By Dr Phil Sacks
Ringworm or Dermatophytosis is the most common infectious skin disease in cats. It is caused by a fungal agent that affects the keratin of the epidermis and the hair follicle e. A worm does not cause it. The fungi feed upon the dead cells of skin and hair causing, in people, a classic round, red lesion with a ring of scale around the edges and normal recovering skin in the centre. Because the ring of irritated, itchy skin looked like a worm it was called ringworm. Dermatophytes invade hair shafts and cornified epithelium. This destroys the hair shaft and disrupts normal keratinisation. Clinically, hair loss and scaling result.
Dermatophytosis is an important zoonotic skin disease in cats and is highly contagious. Within the medical community, there is increasing awareness of this disease, most likely because of the increasing number of immunocompromised cat owners. And treatment must be effective, or the disease will continue to spread. Any cat that comes into contact with infective material is at risk for developing dermatophytosis, but it is most likely to be encountered in at-risk groups: cats from multicat facilities, stray or feral cats, young or old cats, and debilitated cats with poor immune systems. It is well recognized that the immaturity of the host immune system in the neonate or juvenile may play a prominent role in the susceptibility of infection in the young animal Long coated breeds such as the Persian and Himalayan may have higher degree of suspec
Because feline dermatophytosis is so contagious, any cat obtained from a multicat facility should be considered to be at risk. Toothbrush fungal cultures to screen for dermatophytosis may be part of a new cat examination program, as should routine faecal examinations and feline leukaemia virus and FIV testing. The cost of a fungal culture is minimal compared with the combined cost of treating an infected cat and any people that contract the dermatophytosis from the cat.
Transmission of of ringworm in cats
Dermatophytosis is highly contagious and can be transmitted by direct and indirect contact with contaminated environments. Although exposure to the infective arthrospores of the fungal element may lead to an active infection, normal host defences often will prevent the establishment of clinical disease. An asymptomatic carrier state of M. Canis is well recognized and demonstrates the balance between host and potential pathogen. Dermatophytosis will spontaneously resolve in most healthy cats, but treatment is recommended because the disease is zoonotic and highly contagious.
The classical lesion is the one characterized as circumscribed alopecia (hair loss), erythemic macule (redness) with superficial scale. It is often associated with a bacterial infection
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Asymptomatic carrier cats are identified best through the MacKenzie brush technique that uses a clean (new) toothbrush that is used to groom the cat. The resulting hair and the bristle end of the toothbrush is used to stab in the media of DTM and/or Sabaroud’s dextrose agar.
Diagnosis of Feline ringworm
Direct microscopic examination of clinically infected cats: The procedure usually provides rapid identification. M. Canis ectothrix spores that form large cuffs around hairs. The diagnosis is quick inexpensive and easy to do and recognise for an experienced cytologist. The client is given the result during the consultation. We rarely need to utilise woods lamp or culture. : The Wood’s light examination is only a screening tool and cannot be used to definitively diagnose dermatophytosis. Using woods lamp , False positive and false negative test results can occur for a variety of reasons . We often culture the hair from clinically healthy cats, to determine if they are carriers..
Treatments of Dermatophytosis
Dermatophyte infections in cats usually resolve without treatment within 60 to 100 days if the cats have competent immune systems. With the endpoint to be both a mycological cure and a decontaminated environment
Combination of clipping the coat, topical therapy that the client is able and willing to do, systemic antifungal therapy that the cat can tolerate and the client can afford, reasonable suggestions for environmental decontamination, and a case-by-case plan for monitoring for and preventing reinfection.
- The cats environment: infective material can remain viable in the environment for up to 18 months under optimal conditions of temperature and humidity. In addition, cats shed infected hairs and spores into the environment throughout treatment. Contact with infective material will increase the risk of reexposure, reinfection.
- Clipping – pro and cons: Clipping the coat removes infected hairs and minimizes continued shedding of hair fragments and spores. It also makes topical therapy application easier and allows for more thorough penetration of the topical antifungal agent. In addition, clipping helps shorten the duration of therapy and ultimately decreases the cost of treatment. However, clipping the coat may require sedating the cat and is time-intensive. It may temporarily worsen the infection by causing micro trauma to the skin. And it may result in environmental contamination if appropriate efforts to capture infected hairs and spores are not taken.
- Topical therapy – Shampoo Miconazole, in combination with chlorhexidine, (Malaseb) has been shown to be an effective antifungal wash.
- Oral medications – Itraconazole 5 to 10 mg/kg given orally. This drug is currently our preferred medication for treating feline dermatophytosis. (GRISEOFULVIN -This drug is teratogenic (caused birth defects) and should not be given to pregnant animals. There are also anecdotal reports of its interfering with spermatogenesis, so it is best avoided in breeding males. Other side effects are Bone marrow suppression and neurological side effects are most likely idiosyncratic reactions.)
Briefly – Treatment Recommendations for Feline ringworm:
- Clip the coat.
- Administer topical antifungal therapy twice weekly.
- Administer systemic antifungal therapy.
- Consider fungal vaccination – not yet available.
- Treat cats until you attain a mycological cure – Keep in mind that cats will be clinically cured before they are mycologically cured.
- Response to therapy – antifungal therapy usually show a marked improvement in clinical signs within two to four weeks of starting therapy. It may take months for the entire coat to regrow if the infection is severe and the coat has been clipped. But after treatment is initiated, there is usually a noticeable decrease in pruritus, scaling, erythema, and hyper pigmentation, with hair growth following shortly thereafter.
- Monitor treatment – Start monitoring treatment in cats with dermatophytosis four weeks after initiating therapy by performing fungal cultures.
- Prevent reinfection – Culture all new pets added to the household. Do not allow cats to go outside
- Decontaminate the environment.