Blastomycosis ("blasto") is a fungal infection dogs catch by breathing in spores from damp soil, usually near lakes, rivers and streams in the Ohio, Mississippi, Missouri and Tennessee river valleys, around the Great Lakes and along the St. Lawrence. It most often starts in the lungs, then can spread to the eyes, skin, bones and other organs.
It is serious, but it is treatable: most dogs need several months of antifungal medicine, and dogs caught before the lungs are badly affected usually do well. And no, your dog can't pass it to you or to your other dog. You can, however, breathe in spores from the same ground.
What is blastomycosis?
Blastomycosis is an infection with Blastomyces fungi, mainly Blastomyces dermatitidis. The fungus is dimorphic, which means it lives in two forms. In soil it grows as a mold that releases tiny spores. Once a dog breathes those spores in, the warmth of the body turns them into yeast, and the yeast causes the disease.
Dogs and people are the species it affects most often, the Merck Veterinary Manual notes. Cats, horses and some wildlife get it less often. It is one of a small group of soil fungi that cause whole-body infections in dogs; its western cousin is Valley fever.
Where does blasto live?
In North America, most cases come from:
- the Mississippi, Missouri, Ohio and Tennessee river basins
- the Great Lakes region
- the St. Lawrence River and Seaway
- parts of the Pacific Northwest and western US
- in Canada, mainly Ontario, Quebec and Manitoba
The CDC's map of estimated areas shows the fungus may be in the soil across much of the eastern US. But it is patchy. It grows in small pockets even inside those regions, and it can turn up outside them, so "we don't live in blasto country" is not a reason for your vet to rule it out.
What the fungus likes is wet ground: moist, acidic, often sandy soil full of rotting leaves and wood, the kind you find along riverbanks, lake shores and beaver dams. It has also been found in bird and bat droppings. Merck notes that most dogs with blastomycosis live within about 400 meters (a quarter mile) of a waterway.
Spores get into the air when that soil is disturbed: digging, a dog nosing through leaf litter, construction or excavation near water. Rain, dew and fog may help release them too, and case numbers can jump after unusually wet spells. In one Michigan study, about 70% of dog cases were diagnosed between June and November.
How dogs catch blastomycosis
Almost always by breathing in spores. In the lungs the spores turn into yeast. If the lungs can't contain them, the yeast travels through the blood and lymph to other parts of the body.
Less often, the fungus gets in through a skin wound. Skin sores are far more often a sign that the infection has spread from the lungs.
Signs don't show up right away. Veterinary Partner gives an incubation period of 5 to 12 weeks, so a dog can get sick well after a camping trip or a fall of hunting.
Why dogs get it more than people
Dogs are about 10 times more likely to get blastomycosis than people and about 100 times more likely than cats, according to Veterinary Partner. Why isn't fully known; Veterinary Partner says the immune system may play a role. Exposure is likely part of it too. Picture what a dog does on a walk by the river: nose in the leaves, paws in the mud, digging after a smell. That is likely a lot more spore exposure than its owner gets standing on the path.
Any dog can get it, including small dogs and older dogs. The dogs seen most often are:
- young adult, large-breed dogs; historically, young males of about 1 to 5 years have had the highest risk
- sporting and hunting dogs (retrievers, pointers, hounds), because of where they spend their time
- dogs that live or travel near water in the regions above
Signs of blastomycosis in dogs
The signs depend on which organs are infected, and none of them is specific to blasto. Many dogs first show general signs:
- fever
- tiredness and low energy
- poor appetite
- weight loss
Then come signs from the organs involved.
Lungs. The most common site: Merck reports lung signs in as many as 85% of affected dogs. Look for a cough, faster or harder breathing, and tiring quickly on walks. Severe lung disease can starve the body of oxygen, which is an emergency. A cough has many other causes, too; see common reasons for a dog cough.
The difference between those two X-rays is the kind of change vets look for. Merck describes small nodules scattered through the lungs and denser areas around the airways as the typical pattern.
Eyes. Eye problems show up in about 30% to 50% of dogs. You may see redness, cloudiness, squinting from pain, sensitivity to light or sudden blindness. Inside the eye, blasto can cause uveitis (painful inflammation), glaucoma (high pressure in the eye) and retinal detachment.
Skin. About half of dogs get skin lesions: small lumps, pimples or sores that ooze or drain, often on the nose, face and nail beds. Infected nail beds can make a dog limp.
Lymph nodes. About half of dogs have one or more swollen lymph nodes, for example under the jaw or in front of the shoulders.
Bones. About 1 in 4 dogs limps because of a bone infection or badly infected toes.
Urinary and reproductive tract. Blood in the urine, straining to urinate or enlarged testicles.
Nose. Discharge from the nose, sometimes bloody.
Brain. Uncommon, under 5% of dogs, but serious: seizures, a head tilt, dullness or other neurological changes.
How vets diagnose blastomycosis
Because the signs look like many other diseases, the first clue is often where your dog has been. Tell your vet about walks, hunting trips, camping or a move from a blasto region, even if it was a couple of months ago.
Your vet may use:
- Chest X-rays to look for the lung pattern above. More on what to expect with dog X-rays.
- Cytology or a biopsy: a sample from a skin sore, a swollen lymph node or fluid from the airways, looked at under the microscope. Seeing the thick-walled, budding yeast confirms the diagnosis.
- Urine antigen test: a lab test that finds a piece of the fungus's cell wall in urine. It is the most sensitive test for blasto, and a blood version exists too. It can react to the closely related fungus that causes histoplasmosis, so vets read it alongside the rest of the picture. It can also help track treatment.
- Blood work and other imaging (ultrasound, CT) to check organs and overall health.
Antibody tests can miss infected dogs, so a negative antibody result doesn't rule blasto out.
Treatment for blastomycosis in dogs
Blasto needs antifungal medicine, usually for several months. Your vet chooses the medicine and the dose, and most dogs take it by mouth at home. How long depends on the dog: the Merck Veterinary Manual describes at least two to three months, continued until the disease is no longer active, and some dogs need longer. Your vet decides when to stop based on rechecks, which may include X-rays, blood work and repeat urine antigen tests. Stopping too early raises the chance the infection comes back.
What else to expect:
- Dogs with breathing trouble may need a hospital stay with oxygen, fluids and medicine through a vein until they are stable.
- It can get worse before it gets better. In dogs with severe lung disease, breathing can get worse early in treatment as the fungus dies, so these dogs are watched closely for the first couple of weeks. Call your vet at once if your dog's breathing gets worse. Some dogs don't look better for a week or two.
- Rest. Dogs with lung involvement are usually kept on restricted activity.
- Eyes respond less well, because the medicine doesn't reach the inside of the eye well. A blind, painful eye sometimes has to be removed.
- Skin sores may need cleaning by the vet team.
Don't swap in leftover or over-the-counter antifungal products. The right medicine depends on your dog's case, and your vet checks how it is working at each recheck.
Outlook
Most dogs that are treated recover. Merck reports a clinical cure in about 70% of treated dogs, and Veterinary Partner gives a range of 50% to 75%. The outlook depends mostly on the lungs and the brain:
- Best: mild lung disease
- More guarded: moderate to severe lung disease
- Poorest: brain involvement, or three or more body systems involved
Dogs that are very sick at diagnosis are at the most risk in the first week or so of treatment. Vision after eye involvement is hard to predict.
Blasto can come back. About 1 in 5 treated dogs relapses, sometimes months or even years later, most often within the first six months after treatment ends. Most respond to treatment again. A dog that recovers is not immune, so the same riverbank can infect it twice.
Can people or other pets catch blastomycosis from a dog?
From your dog, practically no. Dogs and people both catch blasto from the environment, not from each other. Merck notes that people don't catch it from animals, and Veterinary Partner says it doesn't spread from dog to person or from dog to dog.
There are rare exceptions. The CDC notes that people have, extremely rarely, been infected through an animal bite. Merck reports veterinary staff infected by needle-stick injuries while sampling skin sores. And Veterinary Partner advises people with a weakened immune system to limit close contact with an infected dog and wear gloves when cleaning draining sores.
The real link is shared ground. If your dog picked up blasto, the spores may be in places you walk, garden or fish too. In a 2021-2022 Wisconsin cluster, five people and six dogs in one riverside neighborhood got sick, and a veterinarian reported the dog cases first. People's symptoms look like a lingering chest infection: fever, cough, shortness of breath, night sweats, muscle aches, weight loss and fatigue, the CDC says. If you get sick, tell your doctor your dog was diagnosed with blastomycosis.
The same goes for other pets in the house. They can't catch it from the sick dog, but they share its walks. Tell your vet if another dog starts coughing, losing weight or limping.
Can you prevent blastomycosis?
Not completely. There is no vaccine, and the fungus can't be removed from the environment. In known blasto areas you can lower your dog's exposure:
- Keep your dog from digging or nosing in damp soil along lakes, rivers, beaver ponds and rotting logs.
- Steer clear of freshly dug or excavated ground near water.
- Think about a leash on wet, wooded trails by the water. In the Wisconsin neighborhood, dogs walked on the riverside trails, especially off leash, were more likely to carry antibodies to the fungus, a sign they had been exposed.
- Tell your vet about trips to blasto areas, so a cough or limp weeks later gets the right test sooner.
A small dog that walks on pavement and mowed parks has a much lower risk than a retriever that spends weekends in river-bottom woods. Neither is zero.
When to call your vet
Call now:
- hard, fast or noisy breathing, or breathing with the mouth open at rest
- breathing that gets worse after treatment has started
- blue, gray or very pale gums
- collapse or seizures
- a suddenly painful, red or cloudy eye, or sudden blindness
See your vet soon:
- a cough that won't go away, weight loss, fever or a poor appetite, especially after time near water in a blasto region
- skin sores or lumps that drain or don't heal, particularly on the nose, face or nail beds
- a limp with no obvious injury, or swollen lymph nodes
- blood in the urine
Mention at your next visit:
- recent trips or hunting in blasto areas, even if your dog seems well
- if your dog is being treated: ask how long treatment will run and when the next recheck and urine test are due
Quick answers
Is blastomycosis contagious? Not from dog to dog or dog to person. Animals and people catch it by breathing in spores from the soil. People who live with a sick dog may have been near the same source, though.
Can a dog survive blastomycosis? Yes, most treated dogs do, especially when the lungs are only mildly affected. Brain involvement and severe lung disease carry the poorest outlook.
How long does treatment take? Usually several months. The Merck Veterinary Manual describes at least two to three months, and your vet sets the end date from rechecks.
Can blastomycosis come back? Yes, in about 1 in 5 treated dogs, mostly within six months of stopping treatment. Most respond to treatment again.
Do cats get it? Yes, but far less often than dogs. Even indoor-only cats have been diagnosed, Cornell notes.
Written and updated by the I Love Veterinary editorial team.
Sources
- Merck Veterinary Manual: Blastomycosis in Animals
- Merck Veterinary Manual: Fungal Infections in Dogs (Blastomycosis)
- Veterinary Partner (VIN): Blastomycosis is a Systemic Fungal Infection Affecting Dogs and Cats
- Cornell University College of Veterinary Medicine: Blastomycosis
- CDC: Where Blastomyces Lives
- CDC: Risk Factors for Blastomycosis
- CDC: About Blastomycosis
- Segaloff et al., Emerging Infectious Diseases (2024): Human and Canine Blastomycosis Cases Associated with Riverside Neighborhood, Wisconsin
- Shelnutt et al., Medical Mycology (2019): Prevalence, distribution, and risk factors for canine blastomycosis in Michigan
This article is for general information and is not a substitute for advice from your veterinarian.
