Bats have a mixed reputation. Aside from their roles and fame in fact and fiction as vampires, they pollinate plants and have a voracious appetite for insects. Apparently little brown bats can eat 1,200 mosquitoes per hour.
On the other hand humans fearfully associate bats with rabies virus, and occasionally with histoplasmosis, a respiratory infection transmitted via the fungus Histoplasma capsulatum - found in bat guano. But Bat Conservation International (BCI) battles their bad reputation, and points out that bats are only responsible for 1 death per year in the United States and rabies is found in only 6 of 45 species continental U.S. bat species, whereas dogs - "man's best friend" - are responsible for more fatal maulings of humans every year.
However bats are increasingly found to harbor fatal viruses. Bats are natural reservoirs for the Nipah and Hendra viruses. Bats have been linked to West Nile Virus. They have also recently been found to be the natural reservoir of the coronavirus that causes severe acute respiratory syndrome (SARS). Researchers found that bats were immune to symptoms of SARS and were the natural reservoir for the virus, whereas civets, the animal suspected of being the reservoir, whose populations were culled by about 10,000 in China in 2004, is only a carrier of the disease. The coronavirus was found in three species of horseshoe bats which is a fruit bat, indigenous to China.(Li et al, Science vol.310 pp. 676).
Bats have long been linked to human cases of Marburg and Ebola viruses, primarily because they're routinely found in places where humans contracted the viruses, a barn in one case, a field in another case. Since the 1970's, following almost every Ebola virus outbreak, scientists have combed the surrounding areas for the source of the virus -- collected thousands of vertebrates and arthropods and tested them for the virus or antibodies to the virus. Most of these studies came to a dead-end -- no virus was found, no immune reaction detected in the specimens. Scientists also tried to infect cells and animals with Ebola virus, to no avail until earlier this year when they managed to infect a couple of bats. But the infection of the bats was not reproducible and the researchers did not have confidence in their data. (Pourrut et al.; Microbes and Infection, Vol. 7, pp 1005-1014)
They pursued that line of research though, and scientists reported last week that three species of fruit bats in Africa carried asymptomatic Ebola virus (Nature 438, 575-576). Researchers tested over a thousand small animals captured from around Ebola sites, assayed serum samples for antibodies to the virus, and spleen and liver samples for viral RNA and RT-PCR nucleotide sequences. Some bats were found to be Immunoglobin-G positive and others screened positive for nucleotide sequence analogy of Ebola viral RNA, however no bats were positive for both. Viral RNA could not be isolated. The authors discuss their results in their article. The fact that they found Ebola in the fruit bats helps solve a piece of the Ebola puzzle, but there are more unsolved questions. For instance, how do the bat's immune systems protect them from viruses and how do spillover events between species trigger the ever transient emergence of Ebola and other viruses?
Although scientists don't understand a lot about bat immunology they do know something about bat ecology. Bats often eat while flying. They eat fruits and insects, extracting the juice and sugars and leaving insect carcasses and partially eaten fruits for animals and humans to pick up and eat. As humans habitats increasingly overlap with bat habitats, chance interactions or disease spillover through other species like pigs or civets becomes more common. The link of the lethal viruses to bats is likely a harbinger of future infectious disease challenges.
The number of pathogens is arguably increasing with changing environmental, ecological and human factors, and many emerging viruses have lethal pathology including severe neurological symptoms. Over half of emerging infectious diseases come from animals (zoonotic), so understanding how the diseases are transmitted between species is critical to controlling them. It is increasingly important for understanding pathogens to probe anthropogenic affects on ecosystems and reconsider our relentless forays into nature and pension for development.
Ebola and other zoonotic pathogens have company among a growing number of emerging and re-emerging diseases. Bats are increasingly found to be the reservoir for viruses that are fatal to humans. While "zoonotic" is a biological term indicating the source of a pathogen, an anthropocentric understanding of the term has the rhetorical effect of pointing at the the reservoir animal. This does not bode well for bats. Animals suspected of harboring disease are culled as the first line of defense for humans.
As an aside, we'll mention the unfortunate events resulting from humans encroachment on ecosystems, which they change it to suit their short-term needs, and at the same time disrupt so that new species take hold and previously established species are forced to adapt - or by chance become natural reservoirs to emerging pathogens like Ebola...isn't the virus actually the result of human activity? Isn't the zoonotic "source" in fact an unfortunate ecological circumstance precipitated by the arrival of humans? Perhaps hard to say, but so far bats seem to conserve themselves through their immunity to the virus. Humans might be better served if thinking in conservation and environmental ecology was as deep as vaccine development and animal control measures.
