Ecological Niche Modeling of Cryptococcus Gattii in British Columbia, Canada

Article excerpt

BACKGROUND: Cryptococcus gattii emerged on Vancouver Island, British Columbia (BC), Canada, in 1999, causing human and animal illness. Environmental sampling for C. gattii in southwestern BC has isolated the fungal organism from native vegetation, soil, air, and water.

OBJECTIVES: Our aim was to help public health officials in BC delineate where C. gattii is currently established and forecast areas that could support C. gattii in the future. We also examined the utility of ecological niche modeling (ENM) based on human and animal C. gattii disease surveillance data.

METHODS: We performed ENM using the Genetic Algorithm for Rule-set Prediction (GARP) to predict the optimal and potential ecological niche areas of C. gattii in BC. Human and animal surveillance and environmental sampling data were used to build and test the models based on 15 predictor environmental data layers.

RESULTS: ENM provided very accurate predictions (> 98% accuracy, p-value < 0.001) for C. gattii in BC. The models identified optimal C. gattii ecological niche areas along the central and south eastern coast of Vancouver Island and within the Vancouver Lower Mainland. Elevation, biogeodimatic zone, and January temperature were good predictors for identifying the ecological niche of C. gattii in BC.

CONCLUSIONS: The use of human and animal case data for ENM proved useful and effective in identifying the ecological niche of C. gattii in BC. These results are shared with public health to increase public and physician awareness of cryptococcal disease in regions at risk of environmental colonization of C. gattii.

KEY WORDS: British Columbia, cryptococcal disease, cryptococcosis, Cryptococcus gattii, ecological niche modeling, fungal disease, GARP, Genetic Algorithm for Rule-set Prediction, geographic information systems, GIS. Environ Health Perspect 118:653-658 (2010). doi:10.1289/ ehp.0901448 [Online 17 December 2009]

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Cryptococcus gattii appeared on Vancouver Island, British Columbia (BC), Canada, in 1999, causing human and animal illness (Duncan et al. 2005; Kidd et al. 2004; MacDougall et al. 2007; Stephen et al. 2002). Cryptococcal disease is initiated by inhalation and infection by microscopic-size fungal propagules (yeast cells or spores - 1-2 [micro]m in size) through environmental exposure. Cryptococcosis is a potentially fatal infection of the lungs and central nervous system. By the end of 2008, > 240 documented human cases and 360 animal cases had been reported in BC. These cases represent the highest incidence rate of C. gattii infection and the only documented multispecies outbreak of cryptococcosis in the world (Bartlett et al. 2007; MacDougall et al. 2007; Stephen et al. 2002).

C. gattii has been isolated in BC from native vegetation, soil, air, and water on the south and central eastern coast of Vancouver Island and, more recently, on the Gulf Islands, the Lower Mainland, and in the states of Washington and Oregon (Bartlett et al. 2007; Byrnes et al. 2009; Datta et al. 2009; Kidd et al. 2004, 2007a, 2007b; MacDougall et al. 2007; Upton et al. 2007). Before its discovery in the temperate rainforest region of northwestern North America, C. gattii infections and environmental exposures were described in tropical and subtropical regions of the world such as Australia, Africa, India, Italy, Papua New Guinea, South America, and southern California (Casadevall and Perfect 1998; Ellis and Pfeiffer 1990; Kwon-Chung and Bennett 1984; Sorrell 2001).

The epidemiology, genetic characterization, geographic distribution, and environmental sampling for C. gattii in BC have been well described elsewhere (Bartlett et al. 2007; Datta et al. 2009; Duncan et al. 2005; Fraser et al. 2005). The purpose of our study was to help public health officials in BC delineate where C. gattii is currently established and forecast areas that could support it in the future. …