The new strain of Cryptoccus gatti

A mysterious new strain of airborne fungus that has mystified scientists is rolling through the Northwestern U.S. and Canada, leaving at least six people dead in its wake.

A study found that the new strain of Cryptoccus gatti, previously native to tropical and subtropical regions like Australia and South America, is spreading through Washington and Oregon and heading towards Northern California, National Geographic reported.

"The alarming thing is that it's occurring in this region, it's affecting healthy people, and geographically it's been expanding," study co-author Edmond Byrnes, a graduate student at Duke University, told the magazine.

Experts are baffled as to how the fungus reached North American and how  it could survive in a colder climate.

Even more worrisome for health experts are reports that the victims had relatively healthy immune systems, according to National Geographic. Twenty one known cases have been recorded in humans, and six have been fatal. 

A 1999 outbreak of a similar strain of the fungus in British Columbia, Canada,  had a much lower mortality rate, killing 19 out of 218 recorded victims.

There is currently no vaccine for the fungus strain, which causes an infection which may not display symptoms -- including a bad cough and shortness of breath - until months after exposure.

"The enhanced virulence of isolates from the outbreak region, when compared with those from other regions, suggests that the genotypes circulating in the Pacific North West are inherently increased in their predilection to cause disease in mammalian hosts," the study authors wrote in the April 22 issue of the scientific journal, PLOS Pathogens.

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