The worst part of sampling a dead gorilla for Ebola, said Dr. William B. Karesh, is the flies.
“You can imagine the sense of panic,” he said. “A hundred thousand ants and carrion flies are coming off the carcass or climbing up your arms. They get inside your hood and are crawling on your face or biting you.
“After half an hour, you have to get out and pull off the hood, clean up and disinfect. It’s not for the faint of heart.”
The task described by Dr. Karesh — a former chief field veterinarian at the Wildlife Conservation Society, which runs New York’s zoos — was part of an unusual research project. Scientists were trying to predict human Ebola outbreaks by detecting them first in apes and other forest animals.
The team recently published a study in the journal Philosophical Transactions of the Royal Society B detailing 12 years of this work in the Republic of Congo.
In some ways the study, which lasted from 2006 to 2018, was a failure. Only 58 samples were gathered from dead animals, and none was positive for Ebola. Therefore, the team’s hypothesis — that animal sampling could be an early warning system for human outbreaks — was not proved.
The good news, however, was that there was no epidemic. From 1994 to 2003, there had been multiple human outbreaks of Ebola in the Republic of Congo or neighboring Gabon. Weeks or months before each one, dead gorillas and chimpanzees were reported, sometimes hundreds of them. (Those die-offs contributed to the classification of western lowland gorillas as critically endangered.)
Sarah H. Olson, a W.C.S. wildlife health specialist working in the Republic of Congo and a co-author of the study, conceded that carcass surveillance had been “a massive, massive challenge.” But other components of the program, she argued, were highly successful.
For example, she said, public-education teams visited hunting villages across a wide swath of the country to explain why it was dangerous to eat or even touch animals found dead. The educators also put up posters and aired radio spots.
Previously, she said, “people saw dead animals as a gift from God, food they didn’t have to work for.”
Many, but not all, human outbreaks of Ebola have been traced to eating carcasses. But the biggest — the West African outbreak that began in late 2013 and killed more than 11,000 people — did not begin this way. That outbreak is thought to have started when a child played inside a tree where Ebola-infected bats roosted and left droppings.
After years of education efforts in the Republic of Congo, “people there adamantly told us they don’t eat carcasses any more,” Dr. Olson said. “That’s a big change.”
The teams also trained local veterinarians and park rangers to don protective gear to do tests safely, and helped the national laboratory in the Republic of Congo’s capital, Brazzaville, improve its Ebola testing for both animals and humans.
The work was supported by the United States Agency for International Development’s $200 million Predict program, a ten-year effort to find animal diseases that could jump to humans.
Other funders included the Fish and Wildlife Service, the German government and several private foundations.
Perhaps the most intriguing part of the study was the authors’ descriptions of how incredibly difficult it is to even find dead animals in a dense rain forest, and then to safely take samples from them.
The first obstacle is that sick animals often crawl off to die in thick brush or near water.
“It’s not like they’re laying out on a golf course,” Dr. Karesh said. “In the first day or two, hunters can pass one by and not even know it’s there. Later, they smell it or they even hear the flies.”
Dr. Alain U. Ondzie, a W.C.S. veterinarian in the Republic of Congo, described a terrifying moment for a team he was leading through the jungle in 2007. They had been walking and camping for eight days and had run out of water.
When they finally came across a rivulet, the porters and trackers threw themselves on the ground to drink. Only then did one spot a dead gorilla in the water just upstream.
“They cried, ‘It’s all over for us — we’ll be dead before we reach a village,’” Dr. Ondzie said. “We were very fortunate later to learn the carcass was not positive.”
The research program relied heavily on asking local hunters to report carcasses. Most hunters are from the Mbenga subgroup of the forest-dwellers known as pygmies. (The term is often seen as pejorative but there is no uniformly accepted substitute; subgroups genetically related to one another are widely scattered across Central Africa, but share no common language or name.)
Many hunting villages are bound in virtual enslavement to local farming villages, Dr. Karesh explained, which complicates relationships with outsiders.
Also, impoverished hunters may not own cellphones, and even if they do, coverage is spotty. (Without phones, hunters send messages by asking passing drivers of logging trucks to relay the word when they reach the next town.)
Despite the inevitable delays, sampling teams must rush to each carcass before scavengers finish it off. Many animals — including leopards, civets and smaller cats, crocodiles, mongoose, palm-nut vultures and even duikers, a kind of antelope — will dismember a carcass, carry off pieces and pick the bones clean.
For a team based in Brazzaville or another city, it may take several days — using a combination of 4x4s, pirogue canoes and walking — to reach a carcass.
On arrival, the team must clear a path to the carcass and then establish a perimeter about 60 feet back while the two designated samplers don hooded Tyvek suits with goggles and three pairs of gloves.
Working under those conditions in tropical heat can be excruciating, as Dr. Karesh explained. Other samplers described fogged goggles and cameras, and sweat running down their arms to form water balloons in the tips of their gloves, reducing their dexterity.
“It can be quite a fiddly thing,” said Dr. Eeva Kuisma, a W.C.S. technical adviser. She recalled being with a team that had to stand up in a dugout canoe trying to keep a test-tube rack balanced as they sampled a dead monkey snagged in an overhanging branch.
Dr. Olson described the delicate task as like using the tweezers in the child’s game Operation, always nervous that the buzzer will go off — but the stakes are higher because the carcasses, like human ones, can teem with live virus for up to a week.
Merely living in the jungle can be nerve-racking, Dr. Kuisma added, because you are always aware that animals you cannot see are watching you.
She had never seen a leopard or been close to a large crocodile, she said, but the animals the trackers feared most were forest elephants, which lurked nearly invisibly in the leafy shadows.
“If you can even see one, you’re too close,” she said. “They’re usually not aggressive, but a mother with a calf may charge you.”
She herself had been charged by a male gorilla she startled while taking a morning jog on a logging road.
“You hear this shout — it’s kind of like a sharp bark,” she said. “And then they charge. It’s usually a mock charge, but that one made me do a 100-meter personal best.”
Despite the difficulties, Dr. Olson argued, ongoing surveillance and education programs are cheap to run and also build trust with local villagers — something vividly lacking in the current Ebola outbreak in the Democratic Republic of Congo, where medical teams and treatment centers have been attacked.
The idea, she said, “deserves a closer look from the international community.”