In 2001, while studying chimpanzees in the Taï National Park in Ivory Coast as a Ph.D. student, Fabian Leendertz watched an alpha male named Leo vomit, climb up on a low branch, then topple over and die. That death and five others in the following months were the first inklings that an unexpected killer was afoot in the rainforest.
Five years later, Leendertz and his team at the Robert Koch Institute in Berlin showed that what killed the chimps was an unusual form of anthrax. Now, in a Nature paper, they present evidence that the microbe causing it, Bacillus cereus, plays a huge role in the ecology of the rainforest, apparently causing a large proportion of all mammalian deaths. It threatens to wipe out the chimpanzees in Taï, the authors warn.
“This needs to be taken seriously,” says Chris Whittier, a veterinary scientist at Tufts University Cummings School of Veterinary Medicine in Grafton, Massachusetts, who was not involved in the work. “I hope this opens up a lot of people’s eyes.” Hunting and deforestation have already brought chimpanzees to the brink of extinction, but “diseases such as anthrax, Ebola, or introduced human respiratory viruses may serve as the final nail in their coffin,” says disease ecologist Tom Gillespie of Emory University in Atlanta.
Back in 2001, Leendertz and his colleagues thought the chimps had died of the familiar form of anthrax, caused by B. anthracis. But that is known to kill many wild animals in arid regions, not rainforests, and in 2010 the researchers linked the deaths to the closely related B. cereus, a common and usually benign soil microbe. (Some strains do cause diarrhea and vomiting in humans.) They also discovered that the Taï strain has acquired two plasmids, or circles of DNA, possibly from B. anthracis, encoding most of the genes that make anthrax a formidable killer.
Leendertz and his team examined samples from bones and carcasses from at least 20 different species, collected in the forest over almost 26 years. They detected the pathogen in 81 of 204 carcasses and 26 of 75 bones. In addition to chimps, six monkey species, duikers, mongooses, and a porcupine died of the disease; overall, it appeared to be responsible for about 40% of observed wildlife deaths. The killer strain is not limited to the Taï forest; Leendertz and others have linked wildlife deaths to B. cereus in Cameroon, the Central African Republic, and the Democratic Republic of the Congo.
Still, Leendertz’s team could only thoroughly examine 15 of the infected carcasses. All showed signs typical of a lethal anthrax infection, such as bleeding and swollen lymph nodes, but in other cases, the only evidence is genetic, meaning that the animals may have carried B. cereus but died of something else, Gillespie notes.
Computer simulations by the team showed that anthrax could wipe out the Taï forest’s population of roughly 400 chimps within the next 150 years. Why it hasn’t already done so is a puzzle, because the wide genetic variation found in bacterial samples from the forest suggests that B. cereus has long been present there. But Leendertz says chimps migrating in from other areas may have made up for past die-offs. With populations dwindling elsewhere, that may not happen in the future.
Protecting the chimpanzees will be difficult. The standard anthrax vaccine also protects against B. cereus; the team vaccinated about 100 animals in 2012 and 2013 and is now monitoring them. But researchers can only reach the few animals that have been habituated to humans. And protection may only last a year, so the chimps would require regular shots. “Vaccinating them too often is a problem since they get shy,” Leendertz says.
A better understanding of how B. cereus spreads might help efforts to fight it, Leendertz says. Animals contract B. anthracis when they inhale or swallow hardy spores released into the soil by cadavers. But B. cereus in Taï “may have a very different ecology and epidemiology,” Gillespie says.
“We are currently investigating in depth the source of infection by testing everything the chimps are eating,” Leendertz says. “We’re also looking at other possible sources, such as arthropods.” One candidate is carrion flies. Leendertz’s team found traces of B. cereus DNA in 17 flies; if they help spread the disease, that might explain how some monkey species that only live in trees become infected. The team found the pathogen’s DNA in 12 out of 103 flies living in the canopy.
There are other questions. Are humans somehow protected from infection? And how far has this pathogen spread? Whatever the answers turn out to be, the new study provides a lesson, says William Karesh of the EcoHealth Alliance in New York City. Most wildlife research has been done in grasslands, where it’s much easier to work, but “there is a vast amount of information still to be learned from rainforests,” Karesh says. “Here, we’re starting to get a peek at that.”