Human researchers are killing wild chimpanzees by inadvertently giving them colds, a new study shows for the first time.

The researchers in Africa now face a dilemma: let tourists and scientists get close to Africa’s great apes and risk spreading diseases, or curb contact with the apes and leave them vulnerable to a bigger threat – poachers.

For scientists to study wild gorillas and chimpanzees, and for eco-tourists to see them, groups of “habituated” apes must let people get within a few metres of them. It has long been suspected that this spreads human respiratory viruses, which apes can catch. Up to half the apes in such groups have died after showing respiratory symptoms.

However, it is hard to pinpoint what has killed a wild ape, says Fabian Leendertz of the Robert Koch Institute in Berlin, Germany, who runs a chimp health project in the Tai forest in Ivory Coast.


Researchers have found human gut bugs such as Escherichia coli in ape droppings – not surprising, as these bacteria persist in the environment, where eco-tourists with diarrhoea deposit them.

Double whammy

But to catch a human respiratory virus you have to be close to someone coughing. Habituated Tai forest chimps had five separate outbreaks of respiratory disease between 1999 and 2005. Nearly all had cold symptoms, and between 3 and 19% died, mainly juveniles.

Leendertz and his colleagues found either human respiratory syncytial virus, or human metapneumovirus in victims, as well as pathogenic bacteria such as Streptococcus. The viruses commonly cause cold symptoms in human adults, but can be serious in children.

“From the virus alone the chimps would have probably not have died,” Leendertz says. But the bacterial infections that accompanied them were deadly. When zoo chimps get these viruses, they also get antibiotics to kill bacteria.

Should sneezing apes in habituated groups also get antibiotics? “We must decide how much we have to invasively treat the wild apes,” says Leendertz. “Do we have to get away from the “hands off” philosophy?”

Protective effect

The answer is probably not to end close encounters between apes and humans, which pay local people to protect the animals from poachers. “In the Tai forest we have the highest density (of chimps) around the research and tourist sites,” Leendertz says. “Without research our site would probably have [no chimps] left.”

Researchers walking across Tai National Park found their likelihood of running into signs of chimpanzees declined the farther they got from the research project or from a tourist site – but their likelihood of encountering signs of poaching increased.

“This is the first time people have quantified and compared the disease effects of research and tourism with the anti-poaching effect,” says study co-author Peter Walsh of the Max Planck Institute for Evolutionary Anthropology.

The researchers now wear heavy N95 hospital face masks in the field. “They’re a bit hot, but OK,” says Leendertz. “We need to find ways to maximise the benefit of research and tourism by minimising the negative effect of disease.”

For the apes, the choice now is between the poacher and the plague.

Journal reference: Current Biology (DOI: 10.1016/j.cub.2008.01.012)