April 23, 2010

When South is North (aka Let’s Blame Global Warming)

Filed under: Health Effects

The story making the rounds today is that global warming may be responsible for unleashing a deadly new disease on the U.S. and Canada.

But, a closer look at the evidence reveals that the global warming link is rather slim to none (and that’s putting things generously).

The story was generated by a just-published article in the journal Pathogens by Edmond Byrnes of the Duke University Medical Center’s Department of Molecular Genetics and Microbiology and several of his colleagues.

Reuters picked up the story, which was then highlighted on the Drudge Report, which, in turn, caused it to go, well, viral.

Actually, “viral” is not quite correct, for the new disease is the result of a fungus—Cryptococcus gattii, to be exact.

For the past 10 years or so, there has been an outbreak of the life-threatening disease caused by inhaling C. gattii spores that has occurred in the coastal regions of the Pacific Northwest. From about 1999 to 2003, the outbreak was primarily confined to Canada’s Vancouver Island, but during 2004 to 2009, instances of the disease spread to the mainland coast of British Columbia, and then southward to coastal Washington and Oregon—all locations with a relatively similar climate of wet, mild winters.

And, as with anything that is considered bad, a recent occurrence coupled with a climate tie-in leads to global warming being to blame.

Here is how the connection is made in the Reuters article:

“From 1999 through 2003, the cases were largely restricted to Vancouver Island,” the report reads.

“Between 2003 and 2006, the outbreak expanded into neighboring mainland British Columbia and then into Washington and Oregon from 2005 to 2009. Based on this historical trajectory of expansion, the outbreak may continue to expand into the neighboring region of Northern California, and possibly further.”

Freezing can kill the fungus and climate change may be helping it spread, the researchers said.


It is funny how “climate change” is causing the outbreaks to spread southward. Just in case anyone needs a refresher on climatology in the Northern Hemisphere, moving southward generally takes you to warmer environs. Figures 1 and 2 show that this generality also applies to the region of the C. gattii outbreaks.

Figure 1 shows where the disease has occurred.

Figure 1. 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 gattii cases (including marine mammals) by place of residence or detection, and locations of environmental isolation of C. gattii during 1999–2008 (from Datta et al., 2009).

Figure 2 shows that annual mean temperature in the city of Vancouver, BC (near the north end of the region of outbreak) and Portland, Oregon (near the south end of the outbreak region). While it is true that this general region is experiencing a warming trend, the average temperature currently in Vancouver is still less than the average temperature in Portland 100 years ago. So, if global warming is to blame for making the climate of Vancouver now warm enough to be favorable for the disease, why on earth hasn’t it been found in Portland for over a century?

Figure 2 (top). Mean annual temperature history from Vancouver, British Columbia, (bottom), mean annual temperature history from Portland Oregon (source, NASA GISS).

If global warming were to blame, the disease ought to be moving northward into regions where it would have otherwise been too cold in years past. Moving southward because of climate change doesn’t make any sense, as the conditions for the occurrence of the disease would have already been well-established to the south. Instead, a southward spread is an indication of a recent introduction of a disease in one location that then spreads to surrounding locations that were already climatologically favorable.

The very same thing happened in the U.S with the West Nile Virus. Some people insist on linking the spread of West Nile Virus to climate change, despite the fact that it was introduced into New York City in 1999 (hardly the warmest place in the U.S.) and then quickly spread southward and westward. In less than 10 years, West Nile Virus was found in virtually all of the contiguous 48 states (Figure 3). The driver of this rapid expansion was not climate change, but that the extant climate/ecology of the U.S. is one in which the virus can thrive. The reason that it was not found in the past was simply because it had not been introduced.

Figure 3. Spread of the occurrence of the West Nile Virus from its introduction to the United States in 1999 through 2007. By 2003, virtually every state in the country had reported the presence of virus. (Source: Centers for Disease Control)

Instead of “climate change” driving the spread of the C. gattii fungus, human mobility probably has a lot to do with it. A report in the Centers for Disease Control (CDC) journal Emerging Infectious Diseases by Sarah Kidd et al. looked at dispersal mechanisms and concluded:

Recent Cryptococcus gattii infections in humans and animals without travel history to Vancouver Island, as well as environmental isolations of the organism in other areas of the Pacific Northwest, led to an investigation of potential dispersal mechanisms. Longitudinal analysis of C. gattii presence in trees and soil showed patterns of permanent, intermittent, and transient colonization, reflecting C. gattii population dynamics once the pathogen is introduced to a new site. Systematic sampling showed C. gattii was associated with high-traffic locations. In addition, C. gattii was isolated from the wheel wells of vehicles on Vancouver Island and the mainland and on footwear, consistent with anthropogenic dispersal of the organism. Increased levels of airborne C. gattii were detected during forestry and municipal activities such as wood chipping, the byproducts of which are frequently used in park landscaping. C. gattii dispersal by these mechanisms may be a useful model for other emerging pathogens.

And how was it introduced to the Pacific Northwest in the first place? Kidd et al. have this to say:

A specific route of C. gattii introduction to the Pacific Northwest has not been established, although 1 hypothesis implicates importation of contaminated trees. The data presented here indicate that C. gattii could also have been introduced by mechanical vectors such as vehicles or footwear, or by wooden pallets or crates that are not routinely inspected for microbial contamination upon entry into Canada. Bird and animal migration may be involved in C. gattii dispersal through passive transport as well. Certainly, a large number of migratory birds pass through the disease-endemic area on Vancouver Island.

This story reminds us of another that played to a lot of fanfare a few years ago—that global warming was killing amphibians around the world, and specifically the endangered Golden Toad found in Costa Rica’s mountain cloud forests. Now, after years of more study, it has been pretty firmly established that it was the chytrid fungus, and not global warming, that was wiping out amphibian populations (including the Golden Toad). And what was identified as a primary culprit for spreading the chytrid fungus? Inadvertently shipping infected frogs and toads around the world for scientific research and “conservationists” traipsing around trying to survey and study wild toad populations and carrying the fungus around on the soles of their shoes. Nice job guys!

So at the end of the day, what are we left with? Several examples of the spread of disease that were initially blamed on global warming (hey why not, everything else is) but when subject to a more thorough and less emotional review, turned out to be explained primarily by global, regional, and local transportation of goods and people and a tie-in to the existing climate.

Apparently, when faced with a new threat, the response is simply to shoot first and ask questions later.


Byrnes, E.J., 2010. Emergence and Pathogenicity of Highly Virulent Cryptococcus gattii Genotypes in the Northwest United States. Pathogens, 6, e1000850.

Datta, K., et al., 2009. Spread of Cryptococcus gattii into the Pacific Northwest Region of the United States. Emerging Infectious Diseases, 16, 1185-1191.

Kidd, S.E., et al., 2007. Cryptococcus gattii dispersal mechanisms, British Columbia, Canada. Emerging Infectious Diseases, 13, 51–57.

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