Microbiologists have found that a new strain of airborne fungus, which has already killed six people in Oregon, is extremely virulent. The fungus, which seems to thrive in the temperate climate of the Pacific Northwest, is expected to spread throughout the region from Northern California to British Columbia.
The new strain is a close cousin of fungus that first appeared on Vancouver Island in British Columbia, Canada, in 1999. Both are known as Cryptococcus gattii (pronounced GAT-e-i). The Vancouver version, subtype VGIIa, has infected 218 people, killing 19. The new Oregon version, VGIIc, has infected 21 people. Both strains produce spores that are inhaled, causing respiratory symptoms similar to pneumonia and severe headaches. Other strains of C. gattii have long been commonplace in Australia, South America, and Africa, though these are generally thought to be less virulent.
This isn’t the only Cryptococcus troublemaker out there. Another relative, known as C. neoformans is a common infection among people with compromised immune systems, such as those suffering from HIV. However, the emerging C. gattii strains, VGIIa and VGIIc, are worrisome because “they infect completely normal individuals,” says Joseph Heitman, a microbiologist and geneticist at Duke University Medical Center in Durham, North Carolina, who led the study.
Heitman’s team has been tracking the C. gattii outbreak since 2001. Among other things, the researchers have been genotyping isolates of the fungus taken from infected people and animals in an effort to chart its spread, test its virulence, and better understand how it originated. Last April, Heitman’s team was the first to identify VGIIc as a unique strain. In their current work, published online today in PloS Pathogens, genotyping on a larger set of samples confirmed that many of the Oregon cases were caused by a genetically distinct fungus, the VGIIc strain.
Lab studies also showed for the first time that like VGIIa, the new VGIIc strain is hypervirulent, as it killed 100% of mice infected with the spores. For now, Heitman says the two strains appear similar in how they cause disease and how they are treated. But because the genes of the organisms will likely acquire unique mutations, it’s important to track both strains.
Heitman and his colleagues are now trying to understand which genetic changes led to this virulence. Early indications, he says, suggest that the VGIIa and VGIIc strains have an elongated tubular structure of their mitochondria, the energy-producing centers inside cells. This change seems to allow the fungi to better reproduce even when captured by macrophage cells in the human immune system or by amoeba in soil.
Karen Bartlett, an environmental health scientist at the University of British Columbia in Vancouver, calls the new study “very important” because it will help public health officials better track the outbreak—one that both she and Heitman expect to spread throughout the Pacific Northwest due to similar climates and plant hosts. C. gattii infections can be effectively treated with antifungal medications, particularly if the proper diagnosis is made early.
Bartlett adds that even though C. gattii can be dangerous, infections are still rare. The VGIIa strain, for example, is ubiquitous throughout much of Vancouver Island. That means that most of the Island's 750,000 residents have been exposed to C. gattii multiple times with no symptoms. She says that researchers are now working to understand what it is that makes certain people susceptible to coming down with an infection.