Ringworm is a fungal skin infection in dogs that can sometimes cause circular or ring-shaped hairless patches. Not all skin conditions that result in patchy or circular hair loss are caused by ringworm, however. Lamisil is the brand name for the anti-fungal medication, terbinafine, available as an over-the-counter treatment for human fungal diseases including athlete's foot, jock itch and ringworm. Lamisil is not labeled for use in dogs. Most cases of ringworm in dogs do not require treatment with medication, however an accurate diagnosis of the condition by a veterinarian is necessary as other skin disorders in dogs will require treatment.
The name "ringworm" is somewhat misleading, as the infection is not caused by a worm but rather by a group of fungi known as dermatophytes. The lesions caused by dermatophyte infections can sometimes be circular and appear like a ring, but oftentimes simply will result in a scaly, patchy hairless area of the dog. The most common species of dermatophytes cultured from skin infections of dogs are Microsporum canis, Trichophyton mentagrophytes and Microsporum gypseum. In contrast, the most frequently isolated species of ringworm infections in humans are Trichophyton rubrum and Trichophyton tonsurans. Humans are also susceptible to infection with the same species of dermatophytes as their dogs, particularly Microsporum canis.
Lamisil is the trade name for a human anti-fungal medication called terbinafine. While the medication is labeled for use against ringworm in humans, it is not labeled for use in dogs. It is not appropriate to extrapolate the use of medication from one species to another. Many medications that are approved for humans are dangerous to give to dogs due to differences in how each species metabolizes the drugs. Terbinafine has been evaluated scientifically as a treatment for ringworm in dogs and was found to be less effective than other commonly used veterinary approved products. Always consult your veterinarian before treating your dog with any medication.
Dogs with ringworm infections may have hairless areas that are scaly and flaky, and they may be itchy. Dermatophytes feed on the dead skin cells that accumulate on the surface of the skin so the infections usually are superficial, stopping when they reach healthy skin cells or inflamed tissue. The infection weakens the hair shaft causing it to become brittle and break off at the surface. Ringworm infections are most common in puppies or young dogs, but may occur in dogs of all ages. In older dogs, ringworm is often associated with other diseases that cause suppression of the immune system or endocrine imbalances such as Cushing's disease or hypothyroidism. Additionally, treatment with corticosteroids for other conditions may suppress your dog's immune system enough to allow a ringworm infection to take hold. Most cases of ringworm are mild and self-limiting, meaning that they require no specific treatment other than attention to hygiene and occasional bathing to prevent the spread of the infection on the dog or to others.
Unfortunately, in most cases you or your veterinarian will not know if your dog is suffering from ringworm just by looking at him. Sometimes a simple screening with a special light can reveal a dermatophyte infection, but other times a fungal culture and microscopic hair examination is necessary. Repeated fungal cultures are the only way to know if the infection is fully resolved as animals can remain asymptomatic carriers after an infection. Also, it is important to remember that not all skin infections are caused by dermatophytes. Dog can suffer from other fungal or yeast infections, bacterial skin infections, parasites such as lice or mites or allergic skin diseases. Most diseases of the skin look the same on the outside regardless of the cause. Many times an accurate diagnosis only can be made by looking at samples under a microscope, performing blood tests and performing cultures to isolate the specific organisms responsible for the infection.
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