A BD infection most commonly refers to blastomycosis, a fungal infection caused by the organism Blastomyces dermatitidis (often shortened to B.d. or BD in medical shorthand). It primarily affects the lungs after a person breathes in fungal spores from contaminated soil, though it can spread to the skin, bones, and other organs. Symptoms typically appear three weeks to three months after exposure and often mimic pneumonia, which makes it easy to misdiagnose.
How You Get Blastomycosis
Blastomyces lives in moist soil and decomposing organic matter like rotting wood and leaves. In the environment, the fungus grows as a mold and releases tiny spores. When soil or plant material is disturbed, whether by construction, digging, hiking, or even strong wind, those spores become airborne. Breathing them in is the primary route of infection. You cannot catch blastomycosis from another person.
The fungus is concentrated in the midwestern, south-central, and southeastern United States, particularly in areas near rivers and lakes where soil stays damp. Wisconsin reports the highest incidence of any state, with some northern counties seeing 10 to 40 cases per 100,000 people each year. People who spend time outdoors in these regions, especially those involved in activities that disturb soil, face the greatest exposure risk.
Symptoms of Blastomycosis
Blastomycosis usually starts in the lungs and produces symptoms that look a lot like pneumonia. The most common signs include:
- Fever
- Cough and shortness of breath
- Night sweats
- Muscle aches or joint pain
- Chest, rib, or back pain
- Weight loss
- Extreme tiredness
Because these symptoms overlap so heavily with bacterial pneumonia, tuberculosis, and even lung cancer, blastomycosis is frequently missed on the first visit. Some people also develop skin lesions, which can appear as raised bumps, blisters, or open ulcers. Skin involvement is often the clue that points doctors toward a fungal cause rather than a bacterial one.
Who Faces the Highest Risk
Anyone who breathes in the spores can develop blastomycosis, but most healthy people either fight off the infection without knowing it or experience only mild illness. The people at greatest risk for severe or widespread disease are those with weakened immune systems. This includes people living with advanced HIV/AIDS, organ transplant recipients, anyone taking corticosteroids or immune-suppressing medications, and people who are pregnant.
When the infection spreads beyond the lungs, it can reach the skin, bones, joints, and in rare cases the brain. Bone and joint involvement is particularly stubborn and more likely to relapse after treatment, often requiring at least a full year of antifungal therapy.
How Blastomycosis Is Diagnosed
Diagnosis relies on a combination of lab tests, with urine and blood antigen testing being the most practical starting point. A urine antigen test picks up the infection in 76% to 93% of cases, while blood antigen testing is somewhat less reliable at 56% to 82%. One important caveat: both tests can cross-react with histoplasmosis, another fungal infection found in overlapping geographic areas. That means a positive antigen test sometimes needs confirmation through a tissue sample or culture.
Doctors may also collect sputum (mucus from the lungs) or take a biopsy of skin lesions to look for the organism directly under a microscope. Growing the fungus in culture remains the definitive proof, though it can take several weeks.
Treatment and Recovery
Mild to moderate blastomycosis is treated with an oral antifungal medication, typically taken for 6 to 12 months. This long treatment window is necessary because fungal infections clear slowly, and stopping too early raises the risk of relapse. The antifungal used in most cases has a success rate of about 90% in clinical trials.
Severe cases, such as those involving widespread disease or serious lung involvement, usually start with a stronger antifungal given intravenously in a hospital for one to two weeks. Once the patient stabilizes, treatment switches to oral medication for the remainder of the 6- to 12-month course. During treatment, blood levels of the medication are checked after about two weeks to make sure enough drug is being absorbed.
Despite effective treatment options, blastomycosis carries real risks. Among patients sick enough to be hospitalized, 8% to 10% die annually in the United States. That figure jumped to 17% in 2021, likely influenced by complications related to the COVID-19 pandemic. Early diagnosis makes a significant difference in outcomes, which is why recognizing the possibility of a fungal infection, especially in people who live or recreate in endemic areas, matters so much.
Why Blastomycosis Gets Misdiagnosed
The biggest challenge with blastomycosis is that it doesn’t announce itself clearly. A person with fever, cough, and fatigue in Wisconsin or Arkansas is far more likely to be diagnosed with bacterial pneumonia or the flu on their first visit. Antibiotics are often prescribed, and when they don’t work, the diagnostic search widens. Skin lesions, when present, sometimes get mistaken for bacterial skin infections like cellulitis, leading to additional rounds of unnecessary antibiotics before the true cause is identified.
If you’ve had a persistent cough and flu-like symptoms that haven’t responded to standard treatment, and you live in or have recently visited the Great Lakes region, Ohio and Mississippi River valleys, or southeastern states, bringing up the possibility of a fungal infection with your doctor can speed up the path to the right diagnosis.