A fungal infection in dogs is an illness caused by fungi, either on the skin’s surface or deep inside the body. These infections fall into two broad categories: superficial infections that affect the skin, ears, and nails, and systemic infections that spread through the lungs, gut, bones, and other organs. Superficial infections like yeast overgrowth and ringworm are extremely common, while systemic infections are less frequent but far more serious.
Superficial vs. Systemic Infections
Superficial fungal infections stay on or near the skin. Ringworm, despite its name, is caused by a fungus that lives in the outer layers of skin, hair, and nails. Yeast dermatitis is another surface-level infection driven by a yeast organism that naturally lives on your dog’s skin but can multiply out of control under the right conditions. These infections are uncomfortable and can look alarming, but they rarely threaten your dog’s life.
Systemic fungal infections are a different story. The principal route of infection is inhalation: dogs breathe in microscopic fungal spores from contaminated soil or decaying organic matter. If the number of spores is small and the dog’s immune system is healthy, the infection may stay confined to the respiratory tract and clear on its own with few or no symptoms. But dogs are usually brought to the vet only after the fungus has already spread through the bloodstream or lymphatic system into other organs, causing visible illness. In some cases, direct wound contamination or ingestion of spores can also start an infection.
Common Types of Fungal Infection
Yeast Dermatitis
This is one of the most frequently diagnosed fungal problems in dogs. The yeast lives naturally on healthy canine skin, but allergies, excess moisture, skin folds, and immune dysfunction can trigger explosive overgrowth. The infection tends to concentrate in warm, moist areas: ear canals, lip margins, groin, armpits, between the toes, skin folds, and around the tail. Severe, persistent itching is the hallmark symptom, often accompanied by a distinctly unpleasant, musty odor that many owners describe as one of the first things they notice.
Affected skin may become greasy or waxy with yellow or slate-gray flaking. Over time, the skin can thicken and darken, taking on a leathery or elephant-like texture. Dogs with nail bed involvement often develop dark brown discoloration around the nails and may chew their paws obsessively. Hair loss, redness, and crusting along the lip margins are also common presentations.
Ringworm
Ringworm produces circular patches of hair loss, often with crusty or scaly edges. It’s highly contagious and one of the few fungal infections dogs can pass directly to humans through skin contact. It spreads easily between pets in the same household as well.
Blastomycosis
This systemic infection is endemic to the Mississippi, Missouri, and Ohio River valleys and parts of the Great Lakes region, particularly areas near low-elevation waterways with sandy soils. Dogs inhale spores from moist soil. Common signs include coughing, rapid or labored breathing, enlarged lymph nodes, eye redness or blindness, and skin lesions. The bones and eyes are frequently affected as the infection disseminates.
Histoplasmosis
Found in similar geographic regions as blastomycosis, histoplasmosis in dogs often targets the gastrointestinal tract. Diarrhea, bloody stool, and weight loss are frequent presenting complaints. The lungs are commonly involved too, so respiratory signs can overlap with digestive symptoms. Direct wound contamination and ingestion of spores play a larger role in histoplasmosis than in some other systemic infections.
Coccidioidomycosis (Valley Fever)
Valley fever is concentrated in the arid southwestern United States. Dogs living in or traveling through desert regions are at risk. Symptoms include cough, difficulty breathing, weight loss, enlarged lymph nodes, and in some cases, seizures when the infection reaches the nervous system.
How Vets Diagnose Fungal Infections
The diagnostic approach depends on whether the infection is on the surface or deeper in the body. For ringworm, vets often start with a Wood’s lamp, an ultraviolet light that causes certain fungal species to glow yellow-green on infected hairs. The lamp needs to warm up for about five minutes before use and the exam takes place in a darkened room. This test catches roughly 66 to 71 percent of positive cases, so a negative result doesn’t rule ringworm out. Vets typically confirm the result with a fungal culture, where a sample is placed on a special growth medium and monitored over about seven days for characteristic color changes and colony growth. PCR testing offers another option, using DNA analysis to identify the exact fungal species quickly.
For yeast dermatitis, vets often collect skin samples by pressing a microscope slide directly against a lesion or using tape to lift surface cells. The yeast organisms are easily visible under a microscope. Systemic infections require different sampling. Lymph node aspirates (a small needle draw from a swollen lymph node), fluid washed from the airways, or impressions from draining skin lesions can all reveal fungal organisms. Blood tests and imaging may also be used to assess how far the infection has spread.
What Treatment Looks Like
Superficial infections are generally treated with a combination of topical therapy (medicated shampoos, creams, or ear drops) and oral antifungal medication. Yeast dermatitis typically requires about three weeks of oral treatment. Ringworm treatment runs longer, usually four to eight weeks of oral medication, and treatment continues until follow-up cultures confirm the fungus is truly gone rather than just visually improved.
Systemic infections demand much longer commitments. Blastomycosis treatment typically lasts 60 to 90 days. Histoplasmosis in dogs requires four to six months of oral medication, sometimes combined with intravenous antifungal therapy for rapidly progressing cases. Coccidioidomycosis is the longest road, with a minimum treatment period of 12 months for dogs with disseminated disease. Some dogs need lifelong medication to keep the infection suppressed.
The most common side effects of oral antifungal drugs are digestive: loss of appetite, vomiting, diarrhea, and lethargy. Liver stress is a known concern with several antifungal medications, so your vet will likely run periodic blood work to monitor liver enzymes throughout treatment. One specific antifungal can cause a severe skin reaction involving ulceration in 5 to 10 percent of dogs when given at higher doses, which is why careful dosing and monitoring matter.
Can Dogs Pass Fungal Infections to People?
Some fungal infections pose a direct zoonotic risk, and some do not. Ringworm is the clearest concern. The fungal species that infect dogs are classified as “zoophilic dermatophytes,” meaning they can jump directly from an animal’s skin or hair to a person’s skin through contact. Children and immunocompromised individuals are especially vulnerable.
Systemic infections like blastomycosis, histoplasmosis, and coccidioidomycosis are not transmitted directly from dog to person. Both species catch these infections independently from fungal spores in the environment. A dog’s diagnosis can serve as a useful warning that the local environment carries risk, but your sick dog is not contagious to you in the way a ringworm-positive dog would be.
Yeast dermatitis is generally not considered a direct zoonotic threat for healthy people. However, there has been at least one documented case where the yeast from pet dogs was transferred to vulnerable newborns in a hospital setting through the hands of healthcare workers, so basic hygiene around immunocompromised individuals is sensible.
Cleaning Your Home During Treatment
Environmental decontamination matters most with ringworm, which sheds spores that can survive on surfaces, furniture, and fabrics for months. The CDC recommends a diluted bleach solution of 1/4 cup (2 oz) bleach per gallon of water. Items should be soaked in or wiped down with this solution and left wet for at least 10 minutes before rinsing. For surfaces where bleach isn’t practical, EPA-registered disinfecting sprays or wipes work if you follow the contact time listed on the label exactly. Bedding, blankets, and soft items should be washed in hot water frequently throughout the treatment period. Vacuuming carpets and upholstered furniture helps remove shed hairs carrying spores.
For systemic infections acquired from outdoor soil, there’s no realistic way to decontaminate the natural environment. Limiting your dog’s access to areas with known fungal activity, such as riverbanks with sandy soil in blastomycosis-endemic regions or dusty desert terrain in Valley Fever zones, is the most practical prevention strategy.