Dogs get fungal infections primarily by breathing in microscopic spores from the environment, though fungi can also enter through skin wounds or, less commonly, by ingestion. The specific source of those spores depends on the type of fungus, but soil, decomposing organic matter, and dust are the most frequent culprits. Some fungal infections stay on the skin’s surface, while others penetrate deep into the lungs and spread throughout the body.
The Three Ways Fungi Enter a Dog’s Body
Fungal spores are everywhere in the natural world, and dogs encounter them constantly. The spores gain entry through three routes: inhalation, direct skin contact, and ingestion. Of these, inhalation is by far the most significant for the dangerous systemic infections that can affect a dog’s lungs, bones, and organs.
When a dog sniffs the ground, digs in soil, or simply walks through a dusty area, it can inhale airborne spores that settle deep in the lungs. From there, the fungus may establish a localized lung infection or, in more serious cases, spread through the bloodstream to other parts of the body. Skin-based entry typically requires some form of damage to the skin’s surface, even something as minor as a small scratch, a clipper nick, or irritation from a harness. Ingestion is the least common route and plays a minor role for most fungal species.
Systemic Infections From Soil and Water
The most serious fungal infections in dogs are systemic, meaning they affect internal organs. These fungi live in specific types of soil and become airborne under the right conditions.
Blastomycosis
The fungus responsible for blastomycosis thrives in moist, acidic soil rich in decaying vegetation. Beaver dams, riverbanks, and lakeshores are classic hotspots. Most dogs diagnosed with blastomycosis live within 400 meters of a waterway. Rain, dew, or fog can dislodge the infectious spores from the soil, sending them into the air where a dog inhales them during a walk or an off-leash adventure. Once inhaled, the incubation period runs 5 to 12 weeks before symptoms appear, which means a dog can seem perfectly healthy for months after the initial exposure.
Valley Fever
Valley fever, or coccidioidomycosis, follows almost the opposite environmental pattern. This fungus grows in the dry, arid soils of the southwestern United States, with the highest case numbers in Arizona, Nevada, New Mexico, parts of California, Texas, southwestern Washington, and Utah. It also occurs in Mexico and Central and South America. The cycle is distinctive: heavy rainfall promotes fungal growth in the soil, and when an extended drought follows, the dried-out soil breaks apart and releases spores into dusty air. Dogs become infected simply by breathing in that dust during walks, hikes, or time spent in the yard. Construction sites, windstorms, and any activity that disturbs desert soil increase the risk.
Histoplasmosis and Other Systemic Fungi
Histoplasmosis spreads through soil contaminated with bird or bat droppings and has the broadest geographic distribution of any systemic fungus in some states. Texas diagnostic data shows that histoplasmosis cases appeared across 95 counties, more than any other systemic fungal disease tracked. Dogs that spend time near chicken coops, old barns, caves, or areas beneath large bird roosts face higher exposure risk. Other systemic fungi like cryptococcus and aspergillus follow their own geographic patterns, with aspergillus cases skewing toward eastern, more humid regions.
Ringworm Spreads Differently
Unlike the systemic infections above, ringworm (dermatophytosis) is a surface-level skin infection, and it spreads through a completely different mechanism. Dogs catch ringworm through direct contact with an infected animal, whether that’s another dog, a cat, or even a rodent. The fungus lives on the skin and in the hair coat, producing spores that can transfer during play, grooming, or shared living spaces.
Picking up ringworm from contaminated objects like bedding, brushes, or grooming clippers is possible but less efficient. Environmental transmission typically requires some form of minor skin damage for the spores to take hold. A dog clipped with contaminated grooming equipment, for example, is more vulnerable than one that simply lies on a contaminated blanket. This is also one of the fungal infections that can pass between dogs and people, so if your dog is diagnosed, basic hygiene like handwashing and laundering shared fabrics matters. Soft surfaces like bedding and towels can be disinfected with a standard washing machine cycle, no bleach required.
Why Some Dogs Get Sick and Others Don’t
Exposure alone doesn’t guarantee infection. Plenty of dogs breathe in fungal spores without ever developing disease. The difference usually comes down to how well the dog’s immune system responds. Dogs with weakened or suppressed immune systems, whether from medications, underlying illness, or simple age, are more vulnerable. Very young puppies whose immune systems are still developing also face higher risk.
The dose of spores matters too. A dog that digs enthusiastically in contaminated soil inhales far more spores than one walking casually past it. Hunting dogs, sporting breeds that spend extensive time in wooded or marshy areas, and dogs that dig obsessively have more opportunities for heavy exposure. Geography is perhaps the biggest factor of all. A dog living in central Arizona faces meaningful Valley fever risk that a dog in Maine simply does not, while a dog near the Great Lakes region is far more likely to encounter blastomycosis.
How Fungal Infections Are Identified
Fungal infections can be tricky to pin down because the early symptoms, like coughing, lethargy, weight loss, or skin sores, overlap with many other conditions. The long incubation period for systemic infections adds another layer of difficulty. A dog that hiked near a river two months ago may not show symptoms until well after the trip is forgotten.
Veterinarians use a combination of approaches to confirm a fungal diagnosis. Imaging like X-rays or CT scans can reveal characteristic lung patterns. Blood tests that detect fungal antigens or antibodies can point toward specific organisms. The most definitive diagnosis often comes from directly examining tissue or fluid samples under a microscope to identify fungal structures. For nasal infections caused by aspergillus, direct visualization of fungal growth inside the nasal passages using a tiny camera is considered the gold standard. No single test catches every case, so vets often combine several methods when a fungal infection is suspected.
Reducing Your Dog’s Exposure
You can’t eliminate fungal exposure entirely, but you can reduce it meaningfully. If you live in a Valley fever region, limiting your dog’s time in dusty, undisturbed desert soil helps, especially after rainy seasons followed by dry spells. Keeping dogs away from digging near waterways in blastomycosis-prone areas lowers that particular risk. Avoiding areas with heavy bird or bat droppings reduces histoplasmosis exposure.
For ringworm, the calculus is simpler: avoid prolonged contact with animals that have patchy hair loss or crusty skin lesions, and make sure grooming tools are cleaned between uses. If you foster or board dogs, keeping new arrivals separated until you’re confident they’re healthy is a practical step. Knowing which fungi are common in your specific region is the single most useful thing you can do, because it tells you which exposures to take seriously and which symptoms to watch for.