Why don’t most viruses make bats sick?
Bats carry a lot of viruses that make humans and other mammals very sick, but don’t make them sick. Here’s what we currently understand about why that is.
It’s a weird aspect of bat biology, viruses that make other mammals very, very ill don’t appear to make them sick.
It’s not that all viruses don’t make all bats sick. Bats can get sick from lyssaviruses – which includes the rabies virus – and a lesser known group of viruses called arenaviruses.
But what about all the rest?
Ebola, Hendra virus, Nipah virus and SARS-CoV-2 can all be found in bats, but they don’t appear to get sick when they’re infected.
The number of viruses that do make bats sick seems to be fewer than what we see with other mammals.
Burnet Institute virologist Dr Joshua Hayward explains what we currently understand about why most viruses don’t make bats sick.
There are two things that set bats apart from other mammalian species, according to what Dr Hayward and the other members of the Retroviral Biology and Antivirals research team led by Professor Gilda Tachedjian, along with their collaborators at the Australian Centre for Disease Preparedness at the CSIRO, have learnt from their research.
“The first is that they’ve got an increased tolerance for viral infection,” he said.
“And the second is that they have unique adaptations in their immune system that helps them manage their viral infections in a manner different to humans and other mammals.”
Both these characteristics are due to the one thing that makes them the most different from all other mammals – their ability to fly.
“The fact that they've evolved to fly has directly led to this distinction that we see in the way that they tolerate their viral infections.”
Flight is very, very energetically expensive, Dr Hayward said.
“Animals that fly have very fast metabolisms, and that generates inflammation inside their cells.”
Inflammation is a complex response to harmful stimuli, but it’s a double-edged sword: while essential for helping us fight infection, it can also cause cellular damage.
“Where bats are concerned, they've evolved to reduce their inflammatory response as a necessary outcome of having evolved to fly,” Dr Hayward said.
"They fly so they have fast metabolisms, so they would be generating a lot of inflammation in their cells, and that would be bad for them.”
Dr Joshua Hayward, Burnet Institute
“But they've turned the knob on their inflammation dial all the way down. And this is important because viral infections also cause inflammation.”
Having a reduced inflammatory response means viruses don’t stimulate the same kind of inflammation in bats that they would in humans and other mammals, meaning they can tolerate viral infection inside their cells without inflammation making things worse.
“What we propose is that because they have this environment in their cells that's a bit more hospitable for viruses, they've had to evolve also additional ways of managing those viral infections which might become persistent in bats.”
And there are changes in other parts of their immune systems that enable them to manage viral infections too.
How are our immune systems different to those of bats?
We last shared a common ancestor with bats around 55 to 60 million years ago, Dr Hayward said, so it’s not surprising that our immune systems are significantly different.
Some of those bat species would have split from each other almost as far back as we diverged from bats.
“So when we talk about bat immune systems we’re talking about lots of different changes, some of which might be present in one species but not in another,” Dr Hayward said.
But the big immune system differences we see between bats and humans are their limited inflammatory response to infection, and the expanded and diversified repertoire of antiviral proteins that they have.
What do antiviral proteins do?
If you think of viruses as being like tiny biological machines whose job is to replicate and spread, antiviral proteins are like tiny spanners you throw into the machine to stop it working.
These antiviral proteins can work in lots of ways, Dr Hayward said.
“They can stop viruses from entering a host cell, they can stop viruses from making new copies of themselves, or they can stop viruses from leaving the cell to infect new ones.”
Dr Hayward’s research has been looking at a particular antiviral protein called tetherin.
We’ve known about tetherin since it was discovered about 15 years ago, Dr Hayward said. And we’ve learnt since then that almost all mammals have a single tetherin gene.
In humans that single tetherin gene makes two different tetherin proteins that are very similar to each other