Animal source of Ebola outbreak eludes scientists

Along with struggling to stem the deadly outbreak of Ebola in West Africa, scientists are racing to figure out where it came from. The outbreak came as a surprise because the strain that is killing people in urban areas in West Africa had previously  struck people in rural, forested parts of Central Africa.

If researchers can figure out which animal carries Zaire ebolavirus, and how the virus made the leap to people in West Africa, they may be able to suggest strategies to prevent or contain future outbreaks.

The outbreak started last December in a village in the rainforest of eastern Guinea. At first, no one recognized the disease as Ebola. Symptoms of Ebola can resemble those of many other diseases, and Ebola had never been seen in West Africa before.

So far fruit bats have taken the brunt of the finger pointing as the possible source of the outbreak, says Kevin Olival, a disease ecologist at EcoHealth Alliance in New York City. Some of the evidence implicating bats comes from a study in the April Viruses by Olival and David Hayman of Massey University in Palmerston North, New Zealand. The researchers mapped the ranges of fruit bat species that might carry Ebola or related viruses. Some of the bats’ ranges include both West African and Central African countries such as Democratic Republic of the Congo, where Ebola has appeared in the past. The finding opens the possibility that the virus could have traversed vast distances via bats.  

Still, Olival says, “the evidence is scant that bats are to blame for the West African outbreak.”

Other animals may also pass along the disease. Great apes and species of forest-dwelling antelope may catch Ebola and infect hunters or anyone who eats tainted bush meat, he says.

Hoping to find the animal source, in April, Fabian Leendertz, an epidemiologist and disease ecologist at the Robert Koch Institute in Berlin, led a 17-member team to Guinea. The group collaborated with ecologists who monitor forest animals and captured bats for testing. He declines to reveal his unpublished findings, but he says there were no obvious epidemics among animals that might have spread the Ebola virus to humans. “We didn’t stumble across any dead animals,” he says.

But that doesn’t rule out local animals as a source of the outbreak. Even though the team pulled together quickly and got its start just a month after the World Health Organization’s first alert on the outbreak , “We were still three months late,” Leendertz laments. “Many things may have changed in the meantime.”

The disease may have burned itself out already within the forest animals; migratory animals carrying the virus may have moved out of the area; or other environmental changes, such as rising or falling moisture levels, may have affected the spread of the virus.

Leendertz thinks one bad bush meat carcass may have sparked the current epidemic. Bats, great apes, other primates and antelopes known as duikers are commonly eaten but also are among the animals most likely to be infected with Ebola, he says. The Guinean government banned eating bush meat at the end of March, but by that time, the disease was already spreading among people.

In the case of bats, Olival says, people could have come into contact with an infected animal’s urine, feces or saliva. As West Africa rainforests are cleared to make way for farms and housing, people may have begun to interact more with bats because the animals may turn to houses as roosting places when trees are destroyed, Olival says.

If bats turn out to be the virus carriers, he says, “the right answer is never to kill all the bats.” That would be an ecological disaster because bats pollinate plants and devour insects. Bat hunts would also only increase human contact with potentially infected animals.

Instead, researchers need to learn more about the environmental and biological conditions that lead to outbreaks. For instance, researchers at the U.S. Centers for Disease Control and Prevention found that bats in caves in Uganda, where, in 2007 and 2008, miners and tourists had contracted the related Marburg virus, give off more virus during twice-yearly birthing seasons.

Most human cases of the disease coincided with those birthing seasons, the researchers reported in PLOS Pathogens in 2012. If fruit bats in West Africa follow a similar pattern, one way to avoid exposure to Ebola would be simply to steer clear of bats during birthing seasons, Olival says.

The Ebola epidemic may have started with a single interaction between a person and an infected bat or bush meat, but researchers agree that humans brought Ebola out of the rainforest to cities. People catch the virus after coming into contact with infected body fluids and then pass the disease on to others through close contact.