Dogs get ringworm by coming into contact with fungal spores, either directly from an infected animal or indirectly from contaminated surfaces and soil. Despite the name, ringworm has nothing to do with worms. It’s a fungal infection that targets the skin and hair, and the spores that cause it are remarkably durable, surviving on surfaces for up to 20 months.
What Ringworm Actually Is
Ringworm is caused by a group of fungi called dermatophytes that feed on keratin, the protein that makes up skin, hair, and nails. In dogs, the most common culprit is a species called Microsporum canis, which primarily spreads between animals. A second common type, Trichophyton mentagrophytes, often comes from contact with rodents or rodent-contaminated environments. A third species, Microsporum gypseum, lives naturally in soil and tends not to spread easily between animals, though dogs that dig frequently can pick it up.
How the Infection Takes Hold
Simply touching a few spores isn’t usually enough to cause an infection. Three conditions need to come together: exposure to a sufficient number of spores, some degree of minor skin damage (a scratch, scrape, or irritation), and moisture on the skin. When those conditions are met, fungal spores can begin germinating and invading the skin and hair shafts within 6 to 8 hours.
Once the fungus establishes itself, it grows outward in a circular pattern on the skin, which is where the characteristic “ring” appearance comes from. Hair becomes brittle and breaks off, creating the patchy, scaly bald spots that owners typically notice first.
The Three Main Routes of Exposure
Direct Contact With Infected Animals
The most common way dogs pick up ringworm is through direct contact with an infected animal’s skin or fur. This can happen during play, grooming, or simply sharing space at a boarding facility, shelter, or dog park. Cats are frequent carriers of M. canis and can pass it to dogs in multi-pet households. Some animals carry fungal spores on their coat without showing any visible symptoms at all. One study examining 154 dogs with no skin lesions found that about 2.6% were carrying dermatophyte fungi, meaning a dog can look perfectly healthy and still be contagious.
Contaminated Objects and Surfaces
Ringworm spores shed from infected animals onto virtually everything they touch: bedding, couch cushions, carpeting, grooming tools, collars, crates, and furniture. These spores can remain infectious for 12 to 20 months on porous surfaces like fabric and carpet. A dog doesn’t need to meet an infected animal to catch ringworm. Sharing a brush, sleeping on the same bed, or even walking through an area where an infected animal rested can be enough. Grooming salons and boarding kennels are common sources because tools and surfaces are shared between many animals.
Soil
The soil-dwelling species M. gypseum can infect dogs that dig in contaminated dirt, particularly if they have small cuts or abrasions on their paws or muzzle. This route is less common than animal-to-animal or object-to-animal spread, but it’s worth knowing about if your dog is an avid digger.
Dogs Most Vulnerable to Infection
Whether a dog actually develops ringworm after exposure depends on several factors: their age, overall health, the condition of their skin, their grooming habits, and their nutrition. Puppies and older dogs with weaker immune systems are more susceptible. Dogs that are malnourished, stressed, or dealing with another illness are also at higher risk because their immune defenses are already stretched thin.
Yorkshire Terriers appear to be particularly prone to persistent, widespread infections. Dogs in crowded environments like shelters and kennels face greater risk simply because of the higher density of animals and shared surfaces. Hot, humid climates also favor fungal growth, making infections more common in warm weather.
Ringworm Can Spread to People
Ringworm is highly contagious between species. You can catch it from your dog through direct skin-to-skin contact or by touching surfaces your dog has contaminated. The risk increases if you have any minor skin abrasions or if your skin is damp from sweating. Children, elderly family members, and anyone with a compromised immune system are most vulnerable. If your dog is diagnosed with ringworm, handling them and then touching your face or other skin before washing your hands is a common way the infection jumps to people.
Cleaning Your Home During an Infection
Because spores survive so long on surfaces, environmental cleanup is just as important as treating the dog itself. Without thorough disinfection, reinfection is likely. Wash all bedding, blankets, and removable fabric covers in hot water. Vacuum carpets and upholstered furniture frequently, and dispose of vacuum bags or empty canisters outside.
For hard surfaces, several disinfectants are effective against ringworm spores. Accelerated hydrogen peroxide products, potassium peroxymonosulfate cleaners, and even some common household sprays containing quaternary ammonium compounds (like Formula 409 or Simple Green) can kill the spores. A dilute bleach solution works too, though concentrated bleach or a strong 1:10 dilution is harsher than necessary for routine cleaning. Grooming tools, food bowls, and crates should all be disinfected regularly throughout treatment.
Focus especially on areas where your dog spends the most time. Restricting an infected dog to rooms with hard, easy-to-clean floors can make the process much more manageable than trying to decontaminate wall-to-wall carpeting throughout an entire home.