Ringworm infection is not caused by a worm at all – it’s actually a skin infection caused by certain kinds of fungus called dermatophytes. The scientific name for ringworm is dermatophytosis, or dermatomycosis. Some of the more common zoonotic species of dermatophytes found in animals include Microsporum canis, Trichophyton verrucosum, T. equinum, and T. mentagrophytes. There are also some dermatophytes that are primarily transmitted from person to person that are not carried by animals. These include the fungi that cause athlete’s foot and jock itch.

Like many fungi, dermatophytes grow best in warm, moist environments, but they can grow almost anywhere on the body. They tend to grow around hairs and in the superficial layers of the skin, and the infection can be quite itchy. A ringworm skin lesion tends to spread out from one point on the skin, causing hairloss as it progresses, resulting in a bald patch (see picture right). The outside (most active) edge of the infection often appears as a red ring, from which "ringworm" gets its name.  The centre of the lesion may begin to heal, and the hair may start to grow back, even as the bald patch gets bigger.  It may take anywhere from a few days to a few weeks for visible lesions to develop after a person or animal has been infected. (Photo credit: A. Yu, Ontario Veterinary College)

Ringworm is quite contagious. The fungi are present in the large numbers on hair and skin cells that are shed by infected individuals. People or animals can be infected through contact with these infected hairs and skin cells, either directly on the affected person or animal (i.e. direct contact), or on things like clothing, blankets, hairbrushes etc. that have touched the affected skin (i.e. indirect contact).

Ringworm occurs all over the world, but no one knows exactly how common it is because there are so many different kinds of fungus that cause it, it’s not reportable, and many cases probably go undiagnosed. Signs of ringworm in animals are often the same as in people, however not every animal that is infected with ringworm develops signs of infection. It has been estimated that ~90% of cats that are carrying dermatophytes do not show any signs of infection, but they can still transmit the fungus to other animals and people. (NB: this does NOT mean that ~90% of cats carry dermatophytes!)  Almost any animal can be infected by at least one dermatophyte or another – cats, dogs, rabbits, rodents… even horses and cattle!

  • Early identification of ringworm is important to reduce the risk of transmission to people and other animals. If your pet develops bald patches, particularly if they’re itchy, it should be examined by your veterinarian to determine if a fungal infection may be present.
  • If you or anyone in your household develops an area of skin that appears infected (especially if it appears as a red "ring"), keep it covered with a piece of clothing or a bandage and see your doctor.
  • Most cases of ringworm can be treated with either topical (e.g. ointments) or oral anti-fungal medication.
  • Clean your pet’s grooming supplies (e.g. brushes, combs) regularly.
  • Always wash your hands after handling your pet.

Watch for another Worms & Germs blog post about ringworm and how to clean up if you or your pet is infected, coming soon!  More information about ringworm and dermatophytes can be found on the CDC’s Dermatophytes website.