National Geographic : 2015 Jul
42 national geographic • July 2015 Marburg—centered on the city of Kikwit in what is now the Democratic Republic of the Congo (DRC). The chain of human-to-human infections, which totaled 315 cases and 254 deaths, began with a man who farmed manioc and made charcoal in a forest area at the city’s edge. Swanepoel flew to Kikwit, joining an in- ternational team of responders. He came down with malaria, went home, recovered, and in early 1996, with the support of the World Health Or- ganization, returned. His primary task was to look for the reservoir host, searching the same ecosystem where the outbreak had begun at the same time of year. “Already by that stage,” he told me, “bats were on my mind.” Swanepoel and his crew at Kikwit took blood and tissue not only from bats but also from a wide selection of other animals, including many insects. Screening those samples back at his lab in Johannesburg, he found no evidence of Ebola. So he tried an experimental approach, one that seemed almost maniacally thorough. Working in NICD’s high-containment suite—biosafety level 4 (BSL-4), the highest—he personally in- jected live Ebola virus from the Kikwit outbreak into 24 kinds of plants and 19 kinds of animals, ranging from spiders and millipedes to lizards, birds, mice, and bats, and then monitored their condition over time. Though Ebola failed to take hold in most of the organisms, a low level of the virus—which had survived but probably hadn’t replicated—was detected in a single spider, and bats sustained Ebola virus infection for at least 12 days. One of those bats was a fruit bat. An- other was an Angolan free-tailed bat, the same little insectivore that would later catch Fabian Leendertz’s attention in Méliandou. It was proof of principle, though not of fact: These creatures could be reservoir hosts. Ten Thousand Haystacks The events in Kikwit highlighted an important difference between Marburg and Ebola viruses that has persisted: Whereas outbreaks of Mar- burg virus disease usually begin around caves and mines, Ebola virus disease outbreaks usu- ally begin with hunting and carcass scavenging, which are forest activities. This suggests the two viruses may emerge from two different kinds of reservoir hosts—or if bats are the hosts, two different kinds of bats, cave roosters and tree roosters. The pattern was reaffirmed during a cluster of Marburg outbreaks from 1998 to 2000, centered on a derelict gold-mining town called Durba, in the DRC. Bob Swanepoel led another expe- dition and found multiple chains of infection, most or all of which started with miners who worked underground. Miners who worked at open pits in the daylight were far more likely to stay healthy. This led Swanepoel to suspect cave-roosting Egyptian fruit bats as the virus source, though he didn’t publish his suspicion at the time. Then, beginning in late 2001 and extending into 2003, another series of small, indepen- dent outbreaks—of Ebola again, not Marburg— afflicted villagers in the densely forested borderlands of Gabon and the Republic of the Congo (which are west of the DRC, on the other side of the Congo River). Roughly 300 people became infected; almost 80 percent died. Mean- while gorillas, chimpanzees, and duikers, small forest antelopes, started turning up dead in the same region. Each human outbreak seemed to start with an unfortunate person, usually a Ebola outbreaks usually occur in isolated villages connected by bad roads and trails, such as this one in the DRC. The 2014 outbreak in West Africa was different— it quickly spread to urban areas. EACH HUMAN OUTBREAK SEEMED TO START WITH ONE UNFORTUNATE PERSON, USUALLY A HUNTER.