Ringworm in dogs is not a worm at all. It’s a fungal infection of the skin and hair caused by microscopic organisms that feed on keratin, the protein that makes up hair, nails, and the outer layer of skin. The fungi burrow into hair shafts and surrounding skin, producing the patchy, scaly lesions most owners notice first. It’s one of the few skin infections dogs can pass directly to people, which is why recognizing and treating it matters beyond your dog’s comfort.
What Causes It
Three fungal species are responsible for most cases in dogs. The most common is one that also infects cats and can live on both animals in the same household. A second type is carried by rodents and small mammals, so dogs that hunt or dig around burrows are more likely to pick it up. The third lives in soil and tends to cause more inflamed, angry-looking lesions when dogs come into contact with contaminated dirt.
Infection doesn’t happen from casual exposure alone. A dog needs contact with enough fungal spores, some degree of minor skin damage (even a tiny scratch or abrasion), and moisture on the skin. Under ideal conditions, spores can begin invading the skin and hair shaft within 6 to 8 hours of landing there. The fungi then coat the outside of the hair with a cuff of spores, weakening and breaking the hair as they grow.
Which Dogs Are Most Vulnerable
Puppies and senior dogs are at the highest risk because their immune systems are either still developing or declining. Dogs on immunosuppressive medications or those with chronic illness are also more susceptible. Healthy adult dogs with strong immune systems often fight off exposure without ever developing visible lesions, which is why ringworm tends to cluster in shelters, breeding facilities, and multi-pet homes where young or stressed animals live in close quarters.
What Ringworm Looks Like
The classic sign is one or more bald, scaly patches with broken hairs. The skin underneath may look red, crusty, or slightly raised. Some dogs develop small acne-like bumps instead of (or alongside) the bald spots. The most commonly affected areas are the face, ear tips, tail, and feet, though lesions can appear anywhere.
Ringworm doesn’t always look dramatic. A single small patch of hair loss near the ear or muzzle can be easy to dismiss as a scrape or hot spot. The “ring” shape people associate with the name is more common in humans than in dogs, so don’t wait for a perfect circle before getting suspicious. Any expanding area of hair loss with flaky or irritated skin warrants a closer look.
How Vets Diagnose It
Diagnosis typically involves a combination of tools rather than a single test. The quickest screening method is a Wood’s lamp, a type of ultraviolet light. Infected hairs from the most common fungal species glow apple-green under this light. However, studies show the lamp catches roughly 70 to 75% of cases caused by that species, and it misses infections from other fungi entirely. A negative Wood’s lamp result does not rule ringworm out.
For a more definitive answer, vets collect hair and skin samples for fungal culture. This involves placing the sample on a special growth medium and watching for characteristic colony growth. Clinically significant results typically appear within 5 to 7 days, and if no growth shows by 10 days, that particular fungus is unlikely to be the cause. Vets may also examine plucked hairs under a microscope to look for spore cuffs coating the hair shaft.
PCR testing offers the fastest turnaround at 2 to 3 days and is extremely sensitive, capable of detecting DNA from a single spore. That sensitivity is a double-edged sword: PCR can confirm or rule out active infection quickly, but it’s not useful for monitoring treatment progress because it picks up dead, noninfectious spores just as readily as live ones. Fungal culture remains the gold standard for confirming cure.
Treatment: Topical and Oral
Most dogs with ringworm need both topical treatment (applied to the skin) and oral antifungal medication. Topical therapy alone may work for a single small lesion, but because spores spread across the coat before lesions even appear, whole-body treatment is generally more effective at preventing new patches and reducing environmental contamination.
The most effective topical option is lime sulfur dip, applied twice weekly with applications spaced 3 to 4 days apart. It sterilizes the coat and provides a residual protective effect for several days between applications. The downside: it smells strongly of sulfur, temporarily stains light-colored fur yellow, and can stain fabrics and surfaces. For small, stubborn lesions in tricky spots like the face and ears, antifungal creams containing clotrimazole or miconazole can be applied daily as a supplement. Chlorhexidine shampoos alone don’t sterilize the coat effectively enough to serve as a primary treatment.
Oral antifungal medications are prescribed alongside topical therapy for most cases. Your vet will select the appropriate drug based on your dog’s size, overall health, and liver function, since some of these medications can stress the liver over the course of treatment. Periodic blood work may be recommended to monitor for side effects during longer courses.
Treatment typically continues for a minimum of 6 to 8 weeks, and your vet will confirm cure through follow-up fungal cultures rather than relying on how the skin looks. Lesions often improve visually well before the infection is truly cleared, so stopping treatment early based on appearance is one of the most common reasons for relapse.
Cleaning Your Home
Fungal spores shed from infected hairs can survive in the environment for months, making thorough cleaning essential to prevent reinfection and protect other household members. The process doesn’t require harsh chemicals, but it does require consistency.
Start by mechanically removing hair and debris from all surfaces. A damp mop or electrostatic sweeper (like a Swiffer) followed by vacuuming works well for floors and hard-to-disinfect areas. Clean the vacuum bag or reservoir after each use. Hard surfaces like crates, countertops, and floors should be cleaned with a good detergent first, then followed with a disinfectant. A diluted bleach solution works, but full-strength or highly concentrated bleach is unnecessarily harsh and not required.
For areas where your dog spends the most time, disinfect twice weekly during the treatment period. Washable items like bedding, blankets, and soft toys should be laundered separately from other household laundry. Don’t overfill the machine, as the mechanical agitation is what removes spores. Hot or cold water both work, and bleach in the wash isn’t necessary. Dry on high heat and clean the lint filter after every load. Dishes and hard chew toys can go through a dishwasher as long as the water temperature reaches at least 110°F.
Can You Catch It From Your Dog
Yes. Ringworm is one of the most common infections that passes between pets and people. You can pick it up through direct contact with your dog, through touching contaminated fur, or from surfaces and objects like bedding where spores have settled. Symptoms in people typically appear 4 to 14 days after exposure and include an itchy, ring-shaped rash with red, scaly, or cracked skin. Hair loss near the rash is common. The rash most often shows up on the face, arms, and legs.
Children, older adults, and anyone with a weakened immune system are especially susceptible. Puppies and kittens are the most common source of transmission to people, partly because they’re handled and cuddled more frequently and partly because they’re more likely to be actively shedding spores. Basic precautions during treatment include washing your hands after handling your dog, keeping the dog out of bedrooms if possible, and staying consistent with both treatment and environmental cleaning until your vet confirms the infection is resolved.