What is Ringworm (Dermatophytosis)?

Dermatophytosis, commonly known as "ringworm" (despite the name, it has nothing to do with worms), is an infection of the skin, hair, and nails caused by dermatophyte fungi, with Microsporum canis being the most common in dogs and cats. These fungi feed on keratin, the protein present in skin and hair, and spread easily through direct contact with an infected animal or with contaminated objects, such as brushes, bedding, and the environment.

The most characteristic sign is a circular or oval patch of hair loss with scaling and, sometimes, a reddish border — hence the popular name "ringworm," even though the cause is a fungus, not a worm. Lesions can appear anywhere on the body and, in some cases, cause little to no itching, which can delay an owner from noticing the problem. Puppies, kittens, senior animals, and immunosuppressed pets have a higher risk of developing the infection.

It is a zoonosis: it can be transmitted to humans, causing similar skin lesions, especially in children. Diagnosis is made using a Wood's lamp exam, fungal culture, or PCR, and treatment combines topical medication and, in more extensive cases, oral antifungal medication, along with decontaminating the environment to prevent reinfection. See a veterinarian if you notice circular patches of hair loss, especially if more than one animal or person in the household has similar lesions.