Ringworm (tinea corporis) is an infection of the skin caused by tiny fungus spores that reproduce in the dead outer layers of your skin. It’s contagious as long as any spores are alive. This means it can spread on contact to other parts of your body, or to other people, even when you’re being treated.

While ringworm may clear up on its own, there’s no set timetable for this. It’s best to seek treatment.

Several different species of fungus can cause tinea.

The name ringworm comes from the circular, ring-like appearance of the red, itchy spots that tinea causes on the skin of your trunk or extremities. Tinea can cause infections on other parts of the body, but then it often doesn’t have the same appearance as ringworm.

Ringworm is highly contagious, and it spreads easily from person to person (and even from animal to person).

Athlete’s foot (tinea pedis) and jock itch (tinea cruris) are caused by the same organism as ringworm. It’s called ringworm when it appears on other parts of your body.

The feet and the crotch area provide a warm, moist environment for the microscopic fungal spores, known as dermatophytes, to live and reproduce. Their food supply is the protein, or keratin, that is found in your skin, especially the dead outer layers.

Athlete’s foot is often spread in showers and locker rooms because the fungal spores can live and reproduce in puddles and wet spots, especially when the water is warm.

Once the fungus has infected your feet, you can spread it to other parts of your own body by touching.

Sharing towels, clothing, and bedding can spread it from person to person.

Ringworm and pets

You can also catch ringworm from an infected pet, though this is less common. But the species of fungi that causes ringworm in pets is different from those common among humans. These fungi may sometimes be transferred to a person in close contact with the pet, but the ringworm is not as likely to spread from that person to another.

Microsporum canis (M. canis) is the most common form of ringworm in pets. Cats and dogs can both carry it, but cats are considered the most important hosts. It’s also regularly found on horses and rabbits. Long-haired breeds such as Persian cats and Yorkshire terriers are reported to be more susceptible.

Both humans and animals can be carriers without showing symptoms of ringworm infection.

The incubation period for ringworm in humans is usually one to two weeks. Because the fungal spores are present before you see the ringworm outbreak, you can catch it from someone even before it shows up on them.

There are also some people and animals who can have ringworm but show no symptoms. They can still transfer the ringworm to you.

M. canis, the common cause of ringworm in pets, can be seen under an ultraviolet light (black light). Under an ultraviolet light, it may appear on the fur within seven days of exposure. But it takes two to four weeks for the symptoms to show up on the animal’s skin. During that time, your pet may be infectious without your seeing any symptoms.

As long as the fungal spores are present in the skin, the ringworm can be spread from person to person, or animal to person. You don’t stop being contagious when you start using antifungal medication. However, once you start treatment, if you cover the lesions you can significantly decrease the risk of spreading them to others.

The condition is contagious until all the spores are eliminated from your skin. It’s difficult even for a doctor to know when all of the fungal spores have been killed.

The fungal spores can also stay alive on clothing, bedding, and elsewhere as long as their food supply (dead skin cells) is present, and they have a moist and warm environment. Spores can live for as long as 12 to 20 months in the right environment.

There’s no set time limit for a ringworm infection. Without treatment, it may go away on its own in a matter of months in a healthy person. Or it may not.

Ringworm on the body is usually treated with a topical ointment such as terbinafine. A four-week course is common, but the time can vary.

With treatment, a ringworm infection on a part of the body without hair (glabrous skin) is likely to clear up within two to four weeks of the start of treatment.

More serious cases, and infections in the scalp, can require oral antifungal pills. In this case, too, you’re contagious until all the fungal spores are eliminated.

The standard treatment for ringworm is a topical antifungal preparation, such as terbanifine (Lamisil AT).

In more serious cases, your doctor may prescribe an oral antifungal, such as terbanafine, itraconazole (Sporanox, Orungal), or fluconazole (Diflucan, Celozole).

Maintaining good general hygiene is the best prevention against ringworm. Ringworm often spreads from your feet or groin area, so preventing athlete’s foot and jock itch can be a first line of defense.

Some tips:

  • Always wear protective footwear in public showers, locker rooms, and pool areas.
  • Dry carefully after showering, especially your toes and groin area.
  • Wear cotton underwear.
  • Never share towels, clothing, or bedding.
  • Avoid skin contact with children and pets who may be infected.