When a deadly tropical infection appeared on the chilly west coast of North America in 1999, its origin was a mystery. Now, researchers believe a giant earthquake 35 years earlier was to blame.
More than 300 people have been hit by a mysterious outbreak of Cryptococcus gattii fungal infection in the Pacific Northwest over the past 20 years, with cases still occurring in humans and wildlife.
Before then, C. gattii had been confined almost entirely to South America, Papua New Guinea and Australia.
The fungus typically infects people through inhalation. It produces a pneumonia-like illness, and may also spread to the brain, causing potentially fatal meningoencephalitis. Published case reports suggest a mortality rate of more than 10 percent.
How this nasty tropical disease reached the cool shores of North America has puzzled epidemiologists since the first cases appeared on Vancouver Island.
Theories to explain its presence have included global warming and the import of tropical eucalyptus trees.
But researchers at the Johns Hopkins Bloomberg School of Public Health and the nonprofit Translational Genomics Research Institute now believe they have the answer: it’s due to a chain of events involving a canal, shipping movements, an earthquake, and the recent evolution of the fungus itself.
The trouble may have started when the Panama Canal opened in 1914, increasing shipping significantly between Atlantic and Pacific ports.
This, the researchers write in mBio, brought C. gattii from south to north, possibly in ships’ ballast tanks. Ships in those days routinely took on water in one port and simply discharged it, without treatment, in another. {%recommended 7268%}
This part of the theory is supported by a “molecular clock” analysis of the DNA sequence of the C. gattii fungus subtypes found in North America, which shows they would have arrived from Brazil or nearby between 60 and 100 years earlier.
So how did C. gattii come to colonise the west coast of Canada and the United States?
This is where the Great Alaskan Earthquake of 1964 comes in.
The 9.2 quake of 1964 is the largest recorded in the northern hemisphere, and its effects were felt as far away as Hawaii. It now seems that one of its unseen effects was to carry the fungus to shore on a series of tsunamis.
The quake had its epicentre in southeastern Alaska but spawned tsunamis throughout the North Pacific. These inundated coastal areas of British Columbia, Washington, Oregon, and California. The affected regions correspond broadly to the locations where C. gattii has been found and human infections have occurred.
But that was in 1964 – decades before the first infection in humans. Why did it take so long to emerge?
The researchers have found evidence that C. gattii can evolve potent defences as a result of being preyed on in the wild by amoebas – defences that can make it more virulent when it infects people.
Report co-author Arturo Casadevall says it appears the fungus may have spent 35 years quietly evolving to a more dangerous form to survive in its new home.
“C. gattii may have lost much of its human-infecting capacity when it was living in seawater, but then when it got to land, amoebas and other soil organisms worked on it for three decades or so until new C. gattii variants arose that were more pathogenic to animals and people.
“The big new idea here is that tsunamis may be a significant mechanism by which pathogens spread from oceans and estuarial rivers onto land and then eventually to wildlife and humans.
“If this hypothesis is correct, then we may eventually see similar outbreaks of C. gattii, or similar fungi, in areas inundated by the 2004 Indonesian tsunami and 2011 Japanese tsunami.”