Blastomycosis in Dogs: Symptoms, Diagnosis, Treatment, and Geographic Guide
This article is reviewed for clinical accuracy. Always consult your veterinarian for diagnosis and treatment.
Key Takeaways
- Blastomycosis in dogs is caused by the dimorphic fungus Blastomyces dermatitidis and the closely related species Blastomyces gilchristii; the term “dimorphic” means that the organism exists in two distinct morphological forms depending on temperature: in the environment (soil, rotting wood, moist organic debris) at ambient temperatures below 35 degrees Celsius, Blastomyces grows as a filamentous mold producing infectious conidia (microscopic spores); when conidia are inhaled by a dog and reach the warm environment of the lung (approximately 37 degrees Celsius), they undergo thermally induced conversion to the yeast form, which is the pathogenic tissue form; yeast cells are large (8 to 15 micrometers in diameter), thick-walled, and reproduce by broad-based budding (the budding daughter cell is attached to the parent cell by a wide base, in contrast to Cryptococcus which buds on a narrow base), which is a key cytological and histological identifying feature.
- Blastomycosis is strongly associated with specific geographic and environmental niches: the Mississippi River and Ohio River drainage basins (which together span from Minnesota and Wisconsin south through Tennessee, Mississippi, Arkansas, and Louisiana), the Great Lakes region (particularly the area around Lake Michigan: Wisconsin, Michigan, Illinois, Indiana), the southern Appalachian Mountains, and the St. Lawrence River valley in Canada; within these broad endemic zones, infection is particularly concentrated near bodies of water, riverbanks, lake shores, beaver ponds, and areas with moist, sandy, or loamy soil rich in organic matter; the fungus grows saprophytically in decomposing organic material and is concentrated near water where soil conditions favor mycelial growth; dogs that hunt, swim, or roam near water in endemic areas carry a significantly elevated risk of exposure compared to dogs that stay in urban or suburban environments.
- Unlike the tick-borne diseases discussed in preceding posts, blastomycosis is acquired by inhalation of environmental conidia, not by arthropod vector; there is no effective vaccine and no preventive medication that reduces the risk of Blastomyces infection; the only practical prevention strategy is reducing exposure by limiting time in high-risk environments (riverbanks, lake shores, beaver pond areas in endemic regions) during periods of high environmental fungal burden; because conidia are microscopic and odorless, dogs (and humans) can be exposed without any awareness of the event; dogs are exposed at a much higher rate than humans because they spend more time with their noses in the soil, and the canine respiratory tract is proportionally larger relative to body mass, allowing more conidia per breath in contaminated environments.
- The lungs are the primary site of infection in almost all cases: inhaled conidia deposit in the terminal airways, convert to yeast, and trigger a pyogranulomatous inflammatory response (a mixed infiltrate of neutrophils, macrophages, epithelioid macrophages, and giant cells attempting to wall off and destroy the organism); the pulmonary lesions produce a characteristic diffuse interstitial to nodular pattern on thoracic radiographs that is frequently mistaken for metastatic cancer (lung metastases from a primary tumor elsewhere) because both appear as multiple discrete nodular opacities throughout the lung fields; this radiographic mimicry is one of the most clinically important features of blastomycosis and is a leading cause of diagnostic delay and misdiagnosis; every dog with a miliary to nodular interstitial pulmonary pattern on thoracic radiographs in an endemic area should have blastomycosis included in the differential diagnosis and should be tested before proceeding with a cancer workup.
- Dissemination from the primary pulmonary focus occurs in a significant proportion of dogs and can affect virtually any organ system; the most commonly affected extrapulmonary sites are: the skin (25 to 50 percent of disseminated cases, producing ulcerated granulomatous nodules that drain serosanguineous fluid), the eyes (20 to 50 percent of disseminated cases, producing uveitis, retinal detachment, secondary glaucoma, and blindness), lymph nodes (hilar and peripheral lymphadenopathy), bone (osteomyelitis producing lameness and bony proliferative lesions, particularly in the distal limbs), and the central nervous system (CNS blastomycosis, associated with the highest mortality of any disease form); the pattern of dissemination in an individual dog is unpredictable and depends on the host immune response and the size of the initial fungal inoculum.
- Itraconazole is the treatment of choice for blastomycosis in dogs at 5 mg/kg orally once daily (or divided twice daily) for a minimum of 60 to 90 days for mild to moderate pulmonary disease and longer for severe or disseminated disease; itraconazole should be given with a fatty meal to maximize absorption (it is a lipophilic drug); the human itraconazole capsule formulation (Sporanox) has significantly better bioavailability than generic itraconazole capsules in dogs, and compounded itraconazole formulations vary widely in bioavailability; for dogs that cannot tolerate itraconazole or have CNS involvement (where itraconazole penetrates poorly), fluconazole (2.5 to 5 mg/kg twice daily) is an alternative with better CNS penetration; amphotericin B (AMB) is reserved for severe, life-threatening pulmonary blastomycosis or CNS disease where rapid fungicidal activity is needed and the dog is hospitalized; the overall prognosis with appropriate antifungal treatment is reasonably good (survival rates of 50 to 75 percent in most series), but CNS blastomycosis and severe respiratory compromise at presentation carry a guarded to grave prognosis.
The 3-year-old Labrador Retriever had been coughing and losing weight for three weeks when the referring veterinarian sent thoracic radiographs along with the referral note. The images showed a diffuse miliary nodular interstitial pattern throughout both lung fields that the referring clinician described as “highly suspicious for pulmonary metastatic disease.” The dog had been healthy until three weeks earlier; the owner reported regular swimming in a lake in northern Wisconsin and hunting excursions along river corridors throughout the summer. On presentation, the dog was thin, febrile (104.1 degrees Fahrenheit), and had a loud, coarse cough. Submandibular and prescapular lymph nodes were enlarged. A single firm, ulcerated skin nodule with serosanguineous discharge was present on the right front paw. Fine-needle aspirate of the skin nodule produced material showing numerous large, thick-walled yeast organisms with characteristic broad-based budding. Urinary antigen testing for Blastomyces (MiraVista Diagnostics urine antigen EIA) returned strongly positive. Itraconazole was started the same afternoon. At the 30-day recheck, the cough had nearly resolved and the skin nodule had healed. Radiographic improvement was confirmed at 60 days.
The Organism: Dimorphic Fungus with Two Lives
Blastomyces dermatitidis and the closely related B. gilchristii (distinguished primarily by genetic analysis rather than morphology or clinical behavior) are thermally dimorphic fungi that exist in fundamentally different forms depending on their environment:
| Feature | Environmental (Mold) Form | Tissue (Yeast) Form |
|---|---|---|
| Temperature | Below 35 degrees Celsius (ambient environment) | 37 to 39 degrees Celsius (mammalian body temperature) |
| Morphology | Septate hyphae (filaments) producing small oval conidia (2 to 10 micrometers) | Large spherical to oval yeast cells (8 to 15 micrometers) with thick double-contoured refractile cell wall |
| Reproduction | Conidiation: produces individual conidia on short hyphal branches (conidiophores) | Broad-based budding: single daughter yeast cells attached to parent by a wide-base connection (2 to 5 micrometers wide); the broad base distinguishes Blastomyces from Cryptococcus (narrow-base budding) |
| Infectivity | Conidia are the infectious particle; inhaled by dog, travel to terminal airways | Non-infectious in the direct contact sense; tissue yeast forms do not typically transmit between animals (no dog-to-dog, dog-to-human direct transmission from tissue contact) |
| Found in | Moist sandy or loamy soil, rotting wood, animal burrows, beaver dams, riverbanks, lake shores in endemic regions | Lungs, lymph nodes, skin, eyes, bone, CNS, and virtually any organ in infected dogs and humans |
Geographic Distribution and Risk Factors
| Region | Highest-Risk States/Areas | Environmental Features |
|---|---|---|
| Mississippi River valley | Minnesota, Wisconsin, Iowa, Illinois, Missouri, Arkansas, Mississippi, Louisiana, Tennessee | Moist bottomland soils, river floodplains, oxbow lakes, beaver ponds, rotting organic matter along river corridors |
| Ohio River valley | Ohio, Indiana, Kentucky, West Virginia, western Pennsylvania | Similar river-bottom habitat; sandy loam soils in riparian zones |
| Great Lakes region | Wisconsin (highest endemic prevalence in the US), Michigan (Upper Peninsula especially), northern Illinois, Indiana, Minnesota | Lake shorelines, sandy beaches, forested lake margins, streams draining into Lake Michigan and Lake Superior |
| Southern Appalachians | North Carolina, Tennessee, Virginia (mountain and foothill regions) | Moist mountain hollows, creek banks, deciduous forest soils with high organic content |
| Canada | Ontario, Quebec, Manitoba (St. Lawrence River and Great Lakes watershed) | Extends the US endemic zone across the international border; significant veterinary and human case burden in Ontario and Manitoba |
Within endemic areas, specific activities and environments create elevated exposure risk:
- Hunting near water: duck hunting, waterfowl hunting, upland bird hunting along river and lake corridors; working dogs with noses in shoreline debris have extremely high exposure potential
- Swimming in lakes and rivers: particularly in shallow sandy-bottomed areas where soil is disturbed
- Digging in soil: dogs that dig extensively near water sources, in areas of recent soil disturbance (construction, excavation, logging), or in wildlife burrow areas are at elevated risk
- Proximity to beaver activity: beaver dams and the wet soil surrounding beaver ponds are repeatedly associated with blastomycosis cases and outbreaks in both dogs and humans
- Breed and sex: large-breed male dogs (Labrador Retrievers, Golden Retrievers, Weimaraners, Pointers, Chesapeake Bay Retrievers) are overrepresented in blastomycosis case series, likely reflecting increased outdoor exposure rather than true breed susceptibility; Sporting and Working breed dogs that hunt or work near water are at highest risk
Clinical Signs by System
| System | Clinical Signs | Frequency |
|---|---|---|
| Respiratory (primary) | Dry or productive cough, exercise intolerance, tachypnea, increased respiratory effort, harsh lung sounds on auscultation; in severe cases, respiratory distress and cyanosis | Present in approximately 65 to 85 percent of cases; the most common initial presenting complaint |
| Systemic/Constitutional | Fever (103 to 105 degrees Fahrenheit), weight loss, anorexia, lethargy; chronic low-grade illness for weeks before diagnosis is common | Present in most cases; weight loss and chronic cough are often the first recognized signs |
| Cutaneous | Firm, raised, ulcerated granulomatous nodules that drain serosanguineous or purulent material; most common on the face, nasal planum, nail beds, and distal limbs; lesions can resemble chronic wounds, abscesses, or skin tumors | 25 to 50 percent of cases with dissemination |
| Ocular | Uveitis (anterior: aqueous flare, hypopyon, hyphema, miosis; posterior: chorioretinitis, retinal detachment); secondary glaucoma; endophthalmitis; blindness | 20 to 50 percent of disseminated cases; ocular blastomycosis can be blinding even with successful systemic treatment if treatment is delayed |
| Skeletal | Lameness, joint swelling, bony pain; osteomyelitis most common in long bones of the distal limbs and in the spine; lytic or proliferative lesions on radiographs | 25 to 35 percent of disseminated cases; bone lesions can be confused with primary bone tumors (osteosarcoma) |
| Lymphatic | Peripheral and visceral lymphadenopathy; enlarged mandibular, prescapular, and popliteal nodes | Common in disseminated disease |
| CNS | Seizures, ataxia, altered mentation, blindness (from optic nerve involvement), vestibular signs, paresis | 5 to 10 percent of cases; associated with poorest prognosis; itraconazole penetrates CNS poorly |
| Urogenital | Prostatitis (male dogs), epididymitis, orchitis; dysuria, prostatomegaly | Uncommon but documented |
Diagnosis
Cytology and Histopathology
The most rapid and definitive diagnosis of blastomycosis is cytological identification of the characteristic yeast form in aspirates or impression smears from affected tissues; fine-needle aspiration of skin lesions, enlarged lymph nodes, or accessible masses often yields diagnostic material; the yeast cells are large (8 to 15 micrometers), round to oval, have a prominent thick double-contoured cell wall (giving them a “halo” appearance on stained smears), and reproduce by broad-based budding (the characteristic diagnostic feature); on Diff-Quik or Wright-Giemsa stained smears the organisms appear as large magenta to blue round structures, often surrounded by neutrophils and macrophages in a pyogranulomatous inflammatory pattern; bronchoalveolar lavage (BAL) cytology can identify organisms in dogs with pulmonary disease who do not have accessible peripheral lesions, but requires general anesthesia and carries risk in respiratory-compromised dogs.
Urine Antigen Test (MiraVista)
The Blastomyces urinary antigen enzyme immunoassay (EIA) offered by MiraVista Diagnostics (Indianapolis, Indiana) is a highly sensitive (approximately 90 percent in dogs with active infection) non-invasive diagnostic test; the fungal cell wall antigen is shed from actively replicating yeast cells and excreted in urine; the test requires a fresh urine sample (not stored or refrigerated for extended periods) submitted to MiraVista; results are typically available within 24 to 48 hours; false-positive results can occur with other systemic fungal infections (Histoplasma, Aspergillus, Paracoccidioides) due to cross-reactive antigens; serial urine antigen testing is also useful for monitoring treatment response, as antigen levels decline with successful therapy and rise with relapse; a dog that completes a treatment course should have antigen testing performed at the end of treatment and at 30-day intervals for 6 months to detect early relapse.
Thoracic Radiography
Thoracic radiographs are abnormal in the vast majority of dogs with blastomycosis, even those presenting primarily with extrapulmonary signs; the classic pattern is a diffuse miliary to nodular interstitial infiltrate with multiple small (2 to 10 mm) soft tissue opacity nodules distributed throughout both lung fields; hilar lymphadenopathy (enlargement of the tracheobronchial lymph nodes at the lung root) is common and produces a widened mediastinum; a patchy alveolar pattern (consolidation) can be seen in severe disease; the nodular interstitial pattern is the radiographic presentation most commonly confused with pulmonary metastatic cancer, and distinguishing between the two requires cytological or antigen testing, not just radiographic interpretation.
Serology
Agar gel immunodiffusion (AGID) serology for Blastomyces antibodies has moderate sensitivity (approximately 40 to 70 percent) in dogs and is much less sensitive than the urine antigen test; a positive AGID provides supportive evidence for blastomycosis, but a negative result does not exclude it; serology is generally considered a secondary diagnostic tool in veterinary medicine compared to cytology and antigen testing; cross-reactivity with other fungal antigens produces false positives.
Culture
Fungal culture of clinical specimens (BAL fluid, tissue aspirates, urine) on Sabouraud dextrose agar or other fungal culture media can recover Blastomyces dermatitidis, but growth is slow (3 to 4 weeks at 25 degrees Celsius) and the mold form is a biohazard (inhalation of cultured conidia can infect laboratory personnel); culture is therefore rarely used for clinical diagnosis but confirms species identity in research settings.
Treatment
Itraconazole (First-Line Therapy)
Itraconazole at 5 mg/kg orally once daily (or 2.5 mg/kg twice daily) is the treatment of choice for blastomycosis in dogs with mild to moderate pulmonary or disseminated disease; the minimum treatment duration is 60 days, but dogs with severe pulmonary disease, bone involvement, or disseminated infection may require 90 to 120 days or longer; treatment should continue for at least 30 days beyond clinical and radiographic resolution; important administration notes:
- Give with a fatty meal: itraconazole is a lipophilic drug whose oral bioavailability is highly dependent on food; giving with a high-fat meal increases absorption approximately 2-fold compared to fasting
- Bioavailability varies by formulation: the brand-name Sporanox capsule formulation has documented bioavailability in dogs and is the standard reference; generic itraconazole capsules and compounded formulations can have significantly lower and more variable bioavailability; switching formulations mid-treatment is discouraged without measuring itraconazole serum concentrations (therapeutic drug monitoring)
- Monitor for hepatotoxicity: itraconazole causes hepatocellular injury in approximately 5 to 10 percent of treated dogs; baseline liver enzyme values (ALT, ALP) should be obtained before starting treatment and rechecked at 30-day intervals during treatment; if ALT rises to more than 3 to 5 times baseline, itraconazole should be discontinued and an alternative antifungal considered
- Monitor for limb edema: cutaneous vasculitis producing limb edema (most commonly of the distal hind limbs) is an itraconazole-associated adverse effect in dogs that resolves with dose reduction or drug discontinuation
Fluconazole (Alternative, CNS Disease)
Fluconazole at 2.5 to 5 mg/kg orally twice daily is an alternative for dogs that cannot tolerate itraconazole due to hepatotoxicity; fluconazole has significantly better CNS penetration than itraconazole (CSF concentrations approach approximately 70 to 80 percent of plasma concentrations) and is the preferred oral azole for CNS blastomycosis; fluconazole is slightly less active against Blastomyces in vitro compared to itraconazole, but clinical outcomes are generally comparable for non-CNS disease; fluconazole does not require food for absorption (it is water-soluble).
Amphotericin B (Severe Disease)
Amphotericin B deoxycholate or the lipid-formulation amphotericin B (AmBisome, Abelcet) is a fungicidal polyene antifungal reserved for dogs with severe, life-threatening pulmonary blastomycosis or CNS disease where rapid fungicidal activity is needed; AMB disrupts the fungal cell membrane ergosterol and is directly fungicidal (as opposed to itraconazole and fluconazole, which are fungistatic); it is given by slow IV infusion, typically three times per week, and requires hospitalization and monitoring; nephrotoxicity (acute tubular injury) is the primary dose-limiting adverse effect; lipid-formulation AMB products have significantly lower nephrotoxicity than conventional AMB deoxycholate; AMB is typically followed by oral itraconazole for long-term consolidation therapy once the dog is stabilized.
Ocular Blastomycosis
Eyes with active uveitis from blastomycosis require both systemic antifungal therapy and topical anti-inflammatory treatment; topical atropine sulfate 1 percent is used to reduce ciliary spasm and prevent posterior synechia formation; topical corticosteroids (prednisolone acetate 1 percent ophthalmic suspension) may be used for anterior uveitis when no corneal ulceration is present; however, topical corticosteroids should not be used if corneal integrity is questionable, as they impair corneal healing and can worsen fungal infections locally; the retina should be examined carefully by indirect ophthalmoscopy for retinal detachment, which can be partial or complete; retinal detachment that is total and of long duration may be irreversible even after successful systemic antifungal treatment; the goal of ocular treatment is to preserve vision, but some dogs lose one or both eyes to blastomycosis despite appropriate treatment; enucleation (surgical removal of the eye) is appropriate for blind, painful eyes with secondary glaucoma that do not respond to medical management.
US Cost Overview for Blastomycosis
| Service | Typical US Cost |
|---|---|
| Veterinary examination | $60 to $150 |
| Thoracic radiographs (two views) | $150 to $350 |
| Fine-needle aspiration with cytology | $100 to $250 |
| Urine Blastomyces antigen EIA (MiraVista) | $60 to $100 |
| Complete blood count and chemistry panel | $150 to $300 |
| Chest CT scan (severe or complex cases) | $800 to $2,000 |
| Bronchoalveolar lavage with cytology | $400 to $900 |
| Itraconazole (Sporanox, 60-day course, 30 kg dog) | $200 to $500 |
| Fluconazole (60-day course, 30 kg dog) | $60 to $180 |
| Amphotericin B (IV, per treatment) | $200 to $600 per session plus hospitalization |
| Monthly liver enzyme monitoring (ALT/ALP) | $80 to $150 per recheck |
| Urine antigen monitoring (post-treatment) | $60 to $100 per test |
| Ophthalmology consultation | $200 to $400 |
Breed Predispositions and High-Risk Dogs
| Breed | Risk Factor | Notes |
|---|---|---|
| Labrador Retriever | Very high | Most commonly diagnosed breed in published case series; hunting and swimming behavior creates maximum environmental exposure in endemic areas |
| Golden Retriever | High | Similar to Labrador; waterfowl hunting and outdoor activity in Wisconsin and Minnesota create significant exposure |
| Weimaraner | High | Bird hunting in endemic states; field dogs with extensive soil contact |
| Pointer / German Shorthaired Pointer | High | Upland and waterfowl hunting dogs with field exposure near water |
| Chesapeake Bay Retriever | High | Waterfowl hunting; strong affinity for water; typically hunted in duck-hunting areas of the Mississippi flyway |
| Treeing Walker Coonhound | Moderate to high | Trailing dogs with extensive ground-level exposure in woodland and riparian environments |
| Doberman Pinscher | Moderate | Overrepresented in some case series; possible increased susceptibility independent of outdoor activity level |
Age-Specific Considerations
Puppies (Under 12 Months)
- Young dogs under 12 months are at elevated risk for severe or rapidly progressive blastomycosis compared to adult dogs, because their innate and adaptive immune responses are less experienced at containing fungal pathogens; the pulmonary pyogranulomatous inflammatory response that walls off Blastomyces in adult dogs is less robust in puppies, potentially allowing more rapid dissemination to extrapulmonary sites; a puppy from an endemic area with a persistent cough unresponsive to antibiotics, weight loss, or any of the characteristic extrapulmonary signs (skin nodules, eye changes, limb swelling from bone involvement) should have blastomycosis considered immediately
- Itraconazole dosing in puppies follows the same weight-based protocol as adult dogs (5 mg/kg once daily with food); however, puppies are still growing and liver enzyme monitoring is particularly important because itraconazole hepatotoxicity in a rapidly metabolizing puppy may manifest more quickly; baseline and monthly ALT/ALP should be followed consistently throughout treatment; if hepatotoxicity develops, fluconazole is the preferred alternative in puppies
- Puppies exposed to environmental Blastomyces through outdoor activity in endemic areas during the hunting season (fall) or late summer should be monitored for signs of respiratory illness for 3 to 6 weeks after significant soil or water exposure; blastomycosis has an incubation period of approximately 5 to 12 weeks from exposure to clinical illness, so respiratory signs appearing weeks after a hunting or camping trip in Wisconsin or Minnesota should raise immediate suspicion
Adult Dogs (1 to 8 Years)
- Adult dogs, particularly large-breed males aged 2 to 6 years, are the most commonly affected group in blastomycosis case series; this reflects both the age at which dogs are most actively used for hunting and field work and the peak immunological competence window during which the immune system can mount a strong but sometimes inadequate pyogranulomatous response that contains pulmonary disease without fully eliminating the organism; adult hunting dogs returning from endemic area trips in fall and early winter should be watched for respiratory signs for 6 to 8 weeks after the last exposure event
- The key diagnostic pitfall in adult dogs with blastomycosis is the radiographic mimicry of pulmonary metastatic cancer; an adult dog with a miliary nodular pulmonary pattern in an endemic area is more likely to have blastomycosis than pulmonary metastatic cancer (particularly if younger than 7 to 8 years and without a known primary tumor), but both are possible; a urine Blastomyces antigen test (same-day result from MiraVista at many clinics) and fine-needle aspiration of any accessible skin or lymph node lesion should precede a cancer staging workup in any dog from an endemic area with this radiographic pattern
- Adult dogs that recover from blastomycosis with appropriate itraconazole treatment generally have good long-term outcomes; however, relapse occurs in approximately 20 to 25 percent of cases, most commonly within the first 6 months after completing treatment; serial urine antigen monitoring at 30-day intervals for 6 months post-treatment is the most sensitive approach to early relapse detection; dogs that relapse can generally be retreated successfully if the relapse is caught early; persistent or recurrent antigen elevation without obvious clinical signs warrants retreatment before clinical relapse becomes severe
Senior Dogs (9 Years and Older)
- Senior dogs with blastomycosis face compounded challenges from age-related immunosenescence (reduced immune system efficacy), concurrent chronic diseases, and reduced physiological reserve for tolerating both the infection and the antifungal treatment; itraconazole hepatotoxicity is a particular concern in senior dogs with pre-existing hepatic disease or concurrent use of other hepatically metabolized medications; a thorough drug interaction review is essential before starting itraconazole in a senior dog on concurrent medications (itraconazole is a potent inhibitor of CYP3A4 and can significantly raise plasma concentrations of many co-administered drugs including cyclosporine, certain antiarrhythmics, and benzodiazepines)
- The radiographic pattern of pulmonary blastomycosis in a senior dog is particularly likely to be attributed to metastatic cancer given the higher prevalence of primary tumors in older dogs; this correct index of suspicion for cancer should not crowd out blastomycosis as a differential, especially in a dog from an endemic area; the urine antigen test is inexpensive and can be run concurrently with a cancer staging evaluation without delaying diagnosis of either condition; a senior dog with both a known primary tumor and a concurrent positive Blastomyces antigen test has two diagnoses and requires treatment for both
- Ocular blastomycosis in senior dogs carries a higher risk of permanent vision loss because pre-existing cataracts, retinal degeneration (progressive retinal atrophy), or nuclear sclerosis may have already compromised vision before the blastomycosis uveitis develops; careful baseline ophthalmologic assessment is important to determine what fraction of vision loss is attributable to the fungal infection versus pre-existing disease, both for prognostic communication with the owner and for treatment planning; even a senior dog with pre-existing mild vision impairment can experience meaningful quality-of-life improvement from preservation of remaining vision through prompt antifungal therapy and topical ophthalmic management
Myths and Facts About Blastomycosis in Dogs
My dog’s chest X-ray shows multiple lung nodules, so the veterinarian is right that it must be cancer.
Pulmonary blastomycosis produces a diffuse miliary to nodular interstitial radiographic pattern that is visually indistinguishable from pulmonary metastatic cancer on routine chest radiographs. Both conditions appear as multiple soft tissue opacity nodules throughout the lung fields. In dogs from endemic areas (Wisconsin, Minnesota, Ohio, Tennessee, and surrounding states), blastomycosis is as likely as or more likely than pulmonary metastatic cancer, especially in younger dogs without a known primary tumor. A urine Blastomyces antigen test should always precede cancer staging in any dog with this pattern from an endemic area.
Blastomycosis can spread directly from my dog to me through contact with the dog’s lesions or respiratory secretions.
Direct dog-to-human transmission of blastomycosis through contact with an infected dog’s skin lesions, respiratory secretions, or body fluids is not documented and is considered extremely unlikely in normal circumstances. The tissue yeast form of Blastomyces is not infectious by contact. Both dogs and humans acquire blastomycosis from the same environmental source (soil containing the mold form and its conidia). The rare documented human infections attributed to dog contact involved direct inoculation of wound material into a skin laceration, not respiratory or casual contact. Normal hygiene is sufficient precaution when caring for a dog with blastomycosis.
Once my dog finishes the itraconazole course and seems better, treatment is done and no further follow-up is needed.
Blastomycosis relapses in approximately 20 to 25 percent of dogs within 6 months of completing treatment. Clinical improvement and resolution of symptoms does not mean the fungal infection has been fully eliminated; viable organisms can persist in pulmonary granulomas and lymph nodes at low levels that are undetectable clinically but sufficient to cause relapse. Urine antigen monitoring at 30-day intervals for at least 6 months after completing itraconazole is the standard approach to detecting early relapse before it becomes severe. Dogs with rising antigen levels should be retreated promptly.
Red Flags: Signs Requiring Same-Day or Emergency Evaluation
- Labored breathing at rest, extended neck posture, open-mouth breathing, or blue-tinged gums in a dog with a cough or suspected blastomycosis: severe pulmonary blastomycosis can cause respiratory failure from massive bilateral pneumonia; respiratory distress in this context requires emergency evaluation and immediate antifungal therapy; some of these dogs need oxygen supplementation and ICU-level monitoring while initiating antifungal treatment
- Sudden onset of vision loss, squinting, or eye redness in a dog being treated for blastomycosis or in an endemic area dog with other signs: ocular blastomycosis can cause rapid progression from uveitis to retinal detachment to permanent blindness within days; early ophthalmologic evaluation (same day if possible) and prompt intensification of antifungal therapy are critical to preserving vision; once complete retinal detachment has occurred, visual prognosis is very poor regardless of treatment
- New neurological signs in a dog on treatment for blastomycosis: seizures, severe ataxia, altered mentation, or sudden blindness during treatment may indicate CNS dissemination or inadequate antifungal penetration into the CNS; itraconazole penetrates the CNS poorly; CNS blastomycosis may require switching to fluconazole (which has better CNS penetration) or adding amphotericin B, and carries the highest mortality of any blastomycosis presentation
- Persistent or worsening respiratory signs after 2 to 4 weeks of appropriate itraconazole treatment: most dogs improve significantly within the first 2 to 4 weeks of treatment; failure to improve or deterioration despite treatment warrants reassessment including repeat radiographs, recheck of serum itraconazole concentrations to verify therapeutic levels, and consideration of whether the diagnosis is correct or whether a second concurrent disease is present
- Any ulcerated, draining skin nodule in a dog from an endemic area, especially combined with cough or weight loss: skin lesions from disseminated blastomycosis are frequently biopsied as suspected skin tumors; before proceeding with surgical excision of any draining nodule in a hunting dog or outdoor dog from the Mississippi River valley, Great Lakes, or Appalachian regions, aspirate the lesion for cytology to check for Blastomyces yeast forms; treating blastomycosis as a skin tumor misses the systemic disease and allows pulmonary and other organ involvement to progress untreated
Frequently Asked Questions About Blastomycosis in Dogs
What is blastomycosis in dogs?
Blastomycosis is a systemic fungal infection caused by Blastomyces dermatitidis and B. gilchristii, dimorphic fungi found in the soil of the Mississippi and Ohio River valleys, Great Lakes region, and southern Appalachians. Dogs inhale microscopic spores (conidia) from contaminated soil, which convert to the yeast form in the lungs and cause a pyogranulomatous pneumonia that can spread to skin, eyes, bone, and the nervous system. It is not contagious between dogs or from dogs to humans.
What are the symptoms of blastomycosis in dogs?
The most common signs are a persistent cough, weight loss, fever, and exercise intolerance from pulmonary disease. Dissemination produces ulcerated skin nodules that drain fluid, eye redness and squinting (uveitis), limb lameness (bone infection), and lymph node swelling. The chest X-ray typically shows a diffuse miliary nodular pattern that closely resembles pulmonary metastatic cancer, which is a leading cause of misdiagnosis in endemic areas.
How is blastomycosis diagnosed in dogs?
Diagnosis is made by finding the characteristic large, thick-walled yeast with broad-based budding on cytology (fine-needle aspirate of skin lesions or lymph nodes) or by the urine Blastomyces antigen EIA test (MiraVista Diagnostics), which has approximately 90 percent sensitivity in dogs with active infection. Chest radiographs show the pulmonary pattern. Serology is less sensitive and less useful than antigen testing in dogs.
What is the treatment for blastomycosis in dogs?
Itraconazole 5 mg/kg orally once daily with a fatty meal for a minimum of 60 to 90 days (longer for severe or disseminated disease) is the standard treatment. Fluconazole is an alternative with better CNS penetration. Amphotericin B is reserved for severe life-threatening cases. Monthly liver enzyme monitoring is essential during treatment. Urine antigen testing at 30-day intervals for 6 months post-treatment monitors for relapse, which occurs in approximately 20 to 25 percent of dogs.
Where is blastomycosis most common in the US?
The highest-risk areas are the Mississippi River valley (Wisconsin, Minnesota, Iowa, Missouri, Tennessee, Arkansas), the Ohio River valley (Ohio, Indiana, Kentucky), the Great Lakes region (especially Wisconsin and Michigan’s Upper Peninsula), and the southern Appalachian Mountains. Wisconsin has the highest reported prevalence of blastomycosis per capita in both dogs and humans in the United States. Dogs that hunt or swim near rivers, lakes, and beaver ponds in these regions are at greatest risk.
Can my dog give me blastomycosis?
No. Direct transmission of blastomycosis from a dog to a human through contact with skin lesions, respiratory secretions, or body fluids is not a documented route of infection. Both dogs and humans acquire blastomycosis from the same environmental source: fungal spores in contaminated soil. Normal hygiene when caring for an infected dog is sufficient. However, if you live in an endemic area and share outdoor environments with your dog, you face the same environmental exposure risk that your dog faced.
What is the prognosis for dogs with blastomycosis?
With appropriate itraconazole treatment, the overall survival rate is approximately 50 to 75 percent in most reported case series. Dogs with mild to moderate pulmonary disease that are treated early generally have the best outcomes. Severe pulmonary compromise at presentation, CNS involvement, and bilateral ocular disease with retinal detachment carry the worst prognosis. Relapse occurs in approximately 20 to 25 percent of survivors within 6 months of treatment completion, so monitoring is important even after clinical recovery.
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