Pathogens of Importance in Cats
- Feline dermatophytosis is a superficial fungal skin disease of cats. The most commonly isolated pathogen is Microsporum canis, however infection with M. persicolor, M. gypseum and Trichophyton spp can occur.
- From a disease control perspective, the most important fungal pathogen of cats and in shelters is M. canis.
- M. canis is not part of the normal fungal flora of cats and isolation on a fungal culture indicates infection, fomite carriage, or cross contamination.
Update on Transmission and Pathogenesis Points
- Transmission of the disease occurs most frequently from cat to cat contact.
- Culture positive status can occur from contact with infective arthrospores on blankets, bedding, toys, brushes, lab coats, leather gloves, or even external parasites.
- Spores can be airborne, but these are readily trapped in heating/cooling furnace filters. Spores readily adhere to dust and fall.
- The incubation period from exposure to clinically obvious lesions is approximately two to four weeks, but there is evidence that cats develop subclinical lesions and are infective much sooner.
- An experimental model of feline skin showed that M. canis arthroconidia started to adhere within two hours of contact and increased for up to six hours post inoculation (Tabart J, Mignon B et al, Veterinary Dermatology 2008). During experimental infection models, infected hairs (Wood’s positive and direct examination positive) were found in less than a week post inoculation.
- Clearly the emerging information on how quickly spores can adhere and germinate emphasizes that prevention of contact with infective spores is important disease control point. In order for an infection to occur, infective spores must contact the skin surface and defeat host protective mechanisms (sebum, normal flora, grooming, and innate immunity).
Re-Thinking “Clinical Presentations” (Moriello-ism!)
- Now that we have a greater understanding of the disease transmission, pathogenesis, immune response, and treatment it is necessary to reconsider the clinical presentations of dermatophytosis.
- Traditional textbooks have long lists of clinical signs and end with a statement that “in cats a skin lesion is ringworm until proven otherwise”. Current developments make this statement outdated.
- Development of clinical signs is directly related to pathogenesis-invasion of hair follicle and heavily influenced by the host’s immune system. Further complicating the situation is the effect of any prior treatment.
- Clinical presentations based upon history and global assessment the cat
- Simple infection: This group consists of otherwise healthy cats or kittens with confirmed infections. Lesions are obvious but limited in extent. Provided the cat/kitten remains healthy, these cats will respond well to almost any therapy protocol.
- Complicated infection: This group consists of cats with wide spread lesions, inflammatory lesions, long-haired/matted hair coats, other illnesses (most notably upper respiratory infections), a history of prior treatment, surrender for “resistant dermatophytosis”, and/or semi-feral or feral cats. In many cases, clipping of the hair coat reveals the true extent of the lesions. These cats are more complicated to treat because of the extent of their lesions and/or because of other health factors. These cats are complicated to treat because antifungal therapy must be coordinated with treatment for other pre-existing diseases.
- Lesions Free but Culture Positive Group: This group of cats may consist of cats mechanically carrying spores on their hair coat (i.e. “dust mops”) or cats with very early lesions that are not easily seen but mature enough to be shedding arthrospores. A major question is how do you know the difference? Fungal culture results are available 7-14 days after collection and the number of colony forming units on the fungal culture plate coupled with a re-examination can help differentiate these cats into:
- The fomite carrier: These cats have no lesions upon re- examination and if cats are otherwise healthy, in a clean environment and are grooming these cats are culture negative upon re-examination. If these cats are group housed they pose a risk of contamination of the environment.
- Infected cats: By the time the culture results are available these cats have lesions under both white light and Wood’s lamp examination.
Repeat of examination in both white light and strong beam flash light, the latter will often reveal skin lesions of all types not seen during bright light examination
Wood’s Lamp: NEW!!!
- Contrary to what has been written, this is a cost and time effective diagnostic procedure.
- In cats, this is a time and cost effective procedure. The major problem with use is not taking enough time to examine the cat and/or not being trained to recognize Wood’s positive hairs.
- It is often said that less than 50% of strains do not fluoresce. This comment was extracted from the human literature and perpetuated into veterinary literature. We do not know this information and scientific data on strains and fluorescence is unknown.
- Crusts do not fluoresce, important to lift crusts and examine hairs beneath
- Cats with no visible lesions on gross examination will often have fluorescing hairs, infection can be confirmed via direct examination
- “Dust mop cats will not fluoresce
- In a study conducted by the author and Drs. Levy (Florida) and Newbury (California and Wisconsin), cats with lesions and fluorescence (proximal hairs) were always Wood’s lamp positive in field work Experimental studies on treatment protocols documented that a decrease in Wood’s positive hairs correlated with response to treatment
- Wood’s lamp examinations are valuable during monitoring of treatment, especially if the cat is not curing
- Bottom line: Wood’s lamp positive hairs examined by direct examination confirm an infection, and Wood’s lamp examination can find sites of infection that are not visible on gross clinical examination.
The Pillars of Treatment
Reasonable Confinement of the Cat/Kitten to an Easily Cleaned Room
- This minimizes spread of infective material in the home
- Clients often protest, simply ask them to do what they would if the cat had diarrhea
- Infective spores will be shed into the environment, it is CRITICAL to explain to understand that these spores do not grow or multiply in the environment, they are just like dust
- Unless it is clearly explained, many people equate “ringworm fungus” with mildew or black mold and become unnecessarily agitated about this risk
- The primary reason that cleaning of the home is necessary is to prevent infective material from collecting on the cat’s hair coat and confounding culture results (false positive culture results)
- Infective material is shed from the hair coat and it must be removed to prevent other cats from becoming fomite carriers and to prevent confusing when doing monitoring of fungal cultures.
- The Triple Cleaning Technique is recommended.
- The KEY STEPS are mechanical removal of the spores and hairs. This can easily be done using a broom, Swiffers (my personal favourite is the Easy Trapper System by 3M) AND washing of the area with a detergent safe to use around cats. It is important to rinse the area with water to remove detergent residue. MY FAVORITE is to use a FLAT MOP. Swiffer Wets are great but 3M makes a flat mop system that can be purchased from Amazon and is great. It cleans well but does not make the floor or area “wet”.
- Disinfectants are the last step/last resort to kill spores that are not removed via “hard cleaning”.
- Disinfectant need to be applied to a properly prepared area-in other words the area looks clean.
- What disinfectants can be used? Lots!!!! Recent studies in the our laboratory have identified that many ready to use products are sporocidal. The key is surface preparation, thorough application and a wetting time of 10 minutes Disinfectant options include Accel (accelerated hydrogen peroxide), Clorox Clean up, Lysol (lactic acid), Simple Green (ethoxylated alcohol mixture 3%), Fantastik (quaternary ammonia 0.22%), and Trifectant. Look to seek if the label says the product is “fungicidal against Trichophyton)
A special note about electric clippers: In order to disinfect clippers, mechanically remove all of the hair and debris. This does involve taking the clipper apart and may require using something small to get into all of the cracks and crevices. Clean the clippers with Accel to remove debis and then spray the inside, outside and the clipper blades with Clippercide (orange can) and let set for 10 min.
Rational Treatment (Combined Systemic and Topical Therapy)
Effective therapy requires removal of infected hairs from the coat that could be shed into the environment. Clipping of the hair coat has been recommended to accomplish this but this is not recommended unless the hair coat is severely matted. The simplest method is to use a flea comb to remove any easily broken or shed hairs before application of a topical antifungal product.
The infection is beneath the skin in the hair follicle and this requires the use of a systemic antifungal drug (itraconazole or terbinafine). The infective spores on the hair coat are not killed by oral antifungal agents. These need to be treated with a topical antifungal rinse that is repeatedly applied. Until reproduction of the spores in the hair follicle stops, spores will continue to be present on the hair coat. Options include: lime sulphur, enilconazole, miconazole shampoo, ketoconazole shampoo, or antifungal mousee products.
- Cats are treated until they are mycologically cured (two negative fungal cultures).
- Depending upon the treatment protocol, the cat can be cured long before all of the hair has regrown OR it can appear normal and still be infected.
- The BEST way to monitor treatment is via weekly fungal cultures. This is the most reliable and fastest way to identify two negative consecutive fungal cultures. It seems to be expensive, but consider the global cost of treatment. More rapid identification of treatment will:
- Decrease the time a cat or kitten is confined
- Decrease the time the client/owner needs to do more intense cleaning
- Decrease the overall number of topical treatments
- Decrease the amount of oral systemic therapy needed
- Prevent overtreatment and minimize adverse reactions to drugs
- Increase treatment compliance
- As cats cure, the number of fungal culture colonies on the fungal culture plate rapidly decreases
- If there is a “lack of response” this can be more readily detected.