Be on the Lookout for Blastomycosis - Dr. Larissa Bowman

As your local pathologist, I have observed an increase in confirmed cases of Blastomycosis among WNC canines this year.  Although dogs ‘typically’ present to their DVMs in late summer to early fall with clinical signs of systemic or cutaneous mycoses, local cases may have been exposed during our recent mild winter/spring. 

Here are some pertinent facts concerning Blastomycosis. 

DISEASE SYNDROME:  Proximity to water and exposure to disturbed soil are significant risk factors, but age, sex, and activities such as hunting, swimming and exposure to beavers are not.  While most cases occur in dogs with extensive outdoor exposure, cases also may be seen in indoor pets.  Blastomycosis is acquired by inhaling fungal spores which induce a respiratory and/or disseminated infection.  The most common clinical findings are nonspecific and include loss of appetite, weight loss, and fever.  Respiratory abnormalities are also common, and radiographs may show nodular or interstitial infiltrates.  Draining skin tracts and lymphadenopathy are commonly observed.  Among fatal cases, the organs most often involved are the lungs, eyes and skin.  Ocular lesions occur in about one third of cases. 

DIAGNOSIS:  Cytology and/or histopathology are considered the gold standard method for diagnosis.  Cytology was positive in 71% of cases in one report, including mostly skin and lymph node samples plus occasional transtracheal washes.  The presence of antigens can be detected in urine and/or serum in dogs with blastomycosis, and a commercial assay has been refined to permit quantification.  The sensitivity is above 90% in urine, but false negative results do occur, mostly in dogs with mild or localized disease.  Thus, a negative result does not exclude the diagnosis.  Repeat antigen testing or demonstration of yeast in body fluids or tissues may be positive in such cases.  Nearly complete cross-reactivity occurs between antigen detection in histoplasmosis and blastomycosis.  The two mycoses cannot be differentiated by antigen detection so there is no need to perform the test for both infections.  Antibody detection permits differentiation of these mycoses in over 90% of cases.

PROGNOSIS:  Although effective therapy is available, one quarter of dogs with blastomycosis die, usually during the first week of treatment (most often due to respiratory failure).  There is a strong correlation between the extent of lung involvement and survival time.  Outcome is especially poor in cases with brain, spinal cord, or ocular involvement.  Amphotericin B has been used for treatment of blastomycosis in dogs and cats and may still be the treatment of choice in severe cases because it induces a more rapid  clinical response.   Itraconazole is currently the treatment of choice for blastomycosis in dogs.  Brand-name itraconazole (Sporanox®, Janssen Pharmaceuticals) or generic itraconazole capsules should be used, as compounded powder formulations have poor bioavailability.  The optimal duration of therapy has not been determined.  Plumb recommends two months for fluconazole but two to three months for itraconazole (42)). Among cases in dogs that survive the initial illness, relapse occurs in about 25% of cases, usually within the first year following therapy.  The antigen test is also used in deciding when to stop therapy and to diagnose relapse. 

Infectious Diseases of the Dog and Cat, C. Greene, 4th ed.
Arceneaux KA, Taboada J, Hosgood G. Blastomycosis in dogs: 115 cases (1980-1995). J Am Vet Med Assoc 1998 Sep 1;213(5):658-64.