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Basics of blastomycosis (Proceedings)

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Aug 01, 2009

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.