As I mentioned in the introductory post, we know incredibly little about the very basics of Marburg virus ecology and epidemiology. The sporadic nature of outbreaks of illness, their occurrence in remote areas of Africa lacking established medical research capabilities, and often in countries experiencing governmental strife and instability, compound the difficulty of determining the ecology of this particular virus. Often, the primary case (the first person in an outbreak known to be infected, and who likely acquired the virus from its wild reservoir) died before questions could be answered regarding his previous whereabouts, diet, and other activities; thus, it was difficult to determine where the case could have contracted the disease. Seasonality may also play a role; if a search for the virus is conducted during the dry season (as many ecological surveys have been), they may miss key pieces of the puzzle of Marburg virus ecology.
Nevertheless, scientists have attempted to make the most of outbreaks when they occur, and have undertaken studies between outbreaks in order to determine where the virus “hides” when it’s not infecting humans, and to find out how the virus moves from wherever it is maintained in nature into human populations. Is it simply airborne? Is it transmitted from butchering infected animals? Is it transmitted by an intermediate, such as an insect vector? The answer to these questions remains, despite years of investigation, a disappointing “we don’t know,” but some answers are slowly emerging. More after the jump…
Bats have been associated with filovirus infection since they were initially discovered. During the 1976 Ebola outbreak, Tadarida (mops) trevori, a species of bats, were found in the roof of the Nzara cotton factory in Sudan; during that outbreak, the index case and two other early cases had worked at this factory. In a second outbreak in Sudan three years later, the index case was again a worker at the Nzara cotton factory. Bats have also been associated with cases of Marburg virus infection. Two cases of Marburg have been linked to a cave on Mount Elgon in Kenya, home to thousands of bats. Additionally, experimental evidence has shown that bats of the Tadarida genus can be infected with Ebola in the laboratory, and transmit it through their guano. However, it wasn’t until 2005 that bats were found with evidence of Ebola infection in the wild, providing evidence that they could indeed act as a reservoir species in nature.
More recently, evidence has been presented that Marburg virus can be found in bats as well. Investigators captured over 1100 bats representing 10 different species in Gabon and the Republic of Congo. The bats were tested using a real-time reverse-transcriptase polymerase chain reaction test (real-time RT-PCR, essentially looking for Marburg-specific nucleotide sequences in ground-up bat liver and spleen). They found that four bats from the same species ( Rousettus aegyptiacus) were Marburg-positive by this method (but at low levels, they mention, with “cycle threshold values >33”).
They then confirmed that these were truly positive by employing a second PCR technique called a nested PCR, where they amplified and then sequenced 2 Marburg-specific genes. 3 out of their original 4 bat samples were found to be positive this way; the fourth probably had too low of a level of viral RNA to be amplified using this test (but was found to be positive multiple times using the real-time assay).
These bats were determined to have come from just two of the sampling sites, so researchers focused on these for further analyses. This time they tested the serum of the bats, looking for antibodies to Marburg (indicative of infection with this virus). They found that 29 of the bats, all Rousettus aegyptiacus, had antibody levels that were significantly higher than background–suggesting prior infection.
So, has the reservoir of Marburg been found? Well, maybe. There are still many unanswered questions. How do the bats become infected themselves? Does it cycle amonst them, or are they becoming infected from another species? Does it cause them any illness? How long do they remain infected? How do they transmit it to humans? Many of these questions can now be examined, as we at least have a starting point as far as a potential reservoir species.
I’ll note that, like the Ebola findings, this species of bat eats fruit. This still leaves open one of my favorite (rather far-fetched) ideas regarding the true filovirus reservoir: that it’s a plant. Sounds strange, but several lines of evidence suggest it could happen.
For one, filoviruses are generally quite pathogenic in vertebrates, killing them quickly rather than allowing for persistence of the virus. Thus, the virus does not appear to be well-adapted for infection in most vertebrate species, and it is possible therefore that the host of the virus is a non-vertebrate species. The appearance of filovirus outbreaks may occur at a similar time as the flowering of a plant. Additionally, a virus that appears to be similar to filoviruses was isolated from a leafhopper (Psammotettix species) from France. Since the bats found are all fruit-eaters, hypothetically they could become infected via their food with the plant virus, and pass it along to humans and other animals. Unlikely, sure, but I love the outside-of-the-box thinking. Either way, hopefully this field will now be able to move faster than it has over the last 40 years, with a potential reservoir species to focus on.
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