NOMENCLATURE
The nomenclature of C. neoformans and C. gattii species complexes (Table 25.1) has the potential to be confusing and controversial, as there have been numerous taxonomic changes over the past 40 yr and it continues to be in a state of flux (Kwon-Chung et al.
2017). Nomenclature is further complicated by speciation of the teleomorph Filobasidiella spp., which remains morphologically distinguishable into two species, Filobasidiella neoformans and F. bacillispora, whereas its corresponding anamorphs (C. neoformans and C. gattii species complexes) are in the process of being divided into seven different species and several hybrids (Hagen et al. 2015). It is useful, however, to be aware of the nomenclature utilised over the past 40 yr and the new proposed classification schemes, as they inform on epidemiology and pathogenesis. It is currently accepted that the species complexes, while distinguishable based on morphological and physiological considerations, are only reliably divisible into further subgroupings using various molecular and mass spectroscopy (MALDI-tof) techniques (Firacative et al. 2012; Kwon-Chung et al. 2017). For clinicians, the key questions in relation to taxonomy are: Is the proposed taxonomy correlated with distinct epidemiology or pathogenesis? and Can the diagnostic laboratory routinely and reliably undertake the identification process to the required level? In other words, will the taxonomy make a real difference in terms of how the case should be managed? If it does have an effect, then it is important that the clinician understands the level of identification that the diagnostic laboratory has used and also to be aware of the historical changes to taxonomy when performing literature reviews of cryptococcosis and its epidemiology. As can be seen in Table 25.1, each species complex is clearly divided into four subcategories, with controversy over further subdivision. Within the C. neoformans species complex, the categories are VNI, VNII,Table 25.1. Historical nomenclature of the Cryptococcus neoformans and C. gattii species complexes, with key taxonomic references
The last column describes the current species complexes used in this chapter.
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VNIII and VNIV; within the C. gattii species complex, the categories VGI, VGII, VGIII and VGIV exist while recently a fifth member of the C. gattii species complex has been described in Zambia in association with the African hyrax (Farrer et al. 2019). There are also several hybrids of variable ploidy among these various subdivisions. The newly proposed nomenclature for seven species, even though not accepted by consensus at the 10th International Conference on Cryptococcus and Cryptococcosis in Brazil in 2017, is now making its way into the literature and thus needs to be understood (Table 25.1).
2.