# Blastomycosis in Dogs: Fungal Infection Guide Blastomycosis in dogs is a serious systemic fungal infection caused by inhaling the spores of *Blastomyces dermatitidis*, a dimorphic fungus that lives in moist, acidic soil near waterways. The infection usually starts in the lungs, but it can spread through the bloodstream to the eyes, skin, bones, joints, lymph nodes, and central nervous system, which is why early recognition and prolonged antifungal treatment matter so much. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## What Causes Blastomycosis in Dogs? The organism behind this disease is *Blastomyces dermatitidis*, one of the dimorphic fungi. "Dimorphic" means the fungus takes two different physical forms depending on its environment. In soil at ambient temperature, it grows as a mold that produces infectious spores called conidia. Once those conidia are inhaled into the warm body of a mammal, the fungus converts to a yeast form that reproduces by budding and resists the host immune response. The mold form does not survive well in the environment as a free-living yeast, and dogs do not pass the infection to each other. A dog with blastomycosis is not contagious to other dogs or to people. The only route of infection is inhalation of conidia from a contaminated environmental source [1]. ### The Environmental Niche *Blastomyces dermatitidis* grows in moist, acidic soil that is rich in organic material, typically near rivers, streams, and lakes. The classic geographic distribution in North America follows the Ohio, Mississippi, and Missouri river valleys, along with the Great Lakes region and parts of the mid-Atlantic and southeastern United States. Wisconsin is a well-documented endemic area. A survey of small-animal veterinary practices in Wisconsin estimated a mean annual incidence of 204 cases per 100,000 dogs in endemic counties and 72 per 100,000 in nonendemic counties, with cases increasing from April through August [2]. A 2021 to 2022 investigation of a cluster of human and canine blastomycosis cases in a riverside neighborhood in Wisconsin found evidence supporting potential exposure from the riverbank, riverside trails or yards, or construction dust, although no single definitive exposure was identified [3]. This pattern matches the known ecology of the fungus: wet soil, disturbed ground, and airborne dust near water all increase the chance that conidia become aerosolized and inhaled. ### Why Some Dogs Are Overrepresented Blastomycosis in dogs is not distributed evenly across the population. A retrospective review of 47 clinical cases described the disease as affecting predominantly young, male dogs of the larger breeds, with clinical signs usually related to weight loss and to respiratory and ocular problems [4]. The overrepresentation of young, large-breed dogs likely reflects behavior and exposure rather than an intrinsic breed susceptibility. Large dogs that spend time outdoors, dig in soil, sniff the ground, and live or travel near waterways have more opportunity to inhale conidia. A more recent study of blastomycosis-associated inflammatory arthritis found that 4 of 12 affected dogs were Labrador Retrievers, with a median age of 5.0 years [5]. ## How the Infection Spreads Through the Body Once conidia reach the alveoli, the yeast form develops and triggers a local inflammatory response. The lungs are the primary site of infection, and many dogs show respiratory signs first. From the lung, yeast cells can enter the lymphatic system and bloodstream and seed almost any organ. This is why blastomycosis is described as a systemic or disseminated disease rather than a simple lung infection. The incubation period is variable and often difficult to pin down because owners rarely know exactly when exposure occurred. Signs may appear weeks to months after inhalation. Some dogs have disease limited to the lungs, while others develop widespread involvement. A dog with disseminated blastomycosis almost always has some degree of lung involvement, even if respiratory signs are not the main complaint [5]. The mermaid diagram below shows the main pathway from environmental exposure to clinical disease and diagnosis. ```mermaid flowchart TD A[Moist acidic soil near water] --> B[Airborne conidia inhaled] B --> C[Conidia reach the lungs] C --> D[Fungus converts to yeast form] D --> E[Local lung infection] E --> F{Does infection spread} F --> G[Lungs only] F --> H[Disseminated disease] H --> I[Eyes skin bones joints] H --> J[Lymph nodes and urogenital tract] H --> K[Central nervous system] G --> L[Diagnostics cytology antigen culture] H --> L K --> M[Prolonged antifungal therapy] L --> M ``` ## Clinical Signs by Body System Blastomycosis can produce a wide range of signs, and the combination of systems involved is often what raises suspicion. The table below summarizes the body systems and the typical signs reported in the veterinary literature. | Body System | Typical Signs | Key Clinical Notes | |--|--|--| | Respiratory | Cough, exercise intolerance, weight loss, labored breathing, stertor, nasal discharge | Most common presentation. Severe cases may need supplemental oxygen [6][7] | | Ocular | Uveitis, blindness, ocular discharge, exophthalmos | Bilateral uveitis is a classic finding in disseminated disease [4][8][9] | | Cutaneous | Draining skin lesions, abscesses, nonhealing wounds | Often on the trunk or limbs, may drain purulent material [8][9] | | Skeletal and articular | Lameness, joint swelling, periarticular osteolysis | Inflammatory arthritis can occur with or without visible organisms in joint fluid [5] | | Lymphatic | Peripheral lymphadenopathy | Enlarged lymph nodes are common and easy to aspirate for cytology [8] | | Urogenital | Prostatic or testicular swelling, urogenital signs | May be underdiagnosed if these tissues are not specifically evaluated [10] | | Neurologic | Seizures, ataxia, paraparesis, neck pain, head tilt | Can mimic meningitis or brain tumors on imaging [11][12][13] | | Cardiac | Arrhythmia, pericardial effusion, intracardiac mass | Rare but documented, including survival with combination azole therapy [9] | ### Pulmonary Disease The lung is the entry point, and respiratory signs are the most common reason owners seek care. Dogs may present with a soft cough, reduced exercise tolerance, weight loss, or increased respiratory effort. In a retrospective study of 139 dogs with pulmonary blastomycosis, 61 percent survived to 30 days, while 27 percent were euthanized because of progressive disease and 10 percent died from respiratory or cardiac arrest [7]. The need for supplemental oxygen was associated with significantly decreased survival [7]. A separate case series of 19 dogs with severe blastomycosis requiring high-flow nasal oxygen therapy found that only 1 of 19 survived to discharge, with 12 euthanized and 6 dying during hospitalization, showing that respiratory failure carries a grave prognosis [6]. ### Ocular Disease Ocular involvement is common in disseminated blastomycosis and often brings owners in before respiratory signs become obvious. Bilateral uveitis and blindness were described in early case reviews [4]. A case report of a 1-year-old golden retriever-poodle crossbred dog described bilateral uveitis with a normal complete blood count and biochemistry panel, which shows that routine blood work can be unremarkable even in disseminated disease [8]. Another report documented intraocular blastomycosis with intracardiac involvement in a Labrador Retriever that regained vision in one eye after combination azole therapy [9]. ### Cutaneous and Skeletal Disease Skin lesions in blastomycosis often look like draining abscesses or nonhealing wounds. A dog may have a swelling on a digit or limb that drains purulent material. In the golden retriever-poodle case, a draining swelling of the left hind fourth digit was one of the presenting complaints [8]. Bone involvement can cause lameness and periarticular osteolytic lesions visible on radiographs. A study of 12 dogs with blastomycosis-associated inflammatory arthritis found that all dogs were lame and 10 of 12 had palpable joint swelling, with intra-articular organisms detected in only 10 of 25 joint fluid samples [5]. This is a critical point: a joint tap can show inflammation without visible yeast, so a negative cytology does not rule out blastomycosis. ### Urogenital Disease Prostatic and testicular blastomycosis are infrequently recognized and may be underdiagnosed because these tissues are not always evaluated. A retrospective case study of eight dogs found that three of four dogs with prostatic blastomycosis had elevated body temperature and all had systemic disease, while all four dogs with testicular blastomycosis had normal body temperature and three had systemic disease [10]. The authors recommended that male dogs with urogenital signs be evaluated with prostatic or testicular cytology or histopathology, since identifying and managing this involvement may improve overall treatment success [10]. ### Neurologic Disease Central nervous system blastomycosis is rare but serious. A study of five dogs with intracranial blastomycosis found variable clinical and laboratory findings, with major presenting complaints including stertor or nasal discharge in two dogs, exophthalmos in one, and seizures in two [11]. Computed tomography revealed a single contrast-enhancing intra-axial mass in one dog, a nasal mass disrupting the cribriform plate in three, and an intracranial mass extending into the orbit and nasal cavity in one [11]. The authors noted that intracranial blastomycosis can mimic other inflammatory disorders or neoplasia [11]. A separate report described a dog with suspected central nervous system localized blastomycosis that had negative urine antigen testing, which shows that antigenuria is not always positive when disease is confined to the nervous system [13]. ## How Blastomycosis Is Diagnosed Diagnosis rests on a combination of clinical signs, imaging, cytology, antigen testing, and occasionally culture. No single test is perfect, and the best approach uses several in parallel. ### Cytology Cytology of lymph node or lung aspirates is often the fastest route to a diagnosis. The characteristic finding is a broad-based budding yeast. The yeast cells are round to oval, have a thick refractile wall, and bud with a wide base rather than a narrow neck, which distinguishes them from other fungi. A retrospective review confirmed that diagnosis was established by microscopic evaluation of aspiration or excision biopsies [4]. Cytology can also be performed on skin lesions, joint fluid, prostatic aspirates, and cerebrospinal fluid. As noted above, joint fluid may show suppurative or pyogranulomatous inflammation without visible organisms, so negative cytology from a joint does not exclude the disease [5]. ### Antigen Testing The *Blastomyces* urine antigen test, often referred to by the manufacturer name MiraVista, is a quantitative enzyme immunoassay that detects a fungal antigen in urine and serum. In a prospective study of 21 dogs with newly diagnosed blastomycosis, urine antigen was positive in all 21 samples at enrollment, giving 100 percent sensitivity, while serum antigen was positive in 18 of 20 samples, giving 90 percent sensitivity [14]. At 2 to 4 months of treatment, urine antigen was more sensitive for clinically detectable disease at 82 percent than serum antigen at 18 percent [14]. The sensitivity of the urine test for clinical relapse was 71 percent with close to 100 percent specificity during post-treatment surveillance [14]. This makes urine antigen the preferred sample for both diagnosis and monitoring. Antigen concentrations also carry prognostic information. A retrospective study of 52 dogs found that dogs with a urine antigen concentration below 5 ng/mL and dogs with mild radiographic lung severity scores had a greater proportion surviving than those with higher concentrations or more severe radiographic scores [15]. Overall survival in that study was 87 percent at discharge, 85 percent at 1 week, 74 percent at 2 months, and 69 percent at 6 months [15]. The authors noted that urine antigen concentration correlated with radiographic lung severity score [15]. A case report of a dog with suspected central nervous system localized blastomycosis had a negative urine antigen test, so a negative result does not completely rule out the disease when the central nervous system is the main site [13]. ### Antibody Testing Antibody tests detect the dog's immune response rather than the fungus itself. An early review found the agar-gel immunodiffusion test helpful in establishing a diagnosis [4]. More recent work evaluated four *Blastomyces* antigens for antibody detection in dogs, and all antigens detected antibody, though efficacy varied. The ERC-2 antigen showed the highest ELISA mean absorbance value at 3.00, followed by T-27 [16]. Test performance varied by the geographic origin of the sample, and the authors concluded that further study is needed to determine clinical utility [16]. Antibody testing is generally less useful than antigen testing for monitoring treatment response because antibodies can persist after clinical remission. ### Culture Fungal culture can confirm the diagnosis and identify the species, but it is used rarely in general practice. The Wisconsin survey found that fungal culture was used rarely for diagnosis, with veterinarians relying more on clinical signs, antigen testing, and in-house cytology [2]. Culture takes weeks to grow and requires appropriate biosafety precautions because the mold form is infectious. It remains a reference method when cytology and antigen testing are inconclusive. ### Imaging Thoracic radiographs are standard for evaluating lung involvement. Radiographic lung severity can be scored on a 0 to 4 scale, and higher scores are associated with lower survival [15]. Computed tomography and magnetic resonance imaging are used when neurologic or nasal involvement is suspected. In the five dogs with intracranial blastomycosis, CT revealed intra-axial masses, nasal masses disrupting the cribriform plate, and an intracranial mass extending into the orbit and nasal cavity [11]. MRI has also been used to characterize central nervous system blastomycosis in dogs [12]. ## Treatment of Blastomycosis in Dogs Treatment is prolonged, expensive, and requires commitment. The standard approach uses azole antifungals, with amphotericin B reserved for severe or refractory cases. ### Itraconazole Itraconazole is the recommended first-line treatment for most dogs with blastomycosis [17]. A retrospective comparison of 144 dogs treated with either fluconazole or itraconazole found that itraconazole achieved clinical remission in 90 percent of cases, compared with 75 percent for fluconazole, though the difference was not statistically significant [17]. Treatment duration was significantly shorter for itraconazole, with a median of 138 days, compared with 183 days for fluconazole [17]. Relapse rates were 18 percent for itraconazole and 22 percent for fluconazole, also not significantly different [17]. The incidence of increased ALT activity was 26 percent for itraconazole and 17 percent for fluconazole, again not significantly different [17]. A case report of localized mandibular blastomycosis in a 5-year-old dog documented complete resolution of clinical signs and oral radiographic changes after itraconazole at 5 mg/kg/day for four and a half months, with the dog free of *Blastomyces* at one year based on urine antigen testing [18]. This illustrates that even localized disease may require months of therapy. The minimum treatment duration is generally 2 to 3 months, but many dogs need longer. Treatment should continue until clinical signs have resolved and antigen testing is negative or markedly reduced, and often for at least one month beyond that point. Central nervous system disease requires longer treatment, often 6 months or more, because azole penetration into the nervous system is limited. ### Fluconazole Fluconazole is an alternative when itraconazole is not tolerated or when cost is a major factor. It achieved remission in 75 percent of dogs in the comparison study, with a longer median treatment duration and a lower total drug cost [17]. Fluconazole penetrates the central nervous system better than itraconazole, so it is sometimes preferred for neurologic disease despite its lower overall efficacy for systemic disease. ### Combination Azole Therapy Combination itraconazole and fluconazole has been used successfully in at least one case of disseminated blastomycosis with intracardiac and intraocular involvement. A 4-year-old spayed female Labrador Retriever with blindness, cutaneous lesions, coughing, inappetence, and lethargy was diagnosed with disseminated blastomycosis and deteriorated during hospitalization with sustained ventricular tachycardia. Echocardiography revealed pericardial effusion, a nodule associated with the left ventricular papillary muscle, and a right atrial mural lesion. Combination azole therapy successfully treated the dog, which regained vision in the left eye and had no residual cardiac disease detectable by electrocardiography or echocardiography [9]. ### Amphotericin B Amphotericin B is reserved for severe cases, including dogs with respiratory failure or rapidly progressive disease. An early review of 22 dogs treated with amphotericin B found that 18 were clinically normal 6 months after initiation of therapy [4]. Amphotericin B is administered intravenously and requires monitoring for kidney toxicity. It is often used as initial therapy to stabilize a critically ill dog before transitioning to an oral azole. ### Supportive Care Dogs with severe respiratory compromise may need oxygen supplementation. A study of 19 dogs treated with high-flow nasal oxygen therapy found a poor to grave prognosis, with only 1 of 19 surviving to discharge [6]. The need for supplemental oxygen was associated with significantly decreased survival in the pulmonary blastomycosis study [7]. Anti-inflammatory medications, including NSAIDs and corticosteroids, did not impact 30-day survival in that same study [7]. Vitamin D status has been investigated as a prognostic marker. A [prospective cohort study](/blog/guides/prospective-cohort-studies-a-guide-to-design-and-implementation) of 19 dogs with blastomycosis found that serum 25-hydroxyvitamin D concentrations at diagnosis were lower than in healthy controls, with a median of 203 nmol/L compared with 259.5 nmol/L [19]. Dogs with baseline concentrations below 180.5 nmol/L had greater odds of death during hospitalization [19]. This is an area of ongoing research rather than a routine clinical test. ## Risk Map for Blastomycosis in Dogs The geographic risk for blastomycosis in dogs follows the environmental distribution of *Blastomyces dermatitidis*. The highest-risk regions in the United States are the Ohio, Mississippi, and Missouri river valleys, the Great Lakes region, and parts of the mid-Atlantic and southeastern states. Wisconsin is a well-documented endemic area, with cases increasing from April through August [2]. A riverside neighborhood cluster investigation in Wisconsin identified potential exposures from the riverbank, riverside trails or yards, and construction dust [3]. Dogs at highest risk are young, large-breed dogs that spend time outdoors in these regions, especially near waterways or in areas where soil is disturbed. Travel history matters. The golden retriever-poodle case had a one-month travel history to Muskoka, Ontario, which is outside the classic US endemic region but still within the broader range of the fungus [8]. Blastomycosis has also been reported in Africa, though the epidemiology differs and the apparent absence of disease in dogs there is noteworthy [20]. ## Clinical Relevance, Limitations and Common Mistakes Blastomycosis is one of the most important systemic fungal infections in dogs in endemic regions, and it carries significant mortality. The Wisconsin survey estimated an overall mortality of 36 percent among diagnosed cases [2]. Survival depends on early recognition, appropriate antifungal therapy, and the extent of organ involvement. Dogs with lower urine antigen concentrations and milder radiographic lung changes have better outcomes [15]. Several common mistakes delay diagnosis. The first is assuming that a normal complete blood count and biochemistry panel rules out blastomycosis. The golden retriever-poodle case had normal blood work at initial presentation despite bilateral uveitis [8]. The second is relying on joint fluid cytology alone. Intra-articular organisms were detected in only 10 of 25 joint fluid samples in one study, so inflammation without visible yeast does not exclude the disease [5]. The third is failing to evaluate the prostate and testes in male dogs with urogenital signs, which may lead to underdiagnosis [10]. The fourth is stopping treatment too early. Relapse rates of 18 to 22 percent have been reported, and urine antigen monitoring helps detect relapse after treatment discontinuation [14][17]. The fifth is expecting a positive urine antigen test in every case. A dog with central nervous system localized disease had negative antigenuria [13]. Individual cases vary widely, and a veterinarian who has examined your dog is the only person who can make treatment decisions for that patient. ## Frequently Asked Questions ### Is blastomycosis contagious from dog to dog? No. Blastomycosis is not transmitted between dogs or from dogs to people. Infection occurs only by inhaling conidia from contaminated soil [1]. ### What are the first signs of blastomycosis in dogs? The earliest signs are often respiratory, including cough, exercise intolerance, and weight loss, or ocular, including uveitis and blindness [4]. Some dogs present with skin lesions or lameness first. ### Can a dog survive blastomycosis? Yes. Itraconazole achieved clinical remission in 90 percent of dogs in one study, and amphotericin B produced clinical normality in 18 of 22 dogs at 6 months in an earlier review [4][17]. Survival depends on the severity and extent of disease. ### How long does blastomycosis treatment last? Treatment typically lasts at least 2 to 3 months, with a median of 138 days for itraconazole in one study [17]. Central nervous system disease often requires 6 months or longer. ### Is there a vaccine for blastomycosis in dogs? No vaccine is available for blastomycosis in dogs. ### Can blastomycosis cause blindness in dogs? Yes. Ocular involvement, including uveitis and blindness, is a common feature of disseminated blastomycosis [4][8][9]. Some dogs regain vision with treatment, as documented in one case of intraocular blastomycosis [9]. ### What does a broad-based budding yeast mean? It is the characteristic microscopic appearance of *Blastomyces dermatitidis* yeast cells, which bud with a wide base rather than a narrow neck. Finding this on cytology supports a diagnosis of blastomycosis [4]. ### Does a negative urine antigen test rule out blastomycosis? No. A dog with central nervous system localized blastomycosis had a negative urine antigen test, so a negative result does not completely exclude the disease [13]. ## Related Articles - [DNA Extraction from Fungi: Protocols for Yeast and Mold](/knowledge/diagnostics/molecular/dna-extraction-fungi-protocols-yeast-mold) - [Dog Ear Fungal Infection Symptoms](/knowledge/veterinary-medicine/symptom-guides/dog-ear-fungal-infection-symptoms) - [Dog Paw Fungal Infection Home Remedy](/knowledge/veterinary-medicine/clinical-methods/dog-paw-fungal-infection-home-remedy) - [Aquarium Fish Fungal Infections: Identification and Treatment](/knowledge/veterinary-medicine/aquarium-fish-care/aquarium-fish-fungal-infections-identification-treatment) - [Itraconazole Fungal Infection Dose: Systemic Fungal Infection Protocol](/knowledge/veterinary-medicine/calculators-clinical-tools/itraconazole-fungal-infection-dose-systemic-fungal-infection-protocol) - [KOH Microscopy: A Rapid Diagnostic Tool for Fungal Infections](/knowledge/diagnostics/imaging/koh-microscopy-a-rapid-diagnostic-tool-for-fungal-infections) - [Dog Antifungal Medication: Options, Doses, and Safety](/knowledge/veterinary-medicine/pet-medication-guide/dog-antifungal-medication-options-doses-and-safety) - [Lingual Tonsils: Anatomy, Location, and Function](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/lingual-tonsils-anatomy-location-and-function) - [Proteus Mirabilis: Characteristics and Infections](/knowledge/veterinary-medicine/veterinary-pathology-microbiology/proteus-mirabilis-characteristics-and-infections) ## Sources 1. [Blastomycosis in Animals - Infectious Diseases - Merck Veterinary Manual](https://www.merckvetmanual.com/infectious-diseases/fungal-infections/blastomycosis-in-animals) 2. [Canine blastomycosis in Wisconsin: a survey of small-animal veterinary practices.](https://pubmed.ncbi.nlm.nih.gov/25187628/) 3. [Human and Canine Blastomycosis Cases Associated with Riverside Neighborhood, Wisconsin, USA, December 2021-March 2022(1).](https://pubmed.ncbi.nlm.nih.gov/39592410/) 4. [Canine blastomycosis: a review of 47 clinical cases.](https://pubmed.ncbi.nlm.nih.gov/7275754/) 5. [Intra-articular organisms are not always cytologically detectable in dogs with blastomycosis-associated inflammatory arthritis.](https://pubmed.ncbi.nlm.nih.gov/41344025/) 6. [Case series of dogs with blastomycosis managed with high-flow nasal oxygen therapy (2019-2023): 19 cases.](https://pubmed.ncbi.nlm.nih.gov/38903690/) 7. [A retrospective study of anti-inflammatory use in dogs with pulmonary blastomycosis: 139 cases (2002-2012).](https://pubmed.ncbi.nlm.nih.gov/28561957/) 8. [Disseminated blastomycosis in a 1-year-old, male golden retriever-poodle crossbred dog.](https://pubmed.ncbi.nlm.nih.gov/35919470/) 9. [Successful treatment of intracardiac and intraocular blastomycosis in a dog with combination azole therapy.](https://pubmed.ncbi.nlm.nih.gov/23690485/) 10. [Blastomyces dermatitidis prostatic and testicular infection in eight dogs (1992-2005).](https://pubmed.ncbi.nlm.nih.gov/22058348/) 11. [Clinical and imaging findings in five dogs with intracranial blastomycosis (Blastomyces dermatiditis).](https://pubmed.ncbi.nlm.nih.gov/21673331/) 12. [Clinical and magnetic resonance imaging features of central nervous system blastomycosis in 4 dogs.](https://pubmed.ncbi.nlm.nih.gov/20738772/) 13. [Negative Antigenuria in a Dog with Suspected Central Nervous System Localized Blastomycosis.](https://pubmed.ncbi.nlm.nih.gov/36584318/) 14. [Serum and urine Blastomyces antigen concentrations as markers of clinical remission in dogs treated for systemic blastomycosis.](https://pubmed.ncbi.nlm.nih.gov/24495193/) 15. [Retrospective analysis of the effects of Blastomyces antigen concentration in urine and radiographic findings on survival in dogs with blastomycosis.](https://pubmed.ncbi.nlm.nih.gov/33604957/) 16. [Potential clinical utility of ERC-2 yeast phase lysate antigen for antibody detection in dogs with blastomycosis.](https://pubmed.ncbi.nlm.nih.gov/30544205/) 17. [Retrospective comparison of the efficacy of fluconazole or itraconazole for the treatment of systemic blastomycosis in dogs.](https://pubmed.ncbi.nlm.nih.gov/21418325/) 18. [Mandibular Blastomycosis in a 5-Year-Old Dog.](https://pubmed.ncbi.nlm.nih.gov/35502840/) 19. [Serum 25-hydroxyvitamin D concentrations and mortality in dogs with blastomycosis.](https://pubmed.ncbi.nlm.nih.gov/34148015/) 20. [Blastomycosis in Africa. A review of known cases diagnosed between 1951 and 1987.](https://pubmed.ncbi.nlm.nih.gov/2682252/)