Up close with a Chytrid infection. Image credit: Wikimedia Commons
"Happiness is like smallpox," Gustav Flaubert once wrote, "if you catch it too soon, it can completely ruin your constitution." Indeed, before the widespread distribution of the vaccine, smallpox was one of the leading causes of death in the Western World.
Most vulnerable, were the naive populations—those which had never before been exposed to the virus. Today, a similarly behaving disease is devastating populations around the world, causing the collapse of entire communities, but it does not affect humans—the victims, in this case, are amphibians.
The mountain yellow-legged frog has nearly been extirpated from Sequoia Kings Canyon National Park by Chytrid infection. Image credit: Wikimedia Commons
Chytridiomycosis is a disease caused by fungal infection and has been identified in amphibians around the world. Surveys have shown that as much as 30 percent of global amphibian populations have been affected and it is believed to be responsible for the extinction of more than 200 frog species.
Chytrid is considered the greatest threat to biodiversity of any single disease but it is still poorly understood by science. The biggest mystery is why some infected populations survive and some are wiped out completely.
Vance Vredenburg, assistant professor of biology at San Francisco State University, explained:
When Chytrid hits naïve host populations, it grows so quickly that the usual checks and balances, which prevent a pathogen from causing extinction, don't have a chance to kick in.
Now, new research has revealed that the key to survival is population density: The fungus needs lots of hosts to infect and reinfect to continue growing, otherwise, it stabilizes.
We found that mass frog die-offs only occur when the severity of the Chytrid infection reaches a critical threshold among the individual frogs...now that we know this limit, which is a specific number of fungal spores per frog, conservation efforts may be able to save susceptible frog species by preventing the disease from reaching this point.
This seemingly simple realization provides conservationists with a crucial tool they can use to preserve infected populations. But, as Vredenburg commented, "these results are about more than just frogs." Instead, "They are about disease, how and why it spreads and how some populations can be wiped out by a disease while others survive."
And this knowledge is important for all species: frogs and humans alike.