A killer fungus that has caused the worst wildlife disease in history, wiping out or driving to the brink of extinction hundreds of amphibian species, has become more widespread across Africa than anyone realized and is probably causing overlooked outbreaks.
First described in the late 1990s, the fungal infection called chytridiomycosis has swept through the diversity-rich continent in little more than two decades, possibly fueled by air travel and the animal and food trades, according to a new study in the journal Frontiers in Conservation Science. Ominous news from the study suggests that one lineage of the fungus assumed to be benign is more common and possibly more virulent than previously thought.
"There is no other pathogen that has caused a disease even close to this," said study co-author Vance T. Vredenburg, associate chair of biology at San Francisco State University. He keeps a jar full of dead frogs to remind him of what it felt like to watch firsthand as the disease destroyed frog populations in the Sierra Nevada mountain range between 2004 and 2008.
While most pathogens sicken or kill a limited number of species, this fungus "is a generalist across all amphibians, and it does extremely well," Vredenburg said. The fungus is considered a significant reason that 41% of amphibians around the world are in danger of extinction.
"We're incredibly lucky it doesn't affect mammals," Vredenburg said.
In terms of scale, the closest any human disease has approached to chytridiomycosis is the Black Death, Vredenburg said, referring to the mid-14th century epidemic of bubonic plague that killed one-third of Europe's population in five years.
The devastation to amphibian species is wreaking havoc on ecosystems around the world. Frogs and salamanders feed on disease-causing mosquitoes and other insects, keeping their populations in check. They also provide food for larger frogs, snakes and some bird species.
The new study's findings are important because "Africa was one part of the world where we've always had a big question mark" about the fungus, said Jamie Voyles, an associate professor of biology at the University of Nevada at Reno who has written about chytridiomycosis but did not participate in this study.
The continent is home to more than 1,200 of the world's 8,592 recognized species of amphibians, making it a key benchmark for measuring the impact of chytridiomycosis. Amphibians, a class of animals able to live both on land and in the water, consist of three groups: frogs, salamanders and the wormlike animals called caecilians.
Vredenburg said researchers did find cause for hope in the fact that "there are quite a few [amphibian] species that get infected and nothing happens." What protects resistant animals, however, remains unclear.
Spores from the waterborne fungus known as Bd, short for the scientific name Batrachochytrium dendrobatidis, seep into the skin of frogs and salamanders. Within 24 hours, the fungus can produce thousands more spores that are released back into the water, where they swim using a threadlike structure called a flagellum. The new spores can infect the same frog as well as others.
The fungus kills by causing an amphibian's skin to grow up to 40 times its normal thickness, making it hard for the animals to breathe and causing them to become dehydrated. Eventually, many get conditions that cause their hearts to stop.
For their study, Vredenburg and his colleagues cast a broad net, searching for signs of the disease in Africa as far back as 1852. They examined 16,900 amphibian specimens, testing almost 3,000 from museums and 1,651 from wild animals in Burundi, Equatorial Guinea, Cameroon and the Democratic Republic of Congo.
Vredenburg and his colleagues found that, starting just before the 1960s, the fungus was present in less than 5% of the samples they studied during every decade. Then its presence abruptly soared to 17.2% in 2000. The study reported that the fungus is spreading especially quickly in Cameroon and Kenya.
In Cameroon, they found that the fungus lineage known as Bd-CAPE was spreading and appears to be more virulent than previously thought.
In Africa, Vredenburg and his colleagues found one country, Equatorial Guinea, where there has been a significant decrease in the prevalence of the fungus. They don't know the reason.
In some cases, its spread can be traced to air travel or shipping, which have increased connections between different countries and between islands.
"As we increase the connectivity," he added, "we're going to disrupt millions of years of evolution between pathogens and hosts."
The huge declines in amphibian populations could wind up changing a fundamental human experience, Voyles said. "Everybody can remember catching frogs or tadpoles as a kid. The joy of discovery is incredible."
For Vredenburg, the spread of the Bd fungus is a sobering reminder that he did not initially realize how great a threat it posed to the frogs he studied in the Sierra Nevada.
"These frog populations are so robust. Come on, they've been here for millions of years in the mountain," he recalls thinking at the time. "Sure enough, the fungus came in and wiped them out. I saw tens of thousands of dead frogs."