Blastomycosis (“blasto”) is an infection caused by a fungus called Blastomyces. Dogs and people catch it by breathing in microscopic spores from damp soil, rotting wood and leaves.
How the fungus works
Outdoors, Blastomyces grows as a mold in moist soil and decomposing plant matter. When that soil is disturbed, it releases spores too small to see. Once a dog or person breathes them in, body heat turns the spores into yeast that grows in the lungs. From there it can spread through the body to the eyes, skin, bones, lymph nodes and brain.
Who gets it
- Dogs are by far the most commonly affected animal, and are estimated to be about 10 times more likely to develop the disease than people.
- People get it too. About half of people who breathe in the spores never get sick; the rest usually develop a pneumonia-like illness.
- Cats are affected far less often than dogs.
How serious it is
- With prompt treatment, roughly 50–80% of dogs recover, though about 1 in 5 relapse.
- In severe cases, up to half of dogs with serious lung disease die within the first week of treatment, so early diagnosis matters.
- In people, the CDC estimates 8–10% of hospitalized patients die each year (17% in 2021).
Why it gets missed
- It looks like other illnesses. In a 2026 study of 54 patients in the Albany, NY area, nearly all were first misdiagnosed, most often with pneumonia or cancer.
- Antibiotics don’t work on it, because it’s a fungus, not a bacterium. A “pneumonia” that doesn’t improve on antibiotics is a red flag.
- Symptoms show up weeks to months after exposure, so few people connect them to a walk by the river.
- Doctors and vets outside the “classic” areas rarely expect it. See Where It Lives.
There is no vaccine, and there is no public test for the fungus in soil.
Blasto Awareness shares public information for awareness only. It is not veterinary or medical advice. If you’re worried about your dog or yourself, talk to your vet or doctor. Sources: CDC, Merck Veterinary Manual, MiraVista Diagnostics, Purina Pro Club, Emerging Infectious Diseases (2026). Full list on the Sources page.