Blastomycosis in Dogs: Signs, Treatment, and What Owners Need to Know
Blastomycosis is one of the most serious fungal infections dogs can contract. It starts in the lungs but can spread throughout the body – to the skin, eyes, bones, and brain. Early recognition, prompt diagnosis, and sustained antifungal treatment are what determine whether a dog survives. Here is what every owner in endemic regions should understand.
What Is Blastomycosis?
Blastomycosis is a systemic fungal infection caused by Blastomyces dermatitidis (and the recently recognized related species Blastomyces gilchristii). The organism lives in moist, sandy, acidic soil enriched with decomposing organic material – particularly near rivers, lakes, wetlands, and beaver activity.
Infection occurs when a dog inhales microscopic spores from disturbed soil. Once inhaled, the spores convert to a yeast form in the warm, moist lung environment and begin to multiply. From the lungs, the infection can spread through the bloodstream to virtually any organ system. Dogs that are large, outdoor-active, and spend time near water in endemic areas are at highest risk.
Blastomycosis is endemic to the central and eastern United States, particularly the Mississippi River valley, Ohio River valley, Missouri River valley, and Great Lakes region. Cases also occur in parts of Canada adjacent to these watersheds. The infection cannot spread between animals or from animal to human – the only source is soil exposure.
Why Dogs Are At Higher Risk Than Humans
Dogs develop blastomycosis at rates 10 times higher than humans in the same geographic areas. The reason is behavioral and anatomical: dogs explore the world with their nose, spending significant time sniffing and digging in exactly the type of moist, rich soil where Blastomyces thrives. Their nose stays close to and in the soil, maximizing spore inhalation. Dogs in endemic areas with outdoor access to river banks, lake shores, and wetlands are at substantially elevated risk compared to urban dogs.
Large-breed sporting and hunting dogs – Labrador Retrievers, Weimaraners, Golden Retrievers, Pointers – are overrepresented in blastomycosis cases, reflecting their high outdoor activity in water-adjacent environments. Male dogs are slightly more affected than females, possibly due to behavioral differences in territorial roaming.
Signs of Blastomycosis: Which Body Systems Are Affected
Blastomycosis is a multisystem disease, but the lung is the primary site of infection. Clinical signs depend on which systems are involved and how severely. Onset is usually gradual over days to weeks, which can make early recognition difficult.
Cough, labored breathing, exercise intolerance, rapid breathing. Chest X-rays typically show a diffuse nodular pattern. Most dogs with blastomycosis show lung involvement.
Raised firm nodules, often on the face, nose, or limbs. Lesions may ulcerate and drain a serosanguineous or purulent discharge. New lesions can appear on treatment.
Uveitis (eye inflammation), cloudiness, redness, pain, and blindness. Ocular involvement is one of the more serious complications – affected eyes may not recover vision even with treatment.
Enlarged lymph nodes, often palpable in the neck, armpits, and groin. Lymphadenopathy is a common early finding and can be the initial clue that raises suspicion for fungal disease.
Bone pain, lameness, or swelling in an affected limb. Blastomycosis can destroy bone tissue and may mimic bone tumors on radiographs, leading to diagnostic confusion.
Seizures, behavioral changes, circling, or cranial nerve deficits. CNS involvement carries the worst prognosis and usually requires more aggressive antifungal treatment including amphotericin B.
Ocular blastomycosis can cause permanent blindness. A dog with cloudy eyes, red eyes, or apparent eye pain alongside other signs of illness in an endemic area should be evaluated urgently. Delaying treatment by even days can mean the difference between preserving vision and losing it. An ophthalmology consultation alongside medical treatment gives the best chance of saving affected eyes.
How Vets Diagnose Blastomycosis
No single test diagnoses blastomycosis definitively in every case. Vets typically use a combination of clinical history, physical exam, imaging, and diagnostic tests.
- Cytology: The fastest and most accessible diagnostic method. Fine-needle aspirates of lymph nodes, skin lesions, or lung washes (transtracheal wash) are examined under a microscope. Blastomyces yeast cells have a distinctive thick double-contoured wall that is recognizable to an experienced cytologist. A positive cytology result is diagnostic.
- Urine antigen testing: The MiraVista Blastomyces urine antigen assay is highly sensitive (around 90%) and specific. A positive urine antigen test in a dog with compatible clinical signs and geographic history is strong evidence for blastomycosis. It is also useful for monitoring treatment response.
- Serology (antibody testing): Less reliable than antigen testing due to variable sensitivity. A negative result does not rule out infection.
- Culture: Definitive but slow (weeks) and dangerous to handle. Not routinely performed.
- Chest radiographs: Typically show a diffuse nodular or interstitial pattern – described as “snowstorm” pattern in classic cases. Important for staging severity and monitoring response.
Treatment: Antifungal Medications
Itraconazole is the standard first-line treatment for blastomycosis in dogs. It is given orally, typically with a fatty meal to improve absorption, once or twice daily depending on the formulation used.
Amphotericin B, given intravenously, is reserved for severe or CNS blastomycosis cases that need rapid antifungal activity. It carries significant nephrotoxicity risk and requires hospitalization with close monitoring. For most dogs, oral itraconazole is appropriate.
One of the most common reasons blastomycosis relapses is premature discontinuation of itraconazole. Treatment should continue for at minimum 60 days and until the dog has been clinically normal for 30 days and antigen levels have returned to negative or baseline. Dogs with severe initial disease, eye involvement, or bone involvement may need 4 to 6 months of treatment. Stopping early because the dog “looks better” frequently leads to relapse within weeks to months.
Severity Classification and Prognosis
Cough and mild exercise intolerance. Chest X-rays show focal or mild diffuse changes. Dog remains alert and maintains appetite with some support. Prognosis with treatment is good – survival rates above 80%.
Skin, lymph node, or eye involvement alongside lung changes. Dog is ill but not in acute respiratory failure. Prognosis is fair to good with sustained treatment – survival around 70-80%.
Open-mouth breathing, cyanosis, CNS signs, or extremely widespread disease. These dogs require hospitalization, oxygen support, and IV antifungals. Survival is reduced to 50-60% in acute respiratory distress. CNS involvement worsens prognosis further.
Some dogs experience a paradoxical worsening of respiratory signs in the first 24 to 48 hours after starting itraconazole. This is thought to result from the inflammatory response triggered by dying fungi. Dogs with marginal respiratory function at the start of treatment may deteriorate rapidly during this window. Hospitalization and close monitoring during the first days of treatment is recommended for any dog with significant respiratory compromise at presentation.
Monitoring Treatment Response
Monitoring is essential because blastomycosis treatment is long and relapse is common. A typical monitoring plan includes:
- Recheck exam and body weight every 2 to 4 weeks during treatment
- Liver enzyme (ALT) monitoring every 4 to 6 weeks – itraconazole can cause hepatotoxicity
- Urine antigen testing at 1 to 2 month intervals to track antigen decline
- Repeat chest radiographs at treatment midpoint and before discontinuing treatment
- Ophthalmology rechecks every 2 to 4 weeks if eyes were affected
A declining antigen level and improving clinical signs together indicate treatment is working. If antigen levels plateau or the dog is not improving clinically after 4 to 6 weeks, the vet should reconsider the diagnosis, check for drug absorption issues, or consider switching or adding antifungal therapy.
Relapse and Long-Term Outlook
About 20 percent of dogs that are successfully treated for blastomycosis relapse within 6 to 12 months. Relapse is more likely in dogs that had severe initial disease, had eye or CNS involvement, or stopped treatment early. A dog that relapses should be retreated with itraconazole for at minimum 90 days, and some cases may require indefinite suppressive therapy.
Dogs that survive blastomycosis and complete treatment can live normal lives. Residual lung scarring does not typically cause significant long-term respiratory problems. Eyes that were affected may have permanent scarring or vision loss even after the active infection is controlled. Bone lesions heal slowly and may remain visible on X-rays for months after cure.
Reducing Exposure Risk
There is no vaccine for blastomycosis. Prevention focuses on reducing exposure in endemic areas:
- Avoid allowing dogs to dig or root in moist soil near water bodies in endemic regions
- Keep dogs out of areas with disturbed, freshly turned soil near rivers or lakes
- Be especially cautious during dry spells when previously wet soil is disturbed
- Hunting and sporting dogs in endemic areas should be monitored closely for early signs of illness after field work
- Know the signs – early treatment dramatically improves outcomes
Frequently Asked Questions
Time Is the Critical Variable
Blastomycosis is a serious infection, but it is also a treatable one. The dogs that fare best are those whose owners recognized something was wrong, sought veterinary care quickly, and committed to the full course of treatment. In endemic regions, a dog with respiratory signs, skin nodules, or unexplained eye changes deserves urgent evaluation for fungal disease – not just antibiotics for a routine respiratory infection.