Ringworms in Dogs and Cats
What Is Ringworm?
Ringworm is not a worm - it is a fungus. It often assumes a ring-like, scaly, reddened shape on your pet’s skin. There are three major types, Microsporum canis, Microsporum gypseum, and Trichophyton mentagrophytes. In dogs and cats microsporum are the most common forms I encounter. The groups of three are also referred to as dermatophytes.
What Animals Can Get Ringworm?
All mammals can contract ringworm – including you! I see it most frequently in cats but I also commonly see it in rabbits, dogs, chinchillas and hedgehogs. All animals, unless they are immuno-supressed, eventually become immune to ringworm and do not show further signs of the disease. This recovery often takes several months.
Ringworm In General: Cats -especially longhaired cats - often have multiple ringworm sites on their bodies. I see it most often in kittens - particularly those that have been stressed or housed in large colonies. Dogs often have only a single lesion and, again, it is most common in puppies and immature dogs. Cats that recover from ringworm often remain carriers of the fungus with no external signs. Because it can transfer to humans, it is best to wear gloves when treating ringworm or playing with infected pets. I scan all new puppies and kittens for ringworm with my ultraviolet lamp on their first visit to my hospital.
Signs of ringworm are typically circular patches of broken hair in ring-like whorles. These areas usually heal at their centers, growing darker than normal hair. Surrounding this darkened area is a band of inflamed, reddened skin within which the hair is also broken off short. The most common areas for ringworm to occur are the face, ear tips, tail and paws. When these areas are examined with an ultraviolet light source the broken hair shafts often fluoresce. In the few cases where the lesions are itchy, the skin is crusty, bumpy and infected with bacteria.
How Your Pet Catches Ringworm:
Ringworm fungus does not penetrate normal skin. The fungus spores are passed into a scratch or scrape on the same or different animal. The usual source is a carrier pet that shows no signs of the disease. Not all pets in a household that are exposed to ringworm develop the disease. Some pets never become infected while others do become infected but develop no overt signs of the disease. Some of these animals go on to become silent carriers that spread the disease to others. Another common method of transmission is contaminated grooming supplies and electric hair clippers. Almost all dogs and cats that become infected with ringworm eventually cure themselves even if left untreated. Some cases, however, are persistent and do need medical treatment. I treat all cases.
Some cases of ringworm are so classical that diagnosis is quite easy and does not require growing the fungus in the laboratory. I have found that about eighty percent of the cases I treat glow under an ultraviolet light source (wood’s lamp). I have also seen a number of cases, which were compound-infections of ringworm and bacteria or ringworm and mange mites. When I suspect ringworm but the hair shafts do not fluoresce, I pluck some hairs from the spot for further examination. I place them into potassium hydroxide solution to clear them and I look for fungus growing within the hair shafts. If this test is negative and I still suspect ringworm I place some affected hairs in a special fungal isolation jell (sabouraud’s agar) to see if it will grow. The fungus is slow to grow and I wait three to four weeks before I am certain this test is negative.
Treatment of Ringworms:
First, infected pets should be separated from those that show no evidence of the disease. I like to clip the area of the infection and then vigorously scrub it frequently with “tame” iodine (povone iodine, Povidine, Betadine) scrub. Do not use tincture of iodine. Iodine scrub kills fungus (fungicidal) and also removes much of the infected skin flakes that spread the disease.
Vacuum your house and scrub down kennels to remove fungal spores. Throw away the vacuum cleaner bag once you are done. Be sure to wear gloves when you treat or handle infected pets so you don’t become infected or spread it to other pets. We used to treat isolated lesions with benzyl benzoate cream. However, we now know this compound is ineffective. I now use ketaconazole cream. We then treated most of these pets with grisiofulvin (Fulvicin P/G, 10mg/lb/day) tablets, which work, but must be given for extended periods of time (4-8 weeks). Some veterinarians use higher doses. Side effects of the drug are common in cats. I still treat some of my cases with grisiofulvin. It must never be given to pregnant animals or people, cats with immunodeficiency disease or feline leukemia. Griziofulvin stops the fungus from growing (bacteriostatic) but does not kill it. So cures rely on the cat’s own immune system recognizing and destroying the fungus.
In persistent cases I now use one of the imidazoles drugs, itraconazole (Sporanox 0.75-1.5mg/lb/day for twenty days). This drug is considerably safer than grisiofulvin. It is quite expensive in the United States.
An alternative drug that I have no experience with is terbinafine (Lamisil 15mg/lb/day for two weeks). Another imidazoles, fluconazole (Diflucan dogs:4-6mg/lb/day, cats:15-20mg/cat/day) is as effective as itraconazole.
Prevention: ringworm fungus hair loss cat dog skin infection
A vaccine to prevent or lessen the severity of ringworm is marketed by Wyeth’s Fort Dodge division but I have no experience with this vaccine. Its major use is in catteries and sanctuaries that have continuing problems with this fungus. Also, an Israeli veterinarian noticed that cats in catteries in Israel that received a flea-control medication, lufenuron (Program, 50mg/lb every two weeks) had less ringworm. Results of treating ringworm with lufenuron in the United States have not been as dramatic. ringworm fungus hair loss cat dog skin infection
Kittens least likely to have this disease are from individual litters born in a residential setting. That is, the more cats at a home or facility, the more likely ringworm is present. The same advice goes for puppies. Do not share grooming aids, clippers, bedding, cages, etc. between pets. Catteries can cleanse themselves of this problem by doing fungal cultures of each cat in the cattery. This includes those that do not show any symptoms. All positive cats should be treated with itraconazole and one can administer Wyeth’s vaccine as well. Long-haired cats seem to suffer more from this disease. I do not know if this is due to their excessive hair or my observation that longhaired cats tend to be frailer in heath than domestic shorthair cats. ringworm fungus hair loss cat dog skin infection.
A good antifungal disinfecting solution is a one in twenty solution of household bleach and water
Article submitted by: © Ron Hines DVM PhD