A Brutal Disease Kills Monkeys. Flies Could Be Spreading It.
Trilobites
A downside of social living among monkeys and chimpanzees, a new study suggests, is being at greater risk of deadly disease.
In the jungles of Ivory Coast, monkeys and chimps forage for food, sleep in trees and travel in groups. Not far behind follow primatologists, like Jan Gogarten, a postdoctoral researcher at the Robert Koch Institute in Germany.
Dr. Gogarten was spending a lot of time in the jungle tracking mangabey monkeys when his attention was drawn to another constant presence there.
“We had these flies always around,” he said. Dr. Gogarten wondered whether the clouds of these flies could travel long distances along with the primates, and whether they were carrying disease.
Now he and his colleagues have reported, last month in the journal Molecular Ecology, that some flies stayed with a group of mangabeys in Tai National Park for up to 12 days and across significant distances. Some of these flies also tested positive for a bacterium responsible for many gruesome monkey and chimp deaths over the last few decades in the park.
If the flies are one cause of the spread of this disease among the primates, it suggests a downside of social living — more animals clustered together could make an easier target for insects and the diseases they may carry. Studying disease transmissions between primates and flies could also help lead to better understanding of how some diseases end up hopping to humans.
To track the population of flies surrounding the monkeys, the researchers set traps using the primates’ feces or synthetic odor as bait. These traps confirmed that there were far more flies among the animals than elsewhere, and they painstakingly marked hundreds of the flies each day with a different color of nail polish before releasing them.
Each day, they set out the traps again, watching for colorful specimens.
“The first one we caught we were euphoric,” Dr. Gogarten said; 50 more marked flies eventually resurfaced. It suggested that flies were actually following the group, rather than just happening to find it.
During the study, one of the mangabeys died, likely of sylvatic anthrax. This disease, caused by bacteria, results in enormous lesions that are difficult to look at, and it is highly lethal. In fact, sylvatic anthrax has been responsible for more than 38 percent of primate deaths in the Ivorian national park over the last 26 years, the researchers say. At the rate of infection currently present in the park, simulations suggest the chimps who live there may not survive another 100 years.
Exactly how primates are exposed to the bacterium, Bacillus cereus biovar anthracis (Bcbva), is not well understood. They may encounter it in the soil where the carcass of an infected animal lay, or they may get it some other way. In this study, about 5 percent of the flies they tested were positive for the anthrax bacterium; if they spread it onto leaves and other surfaces they land on, or if primates swallow the insects, that may explain one mode of transmission.
There are many benefits to living with a group of family and friends. But if additional research confirms the study’s findings, it also suggests that living in groups makes animals better targets for creatures that might make them sick.
The researchers are also curious to learn whether sylvatic anthrax could spread to humans, a question they are addressing by catching flies in villages near the national park and testing inhabitants to see if they’ve been exposed. As human populations around the park grow, understanding how diseases may hop between flies and primates — both human and nonhuman — will be of greater importance.
To illuminate the role played by the flies, future research may involve marking many more of them and tracking how they may move among groups of primates. Eventually, Dr. Gogarten and his colleagues hope to sketch a map of the jungle and the organisms that travel through it, from monkeys to flies to the bacteria themselves.
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