Clinical Pathology Case Study: Blastomycosis in a dog

Cytologic description and interpretation

The aspirate is highly cellular with a mixture of non-degenerate neutrophils and epithelioid macrophages.  Large numbers of 12-15um round basophilic yeast with a thin wall are present and often found in pairs.

Interpretation: Pyogranulomatous inflammation with Blastomyces dermatitidis yeast (Blastomycosis).

Discussion

Blastomycosis (Blastomyces dermatitidis) is a systemic fungal infection acquired from soil, most notably in river valleys of the Eastern United States and parts of Canada.  After infection establishes itself in the lungs, it disseminates throughout the body.  Skin, eyes and lymph nodes are the most common sites of dissemination in dogs but any tissue can be affected.  While mostly found in dogs, infection has also been reported in cats and other species.

Cytologic identification of the yeast from skin nodules and lymph node aspirates is a quick and reliable method of diagnosis, although organisms are not always present.  Lung washes can also produce a cytologic diagnosis but less reliably: only 40% of confirmed respiratory cases reveal organisms with lung washing.

Other diagnostic options include urine antigen testing (a quantitative, non-invasive and sensitive method) and PCR testing (sensitive method if the sample contains organisms).  Serology is often inadequate for diagnosing Blastomycosis with low sensitivity.  Culture is not recommended since the cultured form is zoonotic and most veterinary laboratories are not licensed for this procedure.

Treatment of Blastomycosis predominantly relies on azole medications and is often prolonged, lasting months.  The prognosis is variable, depending on the severity of the initial infection and the tissues affected.