Bats are unique. For instance, they are the only mammals that can fly. But they also seem to be the mammal that hosts the greatest number of highly virulent viruses that can infect humans. Why?
Rabies. Ebola. Coronavirus. If there's a nasty virus out there waiting to infect humans, there's a good chance that a bat carries it. Why? What is it about bats that makes them host to so many virulent viruses?
A paper by Frans Roes in the journal Biological Theory attempts to answer this seemingly simple question. But it's actually not simple. Therefore, in order to answer it, Roes breaks down the big question into three smaller questions:
- Why are some infectious diseases more harmful than others?
- Why are bats, more than other mammals, a reservoir of virulent zoonotic diseases?
- Why is it that many of the viruses that severely affect other mammals, including humans, are apparently less pathogenic for bats?
The first question is one of the most difficult in all of medical microbiology. The answer depends not just on the nature of the microbe but on the genetics of the individual. The COVID pandemic, for instance, has shown us that some people may have a genetic predisposition to severe disease while others do not. Unraveling these mysteries will be a major focus of personalized medicine in the coming decades.
Roes tackles the question from a different angle: Transmissibility. He concludes that "the easier it is for a parasite to be transmitted from very sick hosts, the more virulent the parasite will be." In other words, a virus will evolve to become deadlier if it is still successful at infecting others. A dead host is literally a dead end for a virus, so a virus wants to hit the "sweet spot" where it can reproduce as much as possible without killing the host too quickly or making him too sick to spread it around.
Roes believes the answer to the second question involves the fact that bats often congregate in giant roosts, sometimes up to one million individuals at a density of 3,000 bats per square meter. Such incredible density favors the evolution of virulent viruses. Even if a bat is too sick to move, the proximity of so many other bats means that others will become infected, no matter how deadly the virus is. Furthermore, bats often share their homes with other bat species, which may favor the evolution of viruses that can jump to other species. Finally, bats' flight likely allows them to come into contact with more species than is typical for other mammals.
The final question is a bit paradoxical. Restated, if bats transmit so many nasty viruses, then why don't the bats get sick? Indeed, they are often asymptomatic. Here, the answer is more speculative than for the other questions. Perhaps bats have evolved immune systems that can handle virulent viruses. Perhaps flight, which increases body temperature, allows bats to fight off viruses easier than other mammals. It's a hard question with no obvious answer.
Putting theory aside, one thing is for certain: There will be "another COVID" among humans, and there is a high likelihood that the guilty species will be a bat.
Source: Roes FL. "On the Evolution of Virulent Zoonotic Viruses in Bats." Biol Theory 15: 223-225. Published online: 16-October-2020. DOI: 10.1007/s13752-020-00363-6