As I've mentioned before, Ebola is a virus near and dear to my heart. (Figuratively, not literally. I'm not quite that enamored of it). In that previous post, I mentioned that we didn't know the reservoir of Ebola in nature. It certainly isn't for lack of trying that it wasn't determined previously. The first field studies took place shortly after the initial 1976 outbreaks in the Democratic Republic of Congo (DRC) and Sudan. In the former, 818 bedbugs, 1500 mosquitoes, 10 domestic pigs, one cow, seven bats, 123 rodents, eight squirrels, six Cercopithecus monkeys, and three small antelopes were sampled--all negative when viral isolation was attempted. In Sudan, almost 500 vertebrates were similarly tested--zilch. Similar results in 1977 and 1979 in the DRC. Following that outbreak, Ebola did not re-surface for a decade and a half, popping up again in a large 1995 outbreak in Kikwit, DRC. A total of 2814 vertebrates and a whopping 27,843 arthropods were captured during respective periods of 3 and 6 months after the end of the outbreak. Viral isolation was attempted from both categories of animals, and vertebrates were also tested for Ebola-Zaire-specific IgG. No positive IgG reactions were obtained, and no viruses were isolated on Vero cells. The vertebrates included 1759 rodents, 539 bats, 114 insectivores, 184 birds, and 127 reptiles and amphibians. The arthropods included 15,118 mosquitoes, 124 blood-sucking flies, 6538 bedbugs, 144 fleas, 103 lice, and 5816 ticks.
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).
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).
A Few Questions
I am fascinated by Ebola and Marburg as well, although retroviruses - and in particular prophages - are of even greater interest to me. As I understand it, the good majority of viruses have what are called a "natural host." This is a species that they have infected for perhaps ten or a hundred thousand years and with which they have entered a coevolutionary relationship. In the case of the natural host, the infection is chronic and oftentimes asymptomatic.
Does it appear that a species of fruit bat is the natural host for ebola? Do we know, for example, whether ebola is asymptomatic in this species of fruit bat? Alternatively, is it possible that different strains of ebola have different species as natural hosts, or would we expect different strains to belong to have the same species of natural host, but belong to different populations?
Additionally, I understand that at least one "species" of virus has made the leap before from plants to humans. Are you aware of several instances of this, and if so, how many? Are you aware of any instances where there were significant symptoms?
Finally, as I understand it, the view that in deep evolutionary time, close to the origins of life itself all viruses shared much the same origins (in an RNA world) appears to be becoming increasingly mainstream. Do you see this happening, and do you share this view?
Mam, I do suspect that a specific species of fruit bats
are linked to Ebola & perhaps HIV.
Perhaps you could correct me if I am wrong, but if the bats were able to clear the virus from their system, would that not preclude them from possibly being a reservoir? The virus may not be persistent enough within the species to act as a reservoir.