Canine blastomycosis

Kris Ruotsalo

Animal Health Laboratory, University of Guelph, Guelph, ON

AHL Newsletter 2019;23(4):19-20.

Blastomycosis is a systemic mycotic infection caused by the dimorphic fungus Blastomyces dermatitidis. This organism grows in acidic soil in proximity to water and within decaying organic matter as a saprophytic mycelial form that reproduces sexually, producing infective spores. Resident environmental organisms are thought to restrict the survival of Blastomyces, with a special set of environmental conditions required for proliferation of the organism; rain or heavy dew appears to facilitate the release of infectious spores. Blastomycosis is primarily a disease of North America, and is considered endemic to the Mississippi, Missouri, and Ohio River valleys, the Mid-Atlantic States, Manitoba, Ontario, Quebec, and southern Saskatchewan. 

Most cases of blastomycosis are acquired by inhalation of spores. The spores enter the terminal airway and establish a primary infection in the lungs. At body temperature in tissues, the organism transforms into the yeast form and replicates asexually. The budding yeast forms are 5 to 20 um in diameter and have a thick, refractile, double-contoured cell wall. The yeast forms are too large to enter the terminal airway in an aerosol, thus transmission through coughing is unlikely. Following establishment of infection in the lungs, the yeast disseminates throughout the body via the lymphatics and vasculature. Preferred sites of infection in the dog include the lung, eyes, and skin, but many other sites including lymph nodes, bones, and subcutaneous tissues may be affected. In dogs, subclinical blastomycosis infection is unlikely.

Direct inoculation of Blastomyces into a wound from the soil may also occur, but is considered less likely. Because of this, cutaneous blastomycosis is usually considered a manifestation of disseminated disease. 

Clinical signs of blastomycosis in dogs can include, but are not restricted to, anorexia, weight loss, cough, dyspnea, ocular disease, lameness, pyrexia, and skin lesions. These signs may have been apparent for days to weeks (occasionally longer) and are of varying severity. CBC and serum biochemistry usually reveal nonspecific findings of mild anemia, leukocytosis, and hyperglobulinemia. Diagnosis is typically made by identification of Blastomyces yeast through cytological or histological preparations. The site of sampling depends upon the clinical presentation, but when appropriate, evaluation of bronchoalveolar lavage fluid, aspiration of enlarged lymph nodes, and impression smears of skin lesions typically reveal a marked inflammatory response (suppurative to mixed inflammation), along with low numbers of characteristic yeast. Serology may occasionally be required to help establish a diagnosis if identification of organisms is not rewarding. Culture is not indicated due to the risk to laboratory personnel from inhalation of the mycelial form of the organism.

It is important to note that the yeast phase of Blastomyces cannot be transmitted from animals to people, or from person to person through aerosols. Care should be taken to avoid bites when handling a dog infected with blastomycosis, and contaminated knives or needles can accidently inoculate veterinarians during biopsy or postmortem procedures.

Recently, two cases of canine infection with Blastomyces dermatitidis were identified in samples submitted to the AHL Clinical Pathology Laboratory. The first case involved a 1-year-old female spayed mixed breed dog that initially presented with a history of lameness. Subsequently she developed respiratory signs and interdigital skin lesions with enlargement of draining lymph nodes. The second case was a 3-year-old, female spayed Boxer with a month-long history of dyspnea that apparently resolved without treatment. She then developed multiple draining cutaneous lesions on the hind limbs and the flank. Fine needle aspiration of lymph nodes and impression smears of skin lesions from both patients revealed marked suppurative inflammation with numerous extracellular yeast.  Modified Wright’s-stained smears obtained from an affected lymph node illustrate the classic refractile, deeply basophilic, thick-walled structure of these yeast (Fig. 1A), along with evidence of broad-based budding (Fig. 1B).

A Blastomyces dermatitidis yeast in a fine aspirate of a lymph node (arrow) (modified Wright’s).	B Broad-based budding of Blastomyces dermatitidis yeast (arrow) (modified Wright’s).

Figure 1. A Blastomyces dermatitidis yeast in a fine aspirate of a lymph node (arrow) (modified Wright’s).  B Broad-based budding of Blastomyces dermatitidis yeast (arrow) (modified Wright’s).

Photos courtesy of Dr. Felipe Reggeti


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3. Brown et al. Epidemiology and geographic distribution of blastomycosis, histoplasmosis, and coccidiomycosis, Ontario, Canada 1990-2015. Emerg Infect Dis 2018;24:1257-1266.