The ecology of Ebola virus encompasses the reservoir question: Where does it hide between outbreaks? Another ecological matter is spillover: By what route, and under what circumstances, does the virus pass from its reservoir into other animals, such as apes and humans? To ask those questions is one thing; to get data that might help answer them is more tricky. How does a scientist study the ecology of such an elusive pathogen? Leroy and his team went into the forest, near locations where Ebola-infected gorilla or chimp carcasses had recently been found, and began trapping animals wholesale. They were groping for a hypothesis. Ebola might abide in one of these creatures—but which one?
In the course of several expeditions between 2001 and 2003, into Ebola-stricken areas of Gabon and the Republic of the Congo, Leroy’s group caught, killed, dissected, and took samples of blood and internal organs from more than a thousand animals. Their harvest included 222 birds of various species, 129 small terrestrial mammals (shrews and rodents), and 679 bats. Back at the lab in Franceville, they tested the samples for traces of Ebola using two different methods. One method was designed to detect Ebola-specific antibodies, which would be present in animals that had responded to infection. The other method used PCR (as it had been used on Kelly Warfield) to screen for fragments of Ebola’s genetic material. Having looked so concertedly at the bat fauna, which accounted for two-thirds of his total collections, Leroy found something: evidence of Ebola virus infection in bats of three species.
These were all fruit bats, relatively big and ponderous, like the flying foxes harboring Hendra virus in Australia. One of them, the hammer-headed bat (Hypsignathus monstrosus), is the largest bat in Africa, as big as a crow. People hunt it for food. But in this case the evidence linking bats and virus, though significant, wasn’t definitive. Sixteen bats (including four hammer-headed) had antibodies. Thirteen bats (again including some hammer-headed) had bits of the genome of Ebola virus, detectable by PCR. That amounted to twenty-nine individuals, representing a small fraction of the entire sample. And the results among even those twenty-nine seemed ambiguous, in that no individual bat tested positive by both methods. The sixteen bats with antibodies contained no Ebola RNA, and vice versa. Furthermore, Leroy and his team did not find live Ebola virus in a single bat—nor in any of the other animals they opened.
Ambiguous or not, these results seemed dramatic when they appeared in a paper by Leroy and his colleagues in late 2005. It was a brief communication, barely more than a page, but published by Nature, one of the world’s most august scientific journals. The headline ran: FRUIT BATS AS RESERVOIRS OF EBOLA VIRUS. The text itself, more carefully tentative, said that bats of three species “may be acting as a reservoir” of the virus. Some experts reacted as though the question were now virtually settled, others reserved judgment. “The only thing missing to be sure that bats are the reservoir,” Leroy told me, during our conversation ten months later, “is virus isolation. Live virus from bats.” That was 2006. It still hasn’t happened, so far as the world knows, though not for lack of effort on his part. “We continue to catch bats—to try to isolate the virus from their organs,” he said.
But the reservoir question, Leroy emphasized, was only one aspect of Ebola that engaged him. Using the methods of molecular genetics, he was also studying its phylogeny—the ancestry and evolutionary history of the whole filovirus lineage, including Marburg virus and the various ebolaviruses. He wanted to learn too about the natural cycle of the virus, how it replicates within its reservoir (or reservoirs) and maintains itself in those populations. Finally, knowing something about the natural cycle would help in discovering how the virus is transmitted to humans: the spillover moment. Does that transmission somehow occur directly (for instance, by people eating bats), or through an intermediate host? “We don’t know if there’s direct transmission from bats to humans,” he said. “We only know there is direct transmission from dead great apes to humans.” Understanding the dynamics of transmission—including seasonal factors, the geographical pattern of outbreaks, and the circumstances that bring reservoir animals or their droppings into contact with apes or humans—might give public health authorities a chance to predict and even prevent some outbreaks. But there exists a grim circularity: Gathering more data requires more outbreaks.