Ringworm is a common yet serious disease found in dogs. Because the lesions are often circular, ringworm was once thought to be caused by a worm curling up in the tissue. However, that is not the case. In fact ringworm is caused by a fungus. The fungus causes an infection in the dead layer of the skin, hair and nails. The fungus is able to feed off of the dead tissue as a source of nutrition. Ringworm is also known as dermatophytosis and there are four types of fungi that can cause this condition.
What are the signs and symptoms of ringworm?
The fungi (plural for fungus) live in hair follicles. As the organism invades and weakens, the hair shafts hairs break off at the skin line. Once the hair follicle is invaded the organism grows downward and eventually destroys the hair follicle. The area may become inflamed with ringworm. Patches of hair loss tend to be circular; as the fungi multiply the lesions may become irregularly shaped and spread over the dog's body. These patches may be associated with scaling and crusting of the skin. The lesions are sometimes itchy but that is not always the case. In fact there are times when animals are infected but show no signs at all. It must be remembered that there are no set clinical signs that are used to diagnose ringworm.
The incubation period is anywhere from 10-12 days. This means that after exposure to the fungus and infection 10-12 days will pass before any visible signs occur.
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How is ringworm diagnosed?
Canine ringworm can be diagnosed by three different methods. In some cases more than one approach is utilized. Diagnosis is done by one or more of the following:
- Identifying the typical ringworm lesions on the skin. This is the least accurate method since other skin diseases may have the same appearance and not all infected dogs have visible lesions.
- Placing the infected hairs under a fluorescent light. We recommend using a Woods Lamp Blacklight for this approach. The lamp should be warmed up for 5 minutes and the test should be conducted in darkness holding the lamp over the potentially affected area for 3-5 minutes. This screening test is useful because ringworm will sometimes fluoresce as a bright green color under ultraviolet light. Failure to fluoresce however does not eliminate ringworm as a potential diagnosis. The accuracy of this tool is about 50% since only 2 of the 4 fungus species fluoresce.
- Culturing the hair for the fungus. After a few hairs are plucked from a lesion it is placed on a special gel and is then watched for fungal growth. Also the color of the gel will change from yellow to red as the fungus grows. These cultures are checked daily. This method is the most accurate way to diagnose canine ringworm however it can take 2-3 weeks to get results.
How is ringworm transmitted?
Transmission occurs by direct contact between infected and non-infected individuals. It can be passed from dogs to cats and vice versa. It may also be transmitted from animals to people as well as from people to animals. Adult humans are fairly resistant to infection unless there is a break in the skin or the immune system is suppressed. Children however are highly susceptible.
Transmission may also occur from an infected environment. The fungal spores can live in bedding or carpet for several months. These spores may be killed with a dilution of chlorine bleach and water (1 cup of bleach to 1 gallon water) where it is feasible to use it.
How is ringworm treated?
There are several methods to treating ringworm in dogs. The specific method recommended by your veterinarian will depend on the severity of the infection, the number of pets involved, possible presence of children in the area, and how difficult disinfecting the dog's environment will be. The most commonly used treatment options include the following:
- Griseofulvin. This prescription comes in tablet form and works deep in the hair follicles to reach the site of active fungal growth. Administered daily, dogs with active lesions should receive the tablets for a minimum of 30 days. Griseofulvin is the only drug approved for the oral treatment of canine ringworm. Although effective, it does have some potentially significant side effects including gastrointestinal issues, possible birth defects in pregnant dogs, and suppression of bone marrow production.
- Miconazole Cream or Miconazole Spray are both effective topical treatments. Miconazole products contain the anti-fungal agent Miconazole Nitrate for the treatment of fungal infections such as ringworm and superficial skin infections caused by yeast. It is safe for use on dogs, cats, and horses. Miconazole is only available with a prescription.
- Malaseb Shampoo is also available only with a prescription. Malaseb's unique formulation provides antibacterial and antifungal agents for optimal therapeutic effectiveness.
- LimePlus Lime Sulfur Dip is another prescription product. Dips should be done twice weekly for the first two weeks and then once weekly for the next 4-6 weeks. Lime Sulfur Dip can also be applied to other pets in the household to prevent spread of infection.
- Shaving of the dog's hair. Primarily only recommended when the infection is extensive, some studies have suggested that clipping may cause microscopic nicks in the skin and serve to further inoculate ringworm into the skin.
Treatment will not produce immediate results; in fact the areas of hair loss may get larger before they begin to get smaller. Within 1-2 weeks the hair loss should stop and there should be no new areas of infestation. However, infected pets may remain contagious for about 3 weeks.
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