In most cases, dogs with ringworm lose hair in small, circular areas. These lesions can occur anywhere on the body, and there are usually multiple hairless patches on an infected dog.
In some cases, the hairless patches develop a scab in the center. They can be itchy if they develop a secondary infection, but most dogs with ringworm are not particularly bothered by it. Hair may begin to regrow in the center of the hairless patches after time, leaving a hairless ring that gives the condition its name.
Despite the name, ringworm is caused by a fungus rather than a parasite. It is highly contagious and can affect many animals, including humans. The spores can also live for some time in the environment, so bedding, grooming tools and other objects touched by an infected animal may be able to spread the disease.
Some dogs do not develop noticeable symptoms or they have unusual symptoms that do not look like typical ringworm. Although noticing the lesions may be enough to start treatment, owners of dogs that may have ringworm should take the animal to a veterinarian to confirm it. The veterinarian can make sure the diagnosis is correct by examining the dog with a device called a Wood's lamp, which is a fluorescent light that makes the fungus glow brightly.