DESCRIPTIONS OF MEDICAL FUNGI
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106<br />
Descriptions of Medical Fungi<br />
Histoplasma capsulatum Darling<br />
WARNING: RG-3 organism. Cultures of Histoplasma capsulatum represent a severe<br />
biohazard to laboratory personnel and must be handled with extreme caution in a<br />
Class II Biological Safety Cabinet (BSCII).<br />
Histoplasma capsulatum has a worldwide distribution, however the Mississippi-Ohio<br />
River Valley in the USA is recognised as a major endemic region. Environmental<br />
isolations of the fungus have been made from soil enriched with excreta from<br />
chicken, starlings and bats. Histoplasmosis is an intracellular mycotic infection of the<br />
reticuloendothelial system caused by the inhalation of the fungus. Approximately 95%<br />
of cases of histoplasmosis are inapparent, subclinical or benign. The remaining 5% of<br />
cases may develop chronic progressive lung disease, chronic cutaneous or systemic<br />
disease or an acute fulminating fatal systemic disease. All stages of this disease may<br />
mimic tuberculosis. Sporadic cases have been reported in Australia.<br />
Morphological Description: Histoplasma capsulatum exhibits thermal dimorphism<br />
growing in living tissue or in culture at 37 O C as a budding yeast-like fungus and in soil<br />
or culture at temperatures below 30 O C as a mould.<br />
Colonies at 25 O C are slow growing, white or buff-brown, suede-like to cottony with<br />
a pale yellow-brown reverse. Other colony types are glabrous or verrucose, and a<br />
red pigmented strain has been noted (Rippon, 1988). Microscopic morphology shows<br />
the presence of characteristic large, rounded, single-celled, 8-14 µm in diameter,<br />
tuberculate macroconidia formed on short, hyaline, undifferentiated conidiophores.<br />
Small, round to pyriform microconidia, 2-4 µm in diameter, borne on short branches or<br />
directly on the sides of the hyphae may also be present.<br />
Colonies at 37 O C grown on brain heart infusion (BHI) agar containing blood are<br />
smooth, moist, white and yeast-like. Microscopically, numerous small round to oval<br />
budding yeast-like cells, 3-4 x 2-3 µm in size are observed.<br />
Three varieties of Histoplasma capsulatum are recognised, depending on the clinical<br />
disease: var. capsulatum is the common cause of histoplasmosis; var. duboisii is the<br />
African type and var. farciminosum causes lymphangitis in horses. Histoplasma isolates<br />
may also resemble species of Sepedonium and Chrysosporium. Traditionally, positive<br />
identification required conversion of the mould form to the yeast phase by growth at<br />
37 O C on enriched media, however for laboratory safety, culture identification by either<br />
exoantigen test or DNA sequencing is now preferred.<br />
Key Features: Clinical history, tissue morphology, culture morphology and positive<br />
exoantigen test or DNA sequencing.<br />
Molecular Identification: A probe for species recognition is commercially available<br />
(Padhye et al. 1992, Chemaly et al. 2001) and Elias et al. (2012) developed a multiplex-<br />
PCR for identification from cultures. Scheel et al. (2014) developed a loop-mediated<br />
isothermal amplification (LAMP) assay for detection directly in clinical samples which is<br />
affordable and useful in resource poor facilities. ITS sequencing may also be used for<br />
accurate identification (Estrada-Bárcenas et al. 2014, Irinyi et al. 2015).<br />
References: McGinnis (1980), Chandler et al. (1980), George and Penn (1986),<br />
Rippon (1988), de Hoog et al. (2000, 2015).