Final Diagnosis -- Blastomyces dermatididis


Blastomyces dermatididis


Blastomyces is a dimorphic fungus endemic to the Mississippi and Ohio River valleys. Severe infections more commonly occur in immunosuppressed patients. Clinically, Blastomyces may enter the lungs to cause an asymptomatic, transient, or slowly progressive disease. Chronic infection may resemble a neoplastic process with a localized nodule and low grade fevers. The disease may become systemic and spread to skin, skeletal system, nervous system, or genitourinary system. With a Grocott stain on fixed tissue, yeast measuring 8 to 15 micrometers with broad based budding may be seen [1].

Cultures often produce a white or tan fluffy colony in 1-2 weeks. The microconidia are lollipop shaped but no macrocondia are formed. Yeast forms can be grown at 37 degrees Celsius on BHI plates to form cream-colored colonies with a granular surface to demonstrate dimorphism. These yeast will have the characteristic broad based budding [2]. AccuProbe, a commercially available nucleic acid probe, is used for confirmation. Blastomyces is sensitive to most antifugals.

One of the major differentials is Histoplasma capsulatum. It is a dimorphic fungus found most commonly in the Mississippi and Ohio River valleys. Following exposure, the clinical presentation will vary depending on the dose of exposure and immune status of the individual. Like Blastomyces, it can be asymptomatic or cause an acute pulmonary infection with fevers and cough. It may also cause a granulomatous lymphadenitis or may disseminate. When seen histologically, they appear as round to oval, intracellular yeast. The yeast are 3-5 micrometers with narrow based budding. Diagnosis is aided by GMS or PAS stains. Like Blastomyces, it forms white to brown fluffy colonies after about 2 weeks. The microconidia are very similar to those of Blastomyces, but unlike Blastomyces, Histoplasma will form tuberculate macroconidia at the conidia edge. Confirmation can be made with AccuProbe with high specificity [3], though cross-reactivity with other species has been seen [4]. Diagnostic issues between Histoplasma and Blastomyces also occur with the urine and serum antigen testing. Cross-reactivity has been documented, with up to 63% of patients with Blastomyces testing positive for the earlier generation Histoplasma antigen test [5]. The more recent generation of Histoplasma antigen test has an improved sensitivity, though cross-reactivity with Blastomyces and other endemic mycoses, such as Coccidioides, Paracoccidiodes, and Penicilliosis marneffei [6].


  1. Henry's Clinical Management by Laboratory Methods. Editors McPherson, R., and Pincus, M. 22nd ed. 2011. Elsevier Saunders, Philadelphia.
  2. Guide to Clinically Significant Fungi. Sutton, D, Fothergill, A, and Rinaldi, M. 1st edition. 1998. Williams and Wilkins.
  3. Hall, G, et al. Evaluation of a chemiluminescent probe assay for identification of Histoplasma capsulatum isolates. J Clin Microbiol. Nov 1992; 30(11): 3003-3004.
  4. Brandt, M., et al. False-Positive Histoplasma capsulatum Gen-Probe Chemiluminescent Test Result Caused by a Chrysosporium Species. J Clin Microbiol. Mar 2005; 43(3): 1456-1458.
  5. Wheat, J, et al. Cross-reactivity in Histoplasma capsulatum variety capsulatum antigen assays of urine samples from patients with endemic mycoses. Clin Infect Dis. 1997 Jun;24(6):1169-71.
  6. Connolly, P., et al. Detection of Histoplasma Antigen by a Quantitative Enzyme Immunoassay. Clin Vaccine Immunol. Dec 2007; 14(12): 1587-1591.

Contributed by Aaron Berg, MD, and William Pasculle, Sc.D.

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