Some of the worst and deadliest virusesplanets find natural refuge inside bats, including Ebola, rabies, Marburg virus, and SARS coronavirus. Many serious epidemics can be traced back to bats, and scientists regularly discover new and new viruses carried by these creatures.
Bats seem especially skilled atharboring and spreading disease. Scientists are trying to find a reason for this in the genome of bats; others believe that the sad fame of bats as carriers of viruses is not justified.
“Are bats special?” I think it's too early to talk about it, ”said Linfa Wong, who leads the research team at CSIRO, an Australian animal health laboratory. He spent the past twenty years studying bats-generated viruses and hunting for characteristics that can make animals such serious virus hosts.
“The issue is so important that we just can't ignore it anymore,” he says.
Bats and other species that are chronicallycarry viruses, rats and mice, known as reservoirs of disease. Most of the time, these reservoirs remain untouched, and infected animals themselves rarely display disease symptoms. But sometimes they leak, allowing the virus to infect a new, much more vulnerable species. It is almost certain that the outbreak of the Ebola virus in West Africa in December began precisely with the bats' light arm. To date, the fever, although exaggerated in the media, has hit at least 8,900 people. Scientists suspect that bats are to blame for this epidemic that hit Guinea, Sierra Leone, and Liberia.
The funny thing is that bats can carry a disproportionate amount of scary viruses. But whether it is so or not is an open question.
Scientists are divided into two camps on this subject. One school of thought says bats-related epidemics are just a numbers game; roughly speaking, there are so many species and individuals of bats that the reproduction of viruses among them does not surprise anyone. Another school suggests that bats are really special, there is something in their lifestyle or physiology that makes them good carriers of viruses.
Wong and his colleagues spent enough timetrying to figure out this difficult question. They began by studying the genome of bats, hoping to find a key in the immune system, for example, a set of genes that belong only to them.
Instead, the team found another subtlety: even though the bat genome contains many ingredients that other mammals have exactly, the former use them differently. In particular, bat genes are rich in proteins that detect and repair damaged DNA - more than expected. Simply put, these genes do something that helps mice survive and multiply, so they are passed on to the next generation.
These results are presented in the journal Science.in December 2012, consistent with previous observations that repairing genes are a common target for the invasion of viruses, part of their evolutionary path. The results are also intertwined with the anecdotal observation that bats rarely (if ever) get cancer - perhaps because repair genes determine any malignant tissue growth.
Since then, Wong and his colleagues have gone even further. New, not yet published results show that, unlike people or ordinary mice, in which antitumor and antiviral defense are activated only in response to a threat, these genes work constantly in bats. This activity maintains the level of any viruses at a point to which they do no harm. In short, evolution has raised the mechanism of bat survival to a maximum.
Wong also suggests a connection with the flight, whichincreases the metabolic rate in the bat many times over in comparison with the state of rest. Such energy production leads to stress that can damage cells and DNA, unless there is a way to quickly detect and repair it.
Perhaps these repair proteins were originallytuned to deal with damage that appeared during the natural behavior of mice (fly every night). The ability to fight deadly viruses could appear later, as a kind of co-evolutionary incident, Wong said.
Another hypothesis reported in EmergingInfectious Diseases in May suggests that flying bats can generate enough heat to mimic a fever. As part of the normal immune response in many animals, fever fights infection, raising body temperature to a level at which pathogens die or mute. By raising the temperature every night, flying can inadvertently kill a certain number of viruses, reducing their population.
Although to verify this idea, noone experiment, some scientists believe that this particular hypothesis can be the cause of the danger of bats. They have evolved so much that they constantly maintain a powerful and active immune system, which few mammals can boast of.
“We don’t have this kind of immune system,”says Angela Louis, an environmental ecologist at the University of Montana and author of a recent study. Once released from over-alert mouse organisms, viruses may not have any problems with weaker immune systems.
Wong is not ready to claim one hundred percent,that bats are excellent hosts for viruses, but he believes that scientific research in this direction does not exclude this possibility, but on the contrary - it is getting closer to it.
Another possible explanation can only be a combination of numbers, they say, the virality of bats is nothing more than statistics.
With over 1200 known species, batsmake up more than 20% of all mammalian species on Earth. Among mammals, only rodents are superior to them (contrary to popular belief, bats are not such mice). In many areas, bats are far superior to rodents in numbers; sometimes millions live in one colony.
The notion that bats arespecial, can be strongly highlighted against the background of the total number of works on the virality of these animals. The more scientists dig, the more viruses they find, roughly speaking, says Kevin Olival, an environmental ecologist at the EcoHealth Alliance.
In 2013, Olival and his colleagues studiedVirom (a collection of viruses in one body) of a giant bat called the Indian bat fox (Pteropus giganteus). In this one form, they found 55 viruses, 50 of which were unknown until this moment. This is approximately equal to the total number of viruses detected in a similar study in 2006, which summarized all the studies conducted at that time. Over the past eight years, this number has doubled or tripled, depending on the criteria used to determine the “known virus”.
Olivel argues that this trend is characteristicnot just for bats. “If you look at the wide variety of viruses in mammals, viruses break down into quite different groups. And not all of these groups we studied thoroughly. "
Olival considers ecology and how these animals live and how people interact with them to be more important issues. Especially in those areas where there are really many bats.