Ebola's backstory: How germs jump species

Fruit bats are associated with an array of deadly viruses, including Nipah, Ebola and Marburg. As the bats' habitat shrinks, the odds increase that bats will cross paths with humans, wild primates and other animals.

By Carol Clark
From Emory Medicine

While virologists study pathogens like Ebola by zooming in on them with an electron microscope, primate disease ecologist Thomas Gillespie climbs 100-foot trees in the tropical forests of Africa to get the big picture view. He tracks pathogens in the wild to learn how they adapt to changing environments and jump between species.

It is physically challenging work that often takes him into remote forests where the wildlife has not yet learned to fear people. A chimpanzee turned Gillespie into a human yo-yo while he was ascending a tree with a rope and harness. “Chimpanzees have 10 times the strength of a man and they can be curious and playful,” he says. “I once had an adult male chimpanzee grab my rope and bounce me up and down.”

Wild primates pose a special risk for zoonotic diseases—those transmissible from animals to humans—due to our close genetic relationship. HIV/AIDS and Ebola are the two most dramatic examples of diseases linked to wild primates, but many other viral, bacterial, fungal, and parasitic pathogens found in apes and monkeys are readily passed to humans.

“The bottom line is that the majority of emerging infectious diseases are coming from wildlife and most of that wildlife is in tropical forests,” says Gillespie, a professor in Emory’s Department of Environmental Sciences and the Rollins School of Public Health. “We can’t afford to just focus on one pathogen or one animal. It’s really important to get a general understanding of the interactions of different species, and how changes in the environment are driving zoonotic disease transmission.”



Gillespie is investigating undisturbed forests, as well as sites where logging and other human activity is under way. He gathers fecal and blood samples from people and animals for analysis while also scouring the forest floor and treetops to learn about the diversity of pathogens in the environment. The data can then be mapped spatially and over time to connect the dots of disease ecology.

For one ongoing study in Uganda, Gillespie and his collaborators are following primates in and around fig trees. The researchers hang out near these ancient forest giants, observing the tableau of life feeding amid the branches and on the ground below.

Fig trees are a keystone species of rainforest ecosystems. Climate change is playing havoc with the seasonal fruiting of other types of trees. But fig trees have co-evolved with specific pollinators—fig wasps—and due to their complex interaction, there is always a fig tree fruiting somewhere in the forest, providing a critical, consistent food source for fruit bats, primates, and ground dwellers like the bush duiker, a shy, dainty antelope that darts amid the forest shrubbery.

Fruit bats, associated with an array of deadly viruses including Nipah, Ebola, and Marburg, are especially specific in their diet. “They’re looking for ripe fruit,” Gillespie says, “and that’s a rare resource in the environment.”

And it’s becoming even rarer. Logging companies are cutting down huge swaths of African forests. Mining operations are moving into new terrain. Villages are expanding, and homes and food crops are eating into the wilderness. All these factors bump up the odds that fruit bats will be living near people, and that the bats will be joined by a variety of other animals while they are feeding from a tree.

“Most viruses can only last outside of a host for minutes or hours, not days,” Gillespie says, “but now we have this changing landscape of food availability. That raises the probability that a gorilla or chimpanzee will eat a piece of fruit that a bat has just defecated on, or has bitten into and discarded.”

Diseases and parasites could be transmitted in this manner. Ebola is one of the rare ones, extremely difficult to find, much less study, in the wild. But Ebola looms large in the public imagination because it is hemorrhagic, capable of causing massive bleeding, and because of its high fatality rate. It is also frightening because it is so mysterious, popping up out of the forest to kill voraciously then disappearing again for years.

The virus was first identified in 1976, following an outbreak in a remote hamlet of Zaire (now the Republic of Congo) near the Ebola River. Subsequent outbreaks have also been associated with forested backwaters and have quickly burned themselves out. That is, until the current outbreak in West Africa. Ebola has now made the leap from rural, forested regions to Africa’s urban areas, where many people live in crowded conditions with poor sanitation.

One of the biggest mysteries is where the virus has hidden between these outbreaks. Evidence of Ebola antibodies, and remnants of Ebola RNA, have been found in the blood of three species of fruit bats, making them prime suspects as an Ebola reservoir: An organism that can carry the pathogen without dying or even becoming sickened by it.

“Fruit bats are the best guess as to the reservoir, but until a live virus is found in their blood, we cannot be sure,” Gillespie says. “What we do know is that bats are an important part of the equation. And gorillas, chimpanzees, and some other animals, like the bush duiker, can get infected with Ebola.”

During the past decade, human Ebola outbreaks in Gabon and Congo have been accompanied by reports of gorilla and chimpanzee carcasses in surrounding forests, and epidemiological studies have connected encounters with dead gorillas, chimpanzees, and bush duikers to human cases.

“A hunter might find a dead gorilla in the forest,” Gillespie says, “and instead of saying, ‘I shouldn’t butcher this animal and eat it, it may have died of an infectious disease,’ he throws up his hands and says, ‘Thank you, God, for this gift!’ ”

Fruit bats are also hunted for food in many parts of Africa.

But you don’t have to be a hunter going deep into a forest to catch Ebola. Now that fruit bats are feeling the squeeze of fewer food sources, they may choose to roost under the eaves of a home, feasting on trees in the village orchard as children play below.

Widespread education about what is safe to eat and what is not, and how to identify animals that may have died from an illness, is becoming a vital part of preventing the spread of these diseases.

Just as people are encroaching on wilderness, pathogens are expanding their range into human habitats.

“We’re changing the environment in ways that may be promoting Ebola,” Gillespie says. “As the human population grows and the demand for resources pushes us into new areas, we’re going to see more diseases emerge. Anytime we alter a pristine natural system there are going to be unintended consequences.”

Photos: Thinkstock

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