I have had several people over the years ask me if ringworm is really caused by worms. The answer is no. Ringworm is caused by a fungal infection. Only a few diseases can be spread from one species of animal to another or from animals to people (zoonotic). Ringworm is one of the diseases that can be contagious between species. Cats and dogs can give ringworm to each other, and both of them can give it to people.
In humans, ringworm causes a lesion on the skin that is usually circular and very itchy (pruritic). In dogs, it causes patchy hair loss (alopecia) that can occur anywhere on the body. The skin in the area of the alopecia is usually flaky. Unlike humans, dogs are usually not pruritic.
Cats are the tricky ones. Some cats will have patchy hair loss. Usually, the hair loss is localized around the mouth, eyes, and on the ears. However, there are cats who are asymptomatic, which means they have the fungus on their fur, but don't have any lesions. These cats are still contagious! So, if you suddenly come down with a ringworm lesion and you recently had contact with a cat, the cat was probably the source, even if it appeared healthy.
The fungus that causes ringworm can also survive very well in the environment, including the dirt. The fungal spores can also travel through the air and hide in places, like air conditioning ducts, for long periods. It is extremely difficult to get rid of ringworm once it has entered an environment. The fungus is very susceptible to household cleaners. So, it is pretty easy to kill it on cleanable surfaces. The problem areas are the places we don't usually clean or are difficult to clean, such as furniture or air ducts.
If your pet is diagnosed with ringworm, limit the areas he has access to and keep him isolated from other pets and children. Ringworm is a treatable disease, but as I said, it is difficult to eliminate it from your environment once it’s there.