Over the past two decades, a deadly fungus previously only found in tropical and subtropical climates has begun infecting humans in other climate zones. So how has this fungus increased its reach?
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In 1999, a highly infectious fungus called Cryptococcus gattii emerged on Vancouver Island in Canada. By 2007, the strain had killed 19 people and sickened more than 300 humans and animals on the island. The fungus has since spread into the United States with dozens of cases reported in California, Idaho, Oregon, and Washington. But where had this infectious fungus come from?
Before the Vancouver Island outbreak, C. gattii didn’t exactly fit the typical profile of a fatal infectious disease for the region. It was known to cause infections in humans and animals but never at the high rate that was reported on Vancouver Island. And previously, those infections were in tropical or subtropical climates where the fungus typically resides in tree hollows. Scientists have been puzzled by where the strain had come from and how it adapted to the cold so well.
Now, a new study by a group of international researchers might have answers to both of those questions. In a paper published in the journal PLoS ONE last week, the team reported that this strain of C. gattii originated in the rainforest of Northern Brazil where the genetic diversity of the pathogen is relatively high. In addition, they believe that global warming trends might be playing a role in the fungus’s spread as well.
Latin America contends with alarmingly high rates of the cryptococcus infections. The region has the third most AIDS-related cryptococcal meningitis infections globally. It is particularly deadly in patients who are immunocompromised–like those suffering from AIDS, for example. If detected early enough though, the infection can be treated successfully with fungicides. But how had the fungus spread from the rainforests of Brazil to a temperate island in Canada?
“The global spread of highly genetically variable C. gattii strains from its cradle in South America should be a concern because genetic variation is considered the basis for adaptation to a changing environment,” says Paulo Ceresini, a researcher at the University of São Paulo State in Brazil and one of the authors of the new paper. “This might explain why C. gattii can occupy so many distinct ecological niches, including several trees species, tree bark and hollow, wood and the forestry environment, water currents, air currents, birds, other animals.”
C. gattii’s high genetic variability has made it adroit at moving into new environments and thus compounded its virulence. But why are C. gattii’s genetics so highly variable? Sexual reproduction is one reason. Unlike most bacterial pathogens that reproduce asexually, fungi can reproduce sexually which means more genetic diversity. (Most sexually reproductive fungi don’t have male or female distinctions but rather positive and negative mating types. For example, C. gattii has two mating types: a and α. To complicate matters even more, the α type can also mate with itself to produce offspring.) Fungal mating increases the overall genetic diversity in the population, which has seemingly facilitated its quick adaptation to cold-weather environments.
Before this recent study, two other teams of researchers had developed two different hypotheses to explain the emergence of C. gattii in non-tropical regions. One stated that the highly infectious strain originated from same-sex mating of two strains in the Australiasian population, while the other stated that it originated from mating within the genetically diverse population in South America.
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To find out which theory was right, Ceresini and colleagues performed a genetic analysis of multiple C. gattii populations from a variety of geographical locations as well as reanalyzing previously published datasets. This allowed them to measure the genetic variation in a set of genes and to create genetic profiles of each strains. By comparing these profiles, the researchers determined that highly virulent strain discovered on Vancouver Island resembled those found in the Brazilian population.
And while the high genetic diversity of C. gattii certainly allowed it to spread into non-tropical regions, global warming also has contributed as well. The authors warn that C. gattii might just be part of the first wave of pathogens whose expansion into new regions will be exacerbated as Earth’s climate continues to change.
“Because species of fungi are adapted to a limited range of temperatures, we expect that the current scenario of global warming will impact the distribution and survival of plant pathogens. Consequently, new pathogens will emerge as a result of climate change,” concludes Ceresini.
Photo credit: Jon Rawlinson via Flickr.