ROCHESTER, N.Y. – Bats are thought to be hosts for a number of deadly human viruses, including Ebola, rabies, and SARS-CoV-2 (the virus responsible for COVID-19). Despite carrying these viruses, however, bats themselves are relatively immune to the ill effects of viruses. In a new review paper published in the journal Cell Metabolism, University of Rochester scientists explain the mechanisms underlying bat virus tolerance and how bat immune systems may provide clues to treating viruses like SARS-CoV-2.
Although the origins of SARS-CoV-2 are not known for certain, the authors of the paper believe that the virus originated in bats and was subsequently transmitted to humans. Interestingly, although bats carry SARS-CoV-2, they seem to suffer no ill effects from it.
Medical marvels: Bats live incredibly long lives
In addition to their virus tolerance, bats are remarkable for another reason: they live much longer than other animals of similar size. In general, body size tends to be closely related to lifespan. Smaller animals have shorter lives, while large ones live longer. Not so for bats. Many bat species can live for 30 to 40 years, which is unusual for their body size.
“We’ve been interested in longevity and disease resistance in bats for a while, but we didn’t have the time to sit and think about it,” says lead author Vera Gorbunova in a statement. Gorbunova is the Doris Johns Cherry Professor of Biology at Rochester. “Being in quarantine gave us time to discuss this, and we realized there may be a very strong connection between bats’ resistance to infectious diseases and their longevity. We also realized that bats can provide clues to human therapies used to fight diseases.”
Gorbunova and fellow scientist Andrei Seluanov have studied disease resistance and longevity in other animals, including naked mole rats, which have exceptionally long lifespans. A common factor across long-living species is inflammation, or rather, a lack thereof. These species tend to have dampened inflammatory responses, particularly when infected with viruses.
“With COVID-19, the inflammation goes haywire, and it may be the inflammatory response that is killing the patient, more so than the virus itself,” says Gorbunova. “The human immune system works like that: once we get infected, our body sounds an alarm and we develop a fever and inflammation. The goal is to kill the virus and fight infection, but it can also be a detrimental response as our bodies overreact to the threat.”
Immune systems of steel
Unlike humans, bats’ bodies are able to reduce viral replication without a strong immune response. This is beneficial because it controls the virus, but does not create a harmful inflammatory response. The researchers suggest that there may be several factors explaining how bats came to develop such unique immune systems throughout evolution.
First, bats are the only mammals with the ability to fly. This ability may have required them to develop tolerance to a variety of factors. They include temperature changes, metabolic changes, and molecular damage. In turn, these adaptations may help them to fight off diseases.
Bats also live in very crowded spaces. They often hang close to one another in caves or trees. This cozy style of living means that viruses and other diseases spread easily in bat colonies.
“Bats are constantly exposed to viruses,” Seluanov says. “They are always flying out and bringing back something new to the cave or nest, and they transfer the virus because they live in such close proximity to each other.”
The researchers explain that bats and viruses are in an evolutionary arms race. As bats’ immune systems evolve to fight a virus, the virus evolves to beat the bats’ immune systems. This cycle continues on and on.
“Usually the strongest driver of new traits in evolution is an arms race with pathogens,” Gorbunova says. “Dealing with all of these viruses may be shaping bats’ immunity and longevity.”
Should we live like Batman?
So, why don’t humans just gather together in crowded spaces to give our immune systems a chance to develop tolerance to viruses?
As the researchers explain, bat immune systems have evolved over thousands of years, not a few months. While our social gathering traits are similar to bats, our immune systems have not yet evolved to fight off viruses in the same way bats can.
“The consequences may be that our bodies experience more inflammation,” Gorbunova says.
“Humans have two possible strategies if we want to prevent inflammation, live longer, and avoid the deadly effects of diseases like COVID-19,” Gorbunova says. “One would be to not be exposed to any viruses, but that’s not practical. The second would be to regulate our immune system more like a bat.”