National Geographic : 2015 Jul
52 national geographic • July 2015 2005 the journal Nature published a paper on these results, written by Leroy but with Swane- poel and Paweska credited as co-authors, titled “Fruit Bats as Reservoirs of Ebola Virus.” That paper, though cautious and provisional, is the primary source for all those careless, overly cer- tain assertions you’ve seen in the popular media during the past year to the effect that Ebola vi- rus resides in fruit bats. Possibly it does. Or not. The paper itself says maybe. “ You tried to isolate live virus?” I asked Le- roy during my stop in Gabon. He’s a courteous, dapper Frenchman, now director of the Cen- tre International de Recherches Médicales de Franceville, who works in a white shirt and dark tie, at least when he’s not wearing a full protec- tive suit in his BSL-4 lab or Tyvek coveralls in the forest. “ Yes. Many, many, many times trying to isolate the virus,” he said. “But I never could. Because it was—the viral load was very, very low.” Viral load is the quantity of virus in the solid tissues or blood of the creature, and it tends to be much lower in a reservoir host than in an animal or person suffering an acute infection. That’s just one of three reasons why finding a reservoir host is difficult, Leroy explained. The second is that, in addition to low viral load within each animal, the virus may exist at low prevalence within a population. Prevalence is the percentage of positive individuals at a given time, and if that happens to be as little as one animal in a hundred, then “the probability to detect and to catch this infected animal is very low.” If a single kind of animal amid the great diversity of tropical forests represents a nee- dle in a haystack, then one infected individual within one population of animals amid such diversity represents one needle in ten thousand haystacks. And the third constraint on the search for a reservoir host? “It’s extremely expensive,” Le- roy said. The Perfect Holiday The cost of field operations in remote forest lo- cations, as well as the competing demands upon institutional resources, has hindered even vet- eran researchers such as Swanepoel and Leroy from mounting long-term, continuous studies of the Ebola reservoir question. Instead there have been short expeditions, organized quickly during an outbreak or just as a crisis was end- ing. But going to the site of a human outbreak to do research on the ecology of the virus is lo- gistically nightmarish and, as I’ve mentioned, offensive to local people. So those expeditions get delayed. The problem with delay is that the prevalence of Ebola virus within its host pop- ulation, the viral load within individual hosts, and the abundance of virus being shed into the environment may all fluctuate seasonally. Miss the right season, and you might miss the virus. Fabian Leendertz tried to address these dif- ficulties by organizing a second field expedi- tion, this one at roughly the same season as the fateful spillover that killed Emile Ouamouno, but a year later and in neighboring Ivory Coast. Angolan free-tailed bats are abundant there too, roosting beneath the roofs of village houses. Their very abundance in such close proximity to people suggests a further perplexing ques- tion, if the little-bat hypothesis is correct: With the virus so near, why don’t spillovers occur far more often? Leendertz wanted to trap those bats, as many as possible, and sample them for A Freetown couple mourns as a burial worker removes their day-old baby’s body. The infant likely died of other causes, but officials ordered that all deaths in heavily affected areas be treated as cases of Ebola. THE URGENCY OF HUMAN NEEDS IN AN OUTBREAK MAKES SCIENTIFIC INVESTIGATIONS DIFFICULT.