Cryptococcus gatii: At the Crossroads of Climate and Infectious Diseases

cryptococcus gatii

Climate change is hitting the headlines again! The fact that this has become one of the issues that the leading candidates in the race for the US Presidential seat are quibbling over, has further increased the airtime dedicated to this matter. I thought this would be the ideal time to reflect on a particular case study where climate and infectious diseases come together: the case of Cryptococcus gatii  infections and the role played by climate in its recent rising trend. Also, this is an opportunity to talk about an emerging fungal infection associated with climate change, and climatic patterns – attributes we usually associate with explosive outbreaks of viral illnesses! The information in this blog post reflect the findings from a not-too-old study published in the US CDC’s journal Emerging Infectious Diseases. Image Credits: US CDC via The Conversation.


The study published in the US CDC’s journal [1] states in its abstract:

Vancouver Island, Canada, reports the world’s highest incidence of Cryptococcus gattii infection among humans and animals. To identify key biophysical factors modulating environmental concentrations, we evaluated monthly concentrations of C. gatti in air, soil, and trees over a 3-year period. The 2 study datasets were repeatedly measured plots and newly sampled plots. We used hierarchical generalized linear and mixed effect models to determine associations. Climate systematically influenced C. gattii concentrations in all environmental media tested; in soil and on trees, concentrations decreased when temperatures were warmer. Wind may be a key process that transferred C. gattii from soil into air and onto trees. C. gattiiresults for tree and air samples were more likely to be positive during periods of higher solar radiation. These results improve the understanding of the places and periods with the greatest C. gattii colonization. Refined risk projections may help susceptible persons avoid activities that disturb the topsoil during relatively cool summer days.

The authors also identify a few reasons, which make the Vancouver area specially prone to this fungus. In addition, they also find some issues with the data and try to explain the apparent temporal de-synchrony observed between the reported cases and weather patterns:

In British Columbia, Canada, C. gattii exhibits specialized habitat preferences. It thrives in the area of the Vancouver Island rain shadow (i.e., southeast coast of Vancouver Island and the southwest coast of mainland British Columbia), where winter temperatures are predominantly above freezing and summer temperatures are not too hot (15).

Geographic areas and periods with elevated temperatures decreased isolation of C. gattii from tree samples and concentration in soil. The results are consistent with C. gattii serotype B in Colombia, where C. gattii was sampled from the detritus of trees of species with persistent and elevated C. gattiiconcentrations (Eucalyptus camaldulensis and Terminalia cattapa) (18). In that study, the greatest proportions of positive samples were also found during periods of lower temperatures. Similarly, an elevational transect study conducted at elevations of 300–3,000 m found that C. gattii concentrations were greater at high elevations with cold temperatures (12°C–18°C annual average temperatures) than in temperate and tropical regions (19). In the Vancouver Island study area, average annual temperatures in C. gattii–endemic areas were slightly cooler (9.8°C–11.4°C). Outbreaks of C. gattii infection in humans or animals in Western Australia, Mediterranean Europe, and North America have been characterized by dry summers or dry winters with warm but not hot monthly temperatures (<22°C) (20). Laboratory studies of the optimum growth rates for C. gattii and competitors have not been conducted. This knowledge might provide a stronger mechanistic interpretation of temperature associations. According to research of other Basidiomycota, temperature may influence the ecologic niche by regulating the rate of enzyme-catalyzed reactions (21).

According to our results, the highest airborne C. gattii concentrations occur during August–October on sunny days with moderately windy conditions. The greatest risk for exposure to C. gattii from the soil is during relatively cool June and July summer days. Although these associations are consistent, until more research provides information about the infectious dose for humans, the study results characterize the risk for exposure associated with environmental factors, rather than disease risk. Weather and airborne concentrations of C. gattii should be associated with human cryptococcosis incidence; however, onset of documented cryptococcosis cases in British Columbia does not vary by season or month (28,29). The temporal discrepancy may be masked by the long and variable incubation period of this pathogen. Host factors may be stronger predictors of developing disease risk (30). Nonetheless, refined risk projections may benefit susceptible humans and animals living in areas of marginal C. gattii transmission.

Interestingly, C. gatii causes about 25 cases with four or five deaths a year. The numbers are not that high, but given the mode of transmission and the virulence and associated mortality, this could be an underappreciated cause of pulmonary diseases and deaths.

cryptococcus gatii pneumoniaRadiographic image of the posteroanterior (A) and lateral (B) chest of a 27-year-old woman presenting with shortness of breath, muscle aches and cough. Multiple pulmonary nodules with mass-like areas of opacification can be seen. (Image Source: CMAJ [2])

The relative lack of understanding of the disease is also fuelled by the paucity of information and data related to its epidemiology. This was especially the case in the United States, where the surveillance for Cryptococcal infections was not followed up by identification to the level of species. Consequently, there has been a systematic lack of information about the disease.pacific northwest and cryptococcus gatii

Map of the Pacific Northwest, comprising parts of British Columbia, Canada, and the states of Washington and Oregon in the United States, showing human and veterinary Cryptococcus gattiicases (including marine mammals) by place of residence or detection, and locations of environmental isolation of C. gattii during 1999–2008 (strain NIH444 [Seattle] or CBS7750 [San Francisco] not included). Image Credits: CDC EID [3]

Given the impact of climate on the occurrence of this disease, and the fact that its spread might be encouraged by the warming up of erstwhile cold areas, this is an emerging fungal infection that needs to be watched out for!

References

  1. Uejio CK, Mak S, Manangan A, Luber G, Bartlett KH. Climatic influences on Cryptoccoccus gattii populations, Vancouver Island, Canada, 2002–2004. Emerg Infect Dis. 2015 Nov [30 March, 2016]. http://dx.doi.org/10.3201/eid2111.141161 DOI: 10.3201/eid2111.141161
  2. Johannson KA, Huston SM, Mody CH, Davidson W. Cryptococcus gattii pneumonia. CMAJ. 2012 Sep 4;184(12):1387-90. doi: 10.1503/cmaj.111346. Epub 2012 Aug 13. PubMed PMID: 22891210; PubMed Central PMCID: PMC3447050.
  3. Datta K, Bartlett KH, Baer R, Byrnes E, Galanis E, Heitman J, Hoang L, Leslie MJ, MacDougall L, Magill SS, Morshed MG, Marr KA; Cryptococcus gattii Working Group of the Pacific Northwest. Spread of Cryptococcus gattii into Pacific Northwest region of the United States. Emerg Infect Dis. 2009 Aug;15(8):1185-91. doi: 10.3201/eid1508.081384. Review. PubMed PMID: 19757550; PubMed Central PMCID: PMC2815957.
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