Ringworm in Dogs
by Dr. Audrey Harvey, BVSc (Hons)
Despite its name, ringworm in dogs is not actually caused by a worm. This highly contagious skin infection is caused by fungi. The most common culprit in dogs is Microsporum canis, but the two other species that can infect our canine companions are Microsporum gypseum and Trichophyton mentagrophytes. Collectively, they are known as dermatophytes, which means plants that can live on skin.
These fungi live on keratinized tissues, that is the skin, hair and nails. They are all capable of infecting people, causing a characteristic round red scaly skin lesion. They only infect abraded skin, and are more likely to cause problems in those folk with a reduced immune system. These include the elderly, young children, people being treated with chemotherapy, and even those people who are suffering from stress. If you are healthy, and your skin isn't damaged, you're very unlikely to become infected from ringworm in dogs.
How Dogs Get Ringworm Infections
Dogs are often infected by coming into contact with animals that already have ringworm, and are constantly shedding contaminated hair into the environment. However, fungal spores can also be spread between dogs on bedding or brushes. These spores can live in the environment for many years, and can be a constant source of reinfection.
Some dogs are carriers of dermatophytes. They show no signs of infection, but can easily infect other animals. They raise suspicions when animals in their household are repeatedly reinfected with ringworm, in spite of their owner's best efforts at controlling the disease. The only way to check if an animal is a carrier is to take samples of hair and skin, and try to grow fungi from them.
Remember that because dermatophytes can't infect healthy unbroken skin, dogs only develop skin problems due to ringworm if their skin is irritated.
Diagnosing Ringworm in Dogs
Ringworm in dogs doesn't tend to cause the typical red round lesion that we see in people. It usually results in very non-specific patches of grey scaly skin with hair loss in dogs
. The skin usually isn't red, and it isn't itchy. It can look like many other common skin conditions in dogs
Your veterinarian will often have a suspicion that your dog has ringworm, based on the appearance of his skin. If you have signs of ringworm infection on you, then this strengthens these suspicions. In most cases, more testing is needed to confirm the diagnosis.
1. Wood's Lamp
A Wood's lamp is an ultraviolet light. In 50% of Microsporum canis infections, infected skin will fluoresce green under this light. The other dermatophyte species are less likely to fluoresce. One might say that 50% accuracy isn't very good, but this test is quick, easy and inexpensive. If the test is positive, treatment may be started straight away. But, in most cases, your veterinarian will suggest more accurate testing, just to make sure.
•Examination of hair samples under the microscope. Spores aren't easy to see, but if they are found, then it confirms a diagnosis of ringworm in dogs. Usually these samples are stained with special dyes to make it easier to see the spores.
3. Fungal Culture
A sample of skin flakes and hair are put on a special agar culture medium, to see if any ringworm fungi will grow. This confirms a diagnosis of ringworm in dogs, and you can even identify which dermatophyte is the guilty party. The main disadvantage of this procedure is that it can take up to two weeks before any fungi appear on the agar. In the meantime, your dog is spreading fungal spores and may be infecting yourself and your other household pets.
4. Skin Biopsy
Sometimes you have to go as far as a skin biopsy to reach a diagnosis, if none of the other tests are conclusive. This is usually done under local anesthetic, and is relatively quick, with results available in about 48 hours.
Treatment of Dog Ringworm Infections
It seems so unfair; it is not hard to pick up ringworm infection, but it's often very difficult to completely clear it up. There are two parts to treating ringworm infection in your dog: getting rid of the fungi on his skin and hair, and disinfecting his environment. Let's look at these in more detail, one at a time.
Treating Your Dog
If the ringworm lesions are small enough, you may be able to get away with a topical anti-fungal cream to clear up the infection. However, most dogs need to be given oral medication. The main drugs used in dogs are griseofulvin, ketoconazole and itraconazole. They are very effective, but because treatment often needs to be continued for several weeks, the cost can really add up. Griseofulvin can cause birth defects in dogs, so should never be used in pregnant bitches.
Lime sulfur dips or rinses are used twice weekly in conjunction with medication to clear up the infection on the skin. An alternative to lime sulfur if you don't like the smell is a miconazole-chlorhexidine rinse. Both are very effective. Depending on his coat length and thickness, your vet may recommend that your dog be shaved, to make it easier for the anti-fungal rinses to reach his skin. However, shaving may also spread fungal spores throughout the environment, so it isn't always recommended.
Some veterinarians have found lufenuron to be useful in treating ringworm in dogs. This drug is an active ingredient in many flea control products, and works by preventing chitin development in the flea exoskeleton. The immature fleas don't have the chitin egg-tooth that allows them to break out of the egg, and they can't form a pupa. The result is interruption of the flea lifecycle, and a reduction in flea numbers. Dermatophytes also have chitin in their cell wall, and it is thought that large doses of lufenuron may prevent or treat ringworm infection. Although there is no harm in using lufenuron in this way, its efficacy varies. You're better off spending your hard earned money in a more proven treatment.
Disinfecting the environment
This is the hard part of managing ringworm infection in your dog. Because he will be constantly shedding fungal spores, he should be isolated to one part of the house to prevent him spreading spores everywhere.
Diluted bleach in the ratio of one part bleach to 10 parts water will kill 80% of fungal spores. This can be used on any surface that can be bleached, including tiled floors, walls and plastic furniture.
Carpets and soft furnishings should be vacuumed and steam cleaned, and the vacuum bags should be disposed of straight away. Your dog's bedding should be washed in bleach solution, and any equipment such as brushes, collars and food bowls should also be bleached to kill any spores.
Keep your dog out of disinfected parts of your home until he is free of infection. This means until his skin and hair has been cultured, and there has been no fungal growth. He may not be impressed, but it is the quickest way to get rid of ringworm from your home.
Studies have suggested that ringworm in dogs will go away on its own, if left untreated. This can take many months, and all the while your dog is shedding fungal spores everywhere he goes and potentially infecting any other person or animal he comes in contact with. It's much better to aggressively treat any ringworm infection, and prevent the spread of the condition.
Ringworm in dogs tends not to be a severe disease. However it can be annoying, and it can be difficult to eliminate, so be sure to ask a veterinarian if you need more information.
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