Scientists have finally found the cause of a mysterious brain-invading tropical fungus outbreak that killed more than 40 dolphins and porpoises in the Pacific Northwest: humans.
Between 1997 and 2016, scientists found 42 dead dolphins in the Salish Sea around British Columbia and Washington. All had died of an infection from a tropical fungus, Cryptococcus gattii, which had entered their lungs and eventually spread to their brains.
A similar outbreak occurred in humans from 1999 to 2007, when 218 people in British Columbia were infected by the fungus and 19 died from complications of the ensuing disease.
But it remained a mystery how the fungus, which usually lives inside soil and trees in tropical and subtropical climates and doesn't spread between animals, reached a northern climate and infected dolphins out at sea.
A new study published October 22 in the journal Diseases of Aquatic Organisms has finally solved the mystery: Climate change pushed the habitable zone of the fungus farther north, and then human activity from construction and deforestation displaced the fungus from the soil and trees and moved it into the air, where its deadly spores wafted out to sea.
From there, the brain-invading fungal miasma likely settled over the sea’s surface, where it was inhaled by porpoises and dolphins when they came up to breathe. Indeed, the Dall’s porpoise (Phocoenoides dalli), which is particularly prone to engaging in playful surface activities — such as riding in the wakes produced by boats — was identified by the team to have had a 100 times greater risk of catching the disease than the more common harbor porpoise (Phocoena phocoena).
The first probable case of C. gattii infection in the Salish sea outbreak may have occurred in a porpoise in 1997, a full two years before the epidemic's first confirmed human case in 1999, according to the researchers. In the future, identification of such infections in animals could provide better advanced warning of disease outbreaks that have the potential to infect humans, especially as climate change enables those diseases to spread in unprecedented ways.
This is just one example of how the warming climate has caused a fungus to expand its range. Cases of valley fever — a disease caused by the Coccidioides fungus, which commonly resides in the Southwest — tripled in California between 2014 and 2018. And a 2019 modeling study predicts that by 2100 the fungus will have expanded its range as far east as Kansas and as far north as North Dakota.
Another 2019 study, published in the journal mBio, hypothesized that the drug-resistant fungus Candida auris — of which there have been numerous outbreaks in Asia, Europe and the Americas since the first reported case of the fungus infecting a woman in Tokyo in 2009 — was getting more successful at invading warm human bodies because it had adapted to higher temperatures in the wild.
"As we change the environment in unprecedented ways, we could see more diseases that affect people and wildlife," lead author Sarah Teman, a research assistant at the University of California, Davis School of Veterinary Medicine, said in a statement.
Originally published on Live Science.
Sign up for the Live Science daily newsletter now
Get the world’s most fascinating discoveries delivered straight to your inbox.
Ben Turner is a U.K. based staff writer at Live Science. He covers physics and astronomy, among other topics like tech and climate change. He graduated from University College London with a degree in particle physics before training as a journalist. When he's not writing, Ben enjoys reading literature, playing the guitar and embarrassing himself with chess.