Why bats don't get sick - Arinjay Banerjee

If this bat were a human, she'd be in deep trouble. She’s infected with several deadly viruses, including ones that cause rabies, SARS, and Ebola. But while her diagnosis would be lethal for other mammals, this winged wonder is totally unfazed. In fact, she may even spend the next 30 years living as if this were totally normal—because for bats, it is.

So what’s protecting her from these dangerous infections? To answer this question, we first need to understand the relationship between viruses and their hosts. Every virus has evolved to infect specific species within a class of creatures. This is why humans are unlikely to be infected by plant viruses, and why bees don’t catch the flu. However, viruses do sometimes jump across closely related species. And because the new host has no established immune defenses, the unknown virus presents a potentially lethal challenge. This is actually bad news for the virus as well.

Their ideal host provides a steady stream of resources and comes into contact with new parties to infect—two criteria that are best met by living hosts. All this to say that successful viruses don’t typically evolve adaptations that kill their hosts—including the viruses that have infected our flying friend. The deadly effects of these viruses aren’t caused by the pathogens directly, but rather, by their host’s uncontrolled immune response. Infections like Ebola or certain types of flu have evolved to strain the immune system of their mammalian host by sending it into overdrive.

The body sends hordes of white blood cells, antibodies, and inflammatory molecules to kill the foreign invader. But if the infection has progressed to high enough levels, an assault by the immune system can lead to serious tissue damage. In particularly virulent cases, this damage can be lethal. And even when it’s not, the site is left vulnerable to secondary infection. But unlike other mammals, bats have been in an evolutionary arms race with these viruses for millennia, and they’ve adapted to limit this kind of self-damage.

Their immune system has a very low inflammatory response—an adaptation likely developed alongside the other trait that sets them apart from other mammals: self-powered flight. This energy-intensive process can raise a bat’s body temperature to over 40ºC. Such a high metabolic rate comes at a cost; flight produces waste molecules called Reactive Oxygen Species that damage and break off fragments of DNA. In other mammals, this loose DNA would be attacked by the immune system as a foreign invader. But if bats produce these molecules as often as researchers believe, they may have evolved a dampened immune response to their own damaged DNA.

In fact, certain genes associated with sensing broken DNA and deploying inflammatory molecules are absent from the bat genome. The result is a controlled low-level inflammatory response that allows bats to coexist with the viruses in their systems. Even more impressive, bats are able to host these viruses for decades without any negative health consequences. According to a 2013 study, bats have evolved efficient repair genes to counteract the frequent DNA damage they sustain. These repair genes may also contribute to their long lives.

Animal chromosomes end with a DNA sequence called a telomere. These sequences shorten over time in a process that many believe contributes to cell aging. But bat telomeres shorten much more slowly than their mammalian cousins—granting them lifespans as long as 41 years. Of course, bats aren’t totally invincible to disease, whether caused by bacteria, unfamiliar viruses, or even fungi. Bat populations have been ravaged by a fungal infection called white-nose syndrome, which can fatally disrupt hibernation and deteriorate wing tissue.

These conditions prevent bats from performing critical roles in their ecosystems, like helping with pollination and seed dispersal, and consuming pests and insects. To protect these animals from harm, and ourselves from infection, humans need to stop encroaching on bat habitats and ecosystems. Hopefully, preserving these populations will allow scientists to better understand bats’ unique antiviral defense systems. And maybe one day, this research will help our own viral immunity take flight.