Climatic Influences on an Emerging Fungal Infection; British Columbia, Canada

Over the past twenty years, the number and geographic range of documented fungal diseases have sharply increased. Opportunistic fungal infections are important causes of death and illness in people with compromised immune systems. Cryptococcus gattii (henceforth C. gattii), is medically important infection in people with compromised and healthy functioning immune systems. A novel type of C. gattii was first documented in Vancouver, Canada in 1999. The fungus further spread to Washington and Oregon by 2004 and continuously circulates in the region. The highest reported rates of C.gattii human and animal incidence was reported in British Columbia, Canada.

Humans and wildlife incidentally inhale propagules and develop potentially fatal respiratory and nervous systems complications. Individuals, who are older, smoke, take certain medications, and/or have pre-existing health problems may be more likely to suffer severe complications. There is long and variable incubation period between fungal exposure and human case diagnosis. Identifying periods with elevated C. gattii risk is difficult from human cases. The fungi's natural habitat appears to be a broad range of decaying trees and the surrounding soil. Environmental samples may provide more information on changes in C. gattii risk over time.

Environmental C. gattii samples exhibit seasonal changes, although the relationships vary according to subtype, location, and environmental medium. There are fundamental limitations of studying seasonal versus short-term C. gattii dynamics. For example, it is difficult to disentangle which biophysical conditions (e.g. temperature, sunlight, moisture, momentum) most influence C. gattii populations. Shorter-term (e.g. weekly, monthly) biophysical changes may more strongly modulate C. gattii dynamics and potential human exposures. Analyzing more frequent observations can provide additional insight into key processes modulating disease risk.

This study examined the relative importance of biophysical conditions for monthly C. gattii samples from the air, trees, and soil in British Columbia, Canada. First, we examine specific plots with four or more repeated measurements over 2002-2004. Plots were sporadically revisited over time after an initial positive sample. The first research question focuses on biophysical relationships to heavily colonized C. gatti plots. The second question studies the first C. gattii measurement from randomly selected plots. Initial samples may more broadly reflect C. gatti dynamics across a study city. Hierarchical Generalized Linear and Mixed Effects Model associated weather conditions to monthly C. gattii concentrations (soil, air) or C. gattii presence/absence (trees). The random effects models accounted for autocorrelation related to repeat sampling on the same plots/cities over time.

The results suggest that weather conditions systematically influence C. gattii concentrations in the soil, air, and trees. In the soil and on trees, warmer temperatures decrease C. gattii abundances at both the plot and city analysis levels. The result appears to be consistent with a Columbian study where C. gattii thrives in relatively cooler temperatures. Interestingly, wind speeds exhibits different relationships to C. gattii in the soil versus trees. Higher wind speeds may mobilize soil C. gattii and it may subsequently adhere to trees. Trees and air samples are more likely to test positive for C. gattii in months with higher shortwave solar radiation. C. gatti produces melanin which may protect the fungus from ultraviolet radiation and confer a competitive advantage.