New clues from Guinea yield tantalizing pieces of the puzzle
December 30, 2014 |By Dina Fine Maron
Credit: U.S. Fish and Wildlife Service/Ann Froschauer The hollow Cola tree growing in a remote area of southeastern Guinea was once home to thousands of bats routinely hunted and killed by the neighborhood children. It was also a popular spot to play. A year ago, one child in particular lived within fifty meters of the tree: a two-year-old boy who died in December 2013 and later was identified as the first person in west Africa known to have developed Ebola. The tree was one of the few that loomed over his home village of Meliandou, a hamlet of 31 houses. The question that now haunts researchers: were the tree’s occupants behind how that small boy contracted the virus in the first place?
Yet they will never know the answer. By March, Guinean health officials were told that they had an Ebola outbreak on their hands and alerted the public to halt any consumption of bushmeat. Either by coincidence or as a result of that public warning, that tree was burnt down. Thousands of dead bats rained down on the community, but all researchers were left with in the weeks that followed were fragments of bat DNA. By the time a German research group arrived to conduct tests in April 2014 the tree was gone. No bats were left, and thus no answers.
Their scientific sketch of the village is reported in a new study published today in the journal EMBO Molecular Medicine. The fire may have helped keep the virus from spreading but it was also a blow to the research team, which had hoped to test the bats for genetic markers of the Ebola virus. If they had found signs of the virus it would indicate the bats had acquired Ebola, even if they were not necessarily responsible for transferring it to humans. But the fire foiled their plans since no such Ebola genetic information could be garnered from the remains of charred bat. And, making matters worse, there were no other representatives of the same bat species in the area, says Fabian Leendertz, lead author of the research team from the Robert Koch Institute in Berlin, Germany.
The fire was “really unfortunate since we can never pinpoint if [that bat species] was really the reservoir or not,” says Leendertz. His team was able to identify the species of the bat by conducting genetic analysis on fragments of bat DNA – mostly stemming from bat feces – located in soil in and around the tree, but they could not get any information on the virus from those traces of genetic material. The team also culled other bat species from the area, but found those other bats were all Ebola-free. Leendertz was left with only circumstantial evidence to indicate that the insect-eating bat species, called Mops condylurus, may be a candidate to explain how humans contracted Ebola in the first place. “It’s probably the best we can get but we are very unhappy with the data,” he says. And so the search to identify the carrier – or perhaps carriers – of Ebola continues.
Exactly which creature may transfer the Ebola virus to humans has bedeviled scientists since the virus was first identified in 1976. Myriad setbacks, usually related to slow response and notification, have kept scientists from pinning down an answer. Bats have remained a top Ebola carrier suspect because other experimental data have shown that bat species – including the one located in the massive hollow tree in this village – can survive infection with Ebola in the laboratory. But that alone is not definitive since a creature could survive such infection without being the carrier for the disease. In order to better prove that the correct bat reservoir had been identified, the virus would need to be isolated from a bat and grown in a laboratory setting. Still, there are clues that suggest that Mops condylurus could be a strong candidate. This same species of bat was considered a possible suspect for an earlier Ebola virus outbreak. And most scientists believe a close cousin of Ebola, called Marburg virus disease, is transmitted by bats—an observation that bolsters bats’ credentials as the potential animal reservoir for the Ebola virus.
Yet even if bats are the true culprit, researchers still don’t know how the animals transmit the virus to humans. Does the transmission of virus occur when blood spatters into the eyes or cuts during butchering of the bat? Or, perhaps it’s by eating food that the bat had sullied with its own spit, urine or feces? Then again, it could be through some other exposure to the bats’ bodily fluids.
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