Cryptococcus gattii, a tropical pathogen emerging in a temperate climate zone

Pathogenic cryptococci primarily belong to one of five serogroups: A (Cryptococcus neoformans var. grubii), B or C (Cryptococcus gattii), D (Cryptococcus neoformans var. neoformans), or the hybrid AD. The primary route of exposure to human disease is the lung with possible systemic spread to the central nervous system. Until recently, C. gattii was thought to have a limited habitat in the tropics or subtropics, and to inhabit specific tree hosts, the river gum Eucalypts. Beginning in 1999, increasing numbers of cases of human and animal disease associated with C. gattii were confirmed in hosts with no other travel history than to have lived or to have visited the eastern coast of Vancouver Island, Canada. Vancouver Island is in a temperate climate zone.

As C. gattii is a basidiomycetous yeast associated with the environment, a search was initiated for sources of exposure to this organism. Particular emphasis was placed on documenting airborne exposures to account for the human and animal cases of cryptococcosis.

Samples were taken from the environs of clinical cases (human and animal). Swab samples from the bark of trees, air samples from under the tree canopy, and soil samples from the loam surrounding trees were examined for the presence of Cryptococcus using a differntial agar (Staib agar) for recovery of the organism. Trees which were culture positive by swab sample were mapped by GIS coordinates and repeatedly examined on a monthly basis to determine seasonal variation in air or soil concentration. Molecular fingerprinting (RFLP) was performed on human, animal, and environmental isolates.

A stable ecological niche on the east coast of Vancouver Island was found for pathogenic Cryptococcus, with organisms consistently isolated from swab, air and soil samples over a two year period. No environmental samples have yet been positive from the west coast of British Columbia. Airborne organisms were recovered in higher concentration during the spring and summer (range < LOD to 1514 CFU/m3) than fall and winter (range < LOD to 25 CFU/m3) (GM 8.4 vs GM 1.6, respectively, p < 0.001). Soil samples were less variable seasonally, but varied in concentration spatially, with highest concentrations found mid-Island (Duncan n = 35, GM 1054 CFU/gram), as opposed to the northern-most (Courtenay/Comox n = 17, GM 29 CFU/g) or southern-most (Victoria n = 20, GM 37 CFU/g) samples. Clinical cultures from immunocompetent humans and animals, and the majority of environmental isolates belonged to serogroup B. PCR-RFLP analysis of clinical and environemntal isolates revealed two genetic variants, VGII (> 90%) and VGI (< 10%). Cryptococcus was recovered from multiple species of native BC trees, but not from any introduced Eucalyptus species examined. C.neoformans var. grubii and C. gattii have been repeatedly co-isolated from swab, air, and soil from two clusters of trees located on the southern part of Vancouver Island, but only C. gattii has been cultured from tree clusters in the central area of the Island.

This is the first description of recovery of airborne Cryptococcus from a stable environmental niche in a temperate climate zone. The appearance of this organism as a clinical isolate in humans and animals can be established as late 1999, and may be related to climate. A series of drier than normal summers may be related to the high air concentrations of Cryptococcus during the sampling period 2002-2003.