Blastomycosis in Dogs — Management Guide

Practical, evidence-based guide to diagnosis, treatment, and long-term management of blastomycosis in dogs, including itraconazole therapy and urine antigen monitoring.

Quick Overview

• What it is: Blastomycosis is a systemic fungal infection caused by Blastomyces species (typically Blastomyces dermatitidis or B. gilchristii). Dogs inhale infectious spores from the environment; the fungus classically causes pulmonary disease and frequently spreads (disseminates) to skin, eyes, bones, lymph nodes and sometimes the central nervous system (CNS).
• Who's at risk: Dogs with outdoor exposure in endemic regions — notably the Great Lakes region, and the Ohio and Mississippi River valleys — are at highest risk. Sporting and hunting breeds, dogs that swim or dig in soil or freshwater shorelines, and young adult dogs are often overrepresented.
• Prognosis: With prompt diagnosis and appropriate antifungal therapy prognosis ranges from guarded to good for localized or moderately severe disease; severe pulmonary, CNS, or multi-organ disease has a worse prognosis. Many dogs recover with prolonged treatment (commonly 6 months or longer), but relapses can occur.

Pathophysiology (explained simply)

Blastomyces lives in moist soil and decomposing organic material. Dogs inhale airborne spores (conidia). In the lungs, spores convert to the yeast form, multiply, and provoke inflammation. From the lungs the organism often spreads via the bloodstream to other tissues (skin, bones, eyes, lymph nodes, occasionally the brain). Tissue damage is caused by the host inflammatory response and direct fungal invasion.

Key points:

• Primary inhalation → pulmonary infection.
• Hematogenous dissemination is common; cutaneous lesions are frequent and often help clinch the diagnosis.
• Infection is not considered contagious from dog to dog or dog to human in everyday contact; transmission risk is primarily environmental.

Geographic distribution and epidemiology

Blastomycosis is classically endemic in specific areas of North America:

• Great Lakes region (especially along shorelines and river systems)
• Ohio and Mississippi River valleys
• Some parts of the southeastern and south-central U.S.

Breed- and signalment-related risk factors

• Overrepresented: sporting and hunting breeds (Labrador Retrievers, Labrador mixes), although any breed can be affected.
• Age: often middle-aged to young adult dogs, but all ages can be affected.
• Sex: some case series report a male predominance, possibly reflecting behavioral exposure differences.
• Lifestyle: dogs that swim, hunt, or have frequent freshwater/soil exposure are at increased risk.

Clinical signs — pulmonary vs disseminated forms

Pulmonary disease (primary)

• Cough (often chronic)
• Exercise intolerance, tachypnea, increased respiratory effort
• Fever, lethargy, inappetence

• Skin: ulcerated or nodular skin lesions, draining tracts
• Eyes: uveitis, chorioretinitis, vision changes
• Bones/joints: lameness, osteomyelitis
• Lymph nodes: enlargement
• CNS: seizures, ataxia, behavioral changes (less common but serious)

• Mild: cough, minimal systemic signs, stable vital signs
• Moderate: marked cough, exercise intolerance, fever, visible skin lesions or lymphadenopathy
• Severe/critical: respiratory distress, hypoxia, CNS signs, multi-organ dysfunction

Diagnostic approach

When to involve a specialist

• Severe respiratory compromise, CNS involvement, need for amphotericin B or surgical debridement, or when advanced imaging/bronchoscopy is indicated — referral to a specialty (internal medicine/surgery) center is appropriate.

Treatment options

Goal: eradicate infection and limit tissue damage while managing complications.

Medical therapy (first-line)

Surgical therapy

• Rarely required but may be useful to debulk localized masses, manage destructive bone lesions, or drain deep-seated collections. Surgery is adjunctive, not curative alone.

• Oxygen therapy for hypoxemia, IV fluids for dehydration or pre-renal azotemia, analgesia for bone pain, topical ophthalmic therapy for uveitis, and analgesics or antibiotics for secondary infections as indicated.

Monitoring response to therapy

Clinical signs

• Expect clinical improvement (reduced fever, improved appetite, decreased cough) within 7–14 days in many dogs. Failure to improve or worsening suggests severe disease, poor drug absorption, drug resistance, or complications.

• Thoracic radiographs lag behind clinical improvement; expect slow resolution over weeks to months. Repeating radiographs every 4–8 weeks can help assess progress.

• Useful both for diagnosis and for monitoring response.
• Strategy: obtain a baseline urine antigen before or at the start of therapy, then repeat every 4–8 weeks. Declining antigen concentrations correlate with response. Many dogs require months for the antigen to become negative; persistence of antigen does not always mean treatment failure immediately, but rising or plateauing antigen levels may indicate relapse or inadequate therapy.

• Recheck CBC and serum chemistry (especially liver enzymes) every 2–4 weeks for the first 1–2 months of azole therapy, then every 1–3 months thereafter depending on stability.

• Worsening clinical status, persistence or rise in urine antigen, progressive radiographic disease, renal or hepatic toxicity from drugs, or CNS involvement — discuss amphotericin B, other antifungals, or advanced diagnostics with a specialist.

Duration of therapy and relapse risk

• Minimum effective duration is typically long: many clinicians treat for at least 6 months and for at least 1–2 months beyond clinical and radiographic resolution. Others continue until urine antigen is negative and imaging is improved.
• Relapse can occur, particularly if treatment is stopped too early or absorption is poor. Follow-up and serial antigen testing help reduce relapse risk.

Prognosis and quality of life

• Good to fair for dogs with mild to moderate disease that receive appropriate therapy; survival rates vary with severity but many treated dogs do well long-term.
• Guarded to poor for dogs with severe respiratory compromise, CNS involvement, or multi-organ failure.
• With treatment, many dogs regain good quality of life; attention to monitoring, owner compliance with medication, and early re-evaluation for complications are key.

Living with blastomycosis — practical daily tips

• Medication adherence: give antifungals exactly as prescribed. Missing doses increases relapse risk.
• Feeding: give itraconazole with a small fatty meal if using capsule form (fat improves absorption); check with your vet whether your formulation requires food.
• Watch for side effects: loss of appetite, vomiting, jaundice, lethargy — contact your vet if these appear.
• Wounds/skin lesions: keep skin lesions clean and covered if draining; avoid allowing other pets to lick lesions.
• Eye involvement: follow veterinary ophthalmologist recommendations, use prescribed topical therapy as instructed.
• Limit strenuous exercise while recovering from pulmonary disease.
• Human safety: ordinary contact with treated dogs is not a major transmission risk; however, immunocompromised owners should discuss risks with their physician and veterinarian. Avoid handling fungal cultures or lesion drainage without protection.

When to see your vet urgently

Seek immediate veterinary care if your dog has any of the following:

• Marked respiratory distress (open-mouth breathing, blue gums, collapse)
• New or worsening neurological signs (seizures, stupor, severe ataxia)
• Severe vomiting, inappetence, or signs of liver disease (jaundice, dark urine)
• Sudden worsening of skin lesions or a large areas of new swelling or pain
• Possible severe reaction to medication (facial swelling, hives, severe vomiting)

Key takeaways

• Blastomycosis is a potentially serious systemic fungal disease endemic in the Great Lakes and Ohio/Mississippi valley regions.
• Urine Blastomyces antigen testing is a sensitive, noninvasive tool for diagnosis and monitoring, but can cross-react with other fungal antigens.
• Itraconazole is the common first-line therapy (typical dosing ~5–10 mg/kg/day, often divided q12h), with prolonged treatment (commonly many months) and regular monitoring of liver function.
• Severe cases may require amphotericin B and specialist care.
• Regular clinical review, serial imaging, and urine antigen testing are essential to guide therapy duration and detect relapse.

References and further reading

• Merck Veterinary Manual — Blastomycosis (fungal infection) (primary reference): https://www.merckvetmanual.com/generalized-conditions/fungal-infections/blastomycosis
• Centers for Disease Control and Prevention (CDC) — Blastomycosis: https://www.cdc.gov/fungal/diseases/blastomycosis/index.html
• Veterinary infectious disease and internal medicine texts (for detailed dosing, amphotericin B protocols, and specialist guidance).

Frequently Asked Questions