Dogs get ringworm when a type of fungus called a dermatophyte lands on their skin or coat and takes hold, feeding on the protein in hair and the outer layer of skin. Despite the name, no worm is involved. The infection spreads through direct contact with an infected animal, contaminated objects, or even soil, and certain dogs are far more vulnerable than others.
The Fungi Behind the Infection
Three fungal species cause nearly all ringworm cases in dogs. The most common is one that thrives on dogs and cats alike and passes easily between the two. A second species lives naturally in soil and infects dogs that dig in contaminated dirt. The third is carried primarily by rodents and other small animals, meaning dogs that hunt, dig at burrows, or spend time around wildlife face extra exposure. Each of these fungi reproduces through microscopic spores that are remarkably durable, surviving for months on surfaces and in the environment.
How Dogs Pick It Up
Most infections start with contact. A dog touches noses with an infected stray, plays with a carrier cat, or simply brushes against a contaminated surface. Broken hairs shed by an infected animal are a particularly effective vehicle because each fragment carries fungal spores that can survive long after the hair falls off. Furniture, carpet, bedding, grooming tools, and shared collars are all common go-betweens.
The fungus also spreads easily through the broader environment. Spores settle into carpets, upholstery, and crevices in flooring where they can persist for 18 months or longer. This is why ringworm can seem to appear “out of nowhere” in a household, weeks or months after the original source is gone. Dogs that spend time in kennels, shelters, groomers, or doggy daycares face higher odds simply because more animals share the same space and tools.
Soil is an often-overlooked source. One of the three main fungal species is a natural soil organism, and dogs that love to dig are the ones most likely to pick it up this way. Infections from soil tend to cause more noticeable inflammation than those passed between animals.
Why Some Dogs Are More Vulnerable
Exposure alone doesn’t guarantee infection. A healthy adult dog with a fully functioning immune system can often fight off the fungus before it establishes itself. The dogs that get sick tend to fall into a few categories.
Puppies are at the top of the list. Their immune systems are still developing, which means they lack the mature skin defenses that adult dogs use to keep fungal invaders in check. Senior dogs face a similar disadvantage as immune function naturally declines with age. Dogs on immunosuppressive medications, those undergoing chemotherapy, or those with chronic illnesses like Cushing’s disease are also at elevated risk.
Nutrition plays a role too. Dogs with poor diets or those who are malnourished, as is common in strays and recently rescued animals, have fewer resources to mount an effective immune response at the skin level. Stress, whether from rehoming, overcrowding, or illness, can further suppress the immune system and tip the balance in the fungus’s favor.
What Ringworm Looks Like on a Dog
Skin lesions typically appear one to three weeks after exposure. The classic sign is a roughly circular patch of hair loss, often with a grayish, scaly surface that some veterinarians describe as resembling cigarette ash. The edges of the patch may look slightly raised or red. Lesions most commonly show up on the face, ears, paws, and tail, though they can appear anywhere on the body.
Not every case looks textbook, though. Some dogs develop irregular patches of thinning hair rather than neat circles. Others show mild dandruff-like flaking with no obvious bald spots. A few dogs carry the fungus without showing symptoms at all, silently shedding spores and infecting other animals or even people in the household. This is one reason ringworm can be tricky to trace back to its source.
The Zoonotic Risk
Ringworm passes readily from dogs to humans, and the reverse is also possible. Children, elderly adults, and anyone with a weakened immune system are most susceptible. In people, the infection typically appears as the red, itchy, ring-shaped rash that gave the condition its misleading name. If your dog is diagnosed, basic hygiene steps like washing your hands after handling them and laundering shared bedding in hot water significantly reduce the chance of it spreading to family members.
How It’s Diagnosed and Treated
A veterinarian may start with a Wood’s lamp, an ultraviolet light that causes some (but not all) ringworm strains to glow a bright apple-green color. A more reliable method is a fungal culture, where hairs from the affected area are placed on a special growth medium and monitored over one to three weeks for fungal growth. This takes patience, but it gives a definitive answer and identifies the specific fungus involved.
Treatment usually combines a topical approach with an oral antifungal medication. Medicated shampoos or dips help reduce the number of spores on the coat, limiting what the dog sheds into the environment. Oral medication works from the inside to clear the infection from the hair follicles and skin. Treatment typically lasts a minimum of six weeks and continues until follow-up cultures come back negative, not just until the dog looks better. Stopping too early is a common reason for relapse.
Environmental cleanup matters just as much as treating the dog. Vacuuming frequently, washing bedding and soft items in hot water, and disinfecting hard surfaces helps break the cycle. Without addressing the environment, the spores lingering on furniture and floors can reinfect a dog that has just finished treatment.