Culture
Culture of Blastomyces spp. from a clinical specimen is the gold standard diagnostic for blastomycosis. Blastomyces dermatitidis/gilchristii grow on general fungal media such as Sabouraud's dextrose agar or potato dextrose agar, incubated at 25-30 °C (Bradsher 2014b).
Growth of white to buff colonies usually appears within 10-14 days but may require up to 6 weeks incubation (Bradsher 2014b). Microscopically, B. dermatitidis/gilchristii characteristically have conidiophores of varying lengths that run perpendicular to hyphae and terminate with single conidia that resemble “lollipops” (Bradsher 2014b). In contradistinction, the conidia of B. percursus appear in clusters at the end of conidiophores (Dukik et al. 2017). Traditionally, conversion of the mold to a yeast phase at 37°C was performed for confirmation of the identification, but this is rarely done today (Saccente and Woods 2010). Most microbiology laboratories confirm identification of B. dermatitidis/ gilchristii with a DNA probe (AccuProbe; GenProbe Inc., San Diego, CA), but cross-reactions can occur, including with other dimorphic fungi (Saccente and Woods 2010).The diagnostic yield of culture is surmised from a retrospective review of cases of human pulmonary blastomycosis (Martynowicz and Prakash 2002). Blastomyces dermatitidis was isolated from the first sputum sample in 75% of cases, which increased to 81% after a mean of 2.3 samples (Martynowicz and Prakash 2002); the diagnostic yield of bronchial washings was even higher. Nonetheless, the true sensitivities are likely lower since only diagnosed cases (i.e., those with at least one positive test) are included in this type of study.
Despite favorable operating characteristics, culture has important limitations. Slow turnaround times limit reliance on this test for management decision (Bradsher 2014b). In addition, occupational exposure of laboratory workers to highly infectious mycelia is a real concern (Denton et al. 1967). While culture is a standard investigation of humans suspected of blastomycosis (Bradsher 2014b), it is rarely used in veterinary practice (Legendre 2012; Sykes and Merkel 2014). In a survey of small-animal veterinary practices in Wisconsin, 80% of respondents reported that they never used culture for the diagnosis of blastomycosis (Anderson et al. 2014). Similarly, large retrospective case series of dogs with pulmonary blastomycosis from Louisiana (Arceneaux et al. 1998) and Minnesota (Crews et al. 2008a) found that fungal cultures were sent for only 17 of 115 dogs (12%) and 6 of 125 dogs (5%), respectively.
8.4.2