Are fruit bats a reservoir for Ebola?
Around the same time as the Kikwit outbreak, a case of Ebola was retrospectively identified in a primatologist who had been working on the Ivory Coast. So, scientists trudged back out there, collecting 1642 vertebrates (652 bats, 283 rodents, 398 insectivores, 27 monkeys and 282 birds) between 1996 and 1997. One red colobus monkey was seropositive, but virus was never cultured.
Finally, 242 vertebrates (24 bats, 163 rodents and 56 insectivores) were captured in the Central Africal Republic in 1998. Small Ebola-specific genetic sequences were amplified from organs of six mice and a shrew. However, although these results constitute the only biological evidence that Ebola may be present in healthy animals, no firm conclusions as to the reservoir status of these animals can be drawn, given the lack of specific serologic responses, the lack of nucleotide specificities in the amplified viral sequences, the failure of virus isolation, and the non-reproducible nature of the results. Still, it was heartening to finally have some positive results after so many dead ends. (All of this reviewed in Pourrut et al., "The natural history of Ebola virus in Africa." Microb Infect 7:1005-14).
Now, a new study in today's Nature has found similar results in fruit bats.
Like previous studies, the authors trapped a number of species while Ebola epidemics were occurring (this time, in Gabon). In total, 1,030 animals were captured, including 679 bats, 222 birds and 129 small terrestrial vertebrates. They found Ebola-specific IgG in 3 different bat species (a total of 16 positive individuals). Viral RNA was detected in the liver and spleen of a total of 13 of the individuals of these species as well, and found to be the Zaire subtype. (There are 4 known Ebola subtypes: Zaire, Sudan, Ivory Coast, and Reston, listed from highest to lowest virulence in humans). However, the bats that were antibody-positive weren't the same individuals that were PCR-positive. The authors suggest that the IgG+ individuals had likely already cleared the virus, and the PCR+ individuals had not yet mounted a significant immune response to the virus.
Is it biologically plausible that bats are a reservoir of Ebola? In a word, yep. Bats have long been suspected to play a role in Ebola transmission. Studies have shown that the virus can replicate in experimentally-infected bats, and index cases in outbreaks of not only Ebola, but also its filoviral cousin Marburg virus, have been traced to bat-infested locations: caves, warehouses. However, this is the first direct evidence of bat infection in the field. Finally, the authors note that each of the three bat species has a broad geographical range that includes regions of Africa where human Ebola outbreaks occur, and also suggest that changes in the bat's diet during the dry season (when Ebola mortality increases in the great apes) may make them more likely to spread the virus during that time.
Does this close the book on an Ebola reservoir? Hardly. This will, however, give scientists some targets to focus on.
The three species identified include Hypsignathus monstrosus (which has to win some kind of ugly animal award), Epomops franqueti, and Myonycteris torquata, pictured in that order. Future investigation can examine more closely the ecology of this species, and particularly its interaction both with humans and with the great apes in this region. The current study also cannot address how the bats themselves become infected. Is there another reservoir that they acquire the virus from? Is it transmitted between bats via secretions? Feces? Blood? Acquired from nursing on an infected mother, or during birth? Does it come from their diet? All the positive bats were fruit eaters, and it's been previously suggested that Ebola may be a plant virus (though the evidence for that is currently pretty sketchy). Nevertheless, all are potentially testable hypotheses in the field and/or in the lab.
Finally, an awareness campaign can be instituted, as they mention that villagers in the area may eat these bats. Some previous outbreaks seem to have correlated with the consumption of bush meat (in one case, a chimpanzee who had likely died from Ebola); therefore, this is one potential way the virus could enter the human population. Education may potentially ward off a future outbreak. It won't help the gorillas, but it's definitely a start, after a long and mostly fruitless (wink, wink) search.
(Images (c) N. Ebigbo for E. franqueti and (c) J. Fahr for M. torquata).