Blastomycosis is a systemic fungal infection caused by the dimorphic fungi Blastomyces dermatitidis. Blastomycosis affects dogs, cats, horses, sea lions, lions, rhesus monkeys and polar bears. Dogs are the species most often
infected. In endemic areas, dogs are infected at a rate of 10 times that of people. It is suspected that being close to the
ground and sniffing the soil accounts for the increased incidence of blastomycosis in dogs. The geographic distribution of
disease in animals parallels that in people with most cases in the Mississippi, Ohio and Missouri River basins. In the south,
there is no distinct seasonal occurrence but in areas such as Wisconsin disease often follows the fall hunting season. Proximity
to water is a major risk factor in dogs with most infected dogs living within 500 yards of water. Larger dogs are more likely
to become infected probably because they are more likely to roam. Sporting dogs such as Labrador Retrievers are at increased
risk. Some non-sporting breeds such as Doberman Pinschers are at increased risk because of suspected increased breed susceptibility
to infection with Blastomyces. Outbreaks in neighborhoods suggest a common source of infection or "ecologic niche" where many animals come in contact with
the organism and become infected.
Most cases of blastomycosis are acquired by inhalation of the spores from mycelial growth in the environment. The spores enter
the terminal airway and establish a primary infection in the lungs. The size of the yeast when it grows at body temperature
precludes its entering the terminal airway in an aerosol. Most dogs with blastomycosis first develop a fungal pneumonia. The
organism then disseminates throughout the body with skin, lymph nodes, eye and bone often infected. Subclinical infections
are uncommon and apparent localized skin lesions are usually part of a systemic disease. Less common sites of infection are
testes, mammary glands, prostate, heart and brain. Eye involvement produces a uveitis, chorioretinitis, retinal detachment
and secondary glaucoma.
Dogs with blastomycosis usually have signs of anorexia, weight loss, cough, dyspnea, ocular disease, lameness, or skin lesions.
Signs of disease usually have been present for a few days to a week but may have been apparent for up to a year. In some dogs,
the disease process seems to stabilize; animals may show minimal signs for weeks to months, and then the disease suddenly
progresses with worsening of signs.
Eighty-five percent of dogs with blastomycosis have lung lesions with characteristic dry, harsh lung sounds. Dogs with mild
lung disease show exercise intolerance, and severely affected dogs have dyspnea at rest. Coughing is a variable finding. Thoracic
radiographs are indicated for dogs suspected of having blastomycosis because some dogs have lung changes without respiratory
signs. Diffuse, nodular interstitial and bronchointerstitial lung changes are most commonly seen. Other less common manifestations
include well-marginated solitary to multiple cystic or solid nodules to masses. Tracheobronchial lymphadenomegaly occurs in
some dogs. Pleural effusion, pneumomediastinum, and cavitary lung lesions are also observed. Chylothorax and solid fibrous
masses are uncommon manifestations of thoracic blastomycosis. Solid fibrous masses may partially occlude the great vessels.
Up to 40% of dogs with blastomycosis have ocular lesions, the most common of which is uveitis. Early signs of uveitis are
aqueous flare, miosis, blepharospasm, and photophobia . Retinal separation with detachment, retinal granulomas, and vitreal
hemorrhage are also seen. Severe corneal edema may prevent good visualization of the internal ocular structures. Glaucoma
secondary to angle closure occurs in blastomycosis. Periorbital cellulitis and involvement of the nictitating membrane also
occur. Uveitis in conjunction with signs of respiratory or skin disease should alert the clinician to consider blastomycosis.
Early diagnosis and appropriate treatment are essential to preservation of vision in blastomycosis. Anterior segment disease
has a worse prognosis for maintaining vision because glaucoma is a common sequela to ocular blastomycosis.
Skin lesions, found in 20% to 40% of dogs with blastomycosis, may be ulcerated with drainage of a serosanguineous or purulent
fluid. Other lesions may be granulomatous, proliferative, and meaty. There may be welldefined subcutaneous abscesses. Although
the skin lesions may be found anywhere, the planum nasale, the face, and the nail beds appear to be preferred sites.
Bone involvement occurs in up to 30% of infected dogs. Lameness is the primary sign in affected animals and may be the only
sign of disease. Special procedures, such as bone scans, may identify a greater percentage of dogs with bone involvement.
Lesions usually involve the appendicular skeleton; they are usually osteolytic with periosteal proliferation and soft tissue
swelling . A majority of the bone lesions are solitary and occur distal to the stifle and elbow. Fungal osteomyelitis must
be differentiated from primary and metastatic bone tumors and bacterial osteomyelitis.