagious and extremely infectious. However, it is a treatable, curable and non-life-threatening disease. Unlike other zoonotic diseases, such as those caused by fleas, ticks or other common infections, dermatophytosis cannot be prevented with typical veterinary measures such as vaccinations or the application of repellents and parasite control products.
Ringworm in cats with a competent immune system is usually self-limiting and resolves itself. However, in more vulnerable cases, such as kittens, dermatophytosis can develop into a more widespread and prevalent problem.
What are the greatest clinical challenges in the diagnosis of dermatophytosis in cats?
“Dermatophytosis manifests with nonspecific clinical signs, which should be taken into account when making a differential diagnosis of a skin disorder.”
More specifically, the presentation of ringworm includes hair loss, erythema and flaking skin, with or without pruritus. Within the range of presentations, there are three basic types of case concerned with the cat’s general health:
1. Simple infection: visible but limited lesions. The cat is in good overall health apart from the lesions, so it is expected to respond well to treatment.
2. Infection with complications: cats with extensive and/or inflamed lesions, which are more common in those with long, thick hair, coping with other comorbidities (such as upper respiratory diseases). Cases that prove to be refractory to treatment also form part of this group.
3. No lesions but a positive fungal culture: these cats have no observable lesions, but they do transmit infectious spores that are attached to their hair, either mechanically or by releasing arthrospores from initially undetectable incipient lesions.
Alopecia, erythema, excoriations and small crusts associated with folliculitis, with pruritus caused by Microsporum canis in a street cat.
Alopecia with moderate erythema on the nasal bridge of a cat with dermatophytosis caused by Microsporum canis.
Diagnostic aids
Diagnostic tools are very important in cases of suspected dermatophytosis in cats. They include a combination of:
- a full medical history
- physical examination under ambient white light and with a Wood’s lamp
- direct examination of hairs under fluorescent light
- fungal culture with a CFU count (colony forming units)
It is very important to be aware that a negative result in a Wood’s lamp examination does not rule out dermatophytosis. In fact, over 50% of cases classed as negative under a Wood’s lamp give positive results in the fungal culture.
Which factors increase a cat’s risk of contracting ringworm?
As with other self-limiting diseases, and as mentioned above, each cat’s risk of developing ringworm is influenced by its health status. Kittens or very elderly cats, cats with debilitating comorbidities and even those with stress are more susceptible to contracting ringworm.
Furthermore, two factors increase this risk:
- Factors that provoke microtraumas. Increased scratching due to pruritus or scratches, excess moisture after washing, particularly in animals with thick hair, and other poor hygienic practices that increase the risk of microtraumas are directly related to the increased risk of ringworm in cats.
- Factors that limit grooming. Grooming is a natural defence and cleaning mechanism; if cats cannot carry it out as normal because of conditions such as arthritis in senior cats or upper respiratory infections, then this may contribute to fungal infections.
