Blastomycosis is a fungal lung infection caused by breathing in spores of a fungus called Blastomyces, which lives in moist soil and decaying plant matter. About half of people who inhale the spores never develop symptoms. For those who do, illness typically appears between three weeks and three months after exposure, most often resembling pneumonia.
How People Get Infected
Blastomyces thrives in damp soil rich in decomposing wood and leaves, particularly near waterways. When that soil is disturbed, whether by construction, digging, hiking, or even a dog rooting around, tiny fungal spores become airborne. Breathing those spores in is the only way humans get infected.
Blastomycosis does not spread from person to person, and you cannot catch it from a pet. If your dog is diagnosed with blastomycosis, it means the dog was exposed to the same contaminated soil you might encounter, not that the dog poses a direct risk to you.
Where Blastomyces Lives
The fungus is concentrated in the Great Lakes region, the Ohio and Mississippi River valleys, and parts of southeastern Canada. Wisconsin consistently reports the highest case numbers of any U.S. state, with some northern counties seeing 10 to 40 cases per 100,000 people each year. Northern Minnesota and western Ontario are also hotspots. Outside these areas, blastomycosis is uncommon, with the national incidence sitting at roughly 2 cases or fewer per 100,000 people.
Actual infection numbers are higher than what gets reported, because testing is limited and the disease can be mistaken for bacterial pneumonia or even lung cancer on imaging.
Symptoms to Recognize
When symptoms do develop, they center on the lungs. The most common pattern looks like pneumonia that doesn’t improve with standard antibiotics:
- Fever
- Cough and shortness of breath
- Night sweats
- Muscle aches or joint pain
- Chest, rib, or back pain
- Fatigue
- Weight loss
Some people also develop skin lesions: raised bumps, blisters, or ulcers that can appear on the face, arms, or legs. In more advanced cases, the infection can spread beyond the lungs to the bones, joints, or central nervous system. This dissemination is more likely in people with weakened immune systems.
Who Faces the Greatest Risk
Healthy people who live or recreate in endemic areas can and do get blastomycosis, so a strong immune system is not fully protective. That said, severe and disseminated disease is significantly more common in people with compromised immunity. The highest-risk groups include people living with advanced HIV/AIDS, organ transplant recipients, those taking corticosteroids or TNF-inhibitor medications, and people who are pregnant.
Outdoor activities that disturb soil, like hunting, camping, landscaping, and farming in endemic regions, increase the chance of inhaling spores simply by putting you closer to the source.
How Blastomycosis Is Diagnosed
Diagnosis can be tricky because the symptoms overlap with bacterial pneumonia, tuberculosis, and certain cancers. Doctors often suspect blastomycosis only after antibiotics fail to improve a lung infection, especially in someone who lives in or has recently visited an endemic area.
A urine antigen test is one of the fastest and most practical options, detecting fungal proteins with a sensitivity between 76% and 90%. Results come back relatively quickly, making it a useful first step. Growing the fungus from a sputum or tissue sample in a lab (culture) remains the gold standard for confirming the diagnosis, but it takes time: anywhere from one to four weeks, sometimes longer. DNA-based tests and tissue examination under a microscope can also support the diagnosis, though no single test catches every case. Doctors often combine several tests to build confidence in the result.
Treatment and Recovery
All confirmed cases of blastomycosis require antifungal treatment. For mild to moderate lung infections, the standard approach is a daily oral antifungal, typically taken for 6 to 12 months. This oral treatment clears the infection in roughly 90% to 95% of patients.
Severe cases, including those involving the brain, bones, or widespread dissemination, usually start with a stronger intravenous antifungal given in the hospital for one to two weeks. Once the patient stabilizes, treatment switches to oral antifungals for the remainder of the course. Bone or central nervous system involvement often requires longer treatment beyond the standard 6 to 12 months because of higher relapse rates.
The death rate among hospitalized patients runs 8% to 10% annually in the United States, a figure that jumped to 17% in 2021, likely influenced by the pandemic straining hospital resources. Early diagnosis improves outcomes significantly, which is why blastomycosis should be on the radar for anyone in an endemic area whose pneumonia symptoms aren’t responding to typical antibiotics.
Blastomycosis in Dogs
Dogs are especially susceptible because they spend time with their noses close to the ground, sniffing and digging in exactly the kind of soil where Blastomyces grows. Symptoms in dogs include skin sores (often on the face or paws), coughing, wheezing, unexplained limping, difficulty seeing, and weight loss. If your dog is diagnosed, it’s worth considering that you may share the same environmental exposure, but again, the infection cannot pass directly between animals and people.