Bats’ immune defenses may be why their viruses can be so deadly to people

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When it comes to viruses, ones
from bats are weirdly deadly — at least to humans.

The mammals
can carry many viruses with the potential to cause serious diseases in people, including
rabies, Ebola, Nipah, severe acute respiratory syndrome, or SARS, and others. Bats
rarely get sick from those viruses. Why these pathogens tend to be so dangerous when they infect other animals has
been a mystery.          

Previous
work suggests that a bat’s immune system is especially adapted to tolerate
viruses, thanks in part to its ability to limit inflammation. Now a study using cells grown in a
lab hints that to counter a bat’s immune defenses, these
viruses have gotten good at spreading rapidly
from cell to cell. That means that when they get into
animals without a similarly strong immune system, the viruses are particularly
adept at causing serious damage, researchers report February 3 in eLife.

The study
is “an important piece of the puzzle in understanding why viruses [from bats]
may be emerging and impacting people and other animals,” says Kevin Olival, a disease
ecologist with EcoHealth Alliance in New York City, who wasn’t involved in the
research. “There’s a lot we can learn from bats about their immune system and
take some of that information to think about our own health and developing our
own therapeutics” against viruses, he says.  

Scientists
have pinpointed bats as potential sources of several viral outbreaks in humans.
Insect-eating bats may have been the source of the 2014–16 Ebola outbreak in West Africa (SN: 12/31/14). Egyptian fruit bats (Rousettus aegyptiacus) harbor Marburg
virus, a hemorrhagic virus related to Ebola. Other bat species are reservoirs
of SARS-like coronaviruses, possibly including one that sparked an ongoing outbreak in China (SN: 1/24/20).

In the new
study, Cara Brook, an ecologist at the University of California, Berkeley, and
her colleagues investigated how two bat viruses — Ebola and Marburg — might spread
upon infecting one of three types of cells in the lab. One cell type, from
African green monkeys (Cercopithecus
aethiops
), lacks an antiviral immune response. The other two are from bats:
One type from the Egyptian fruit bat sparks an immune response only if infected
with a pathogen, and the other, from black fruit bats (Pteropus alecto), is probably always in an antiviral state and
“perpetually trying to fight viruses,” Brook says.

The team
infected the cells with viruses engineered to be coated with the proteins that either
Ebola or Marburg use to enter and infect cells. The researchers then monitored viral
spread among cells. While the monkey cells were completely destroyed by the
viruses, more of the bat cells survived.

The team
then re-created their lab experiments using mathematical simulations to calculate
how fast the viruses infected other cells and whether antiviral defenses played
a role in their spread. Viruses replicating under pressure from a bat’s immune
system have a high rate of cell-to-cell spread within a host, the simulations showed.
That rapid spread in bat cells helps the viruses combat bat cells’ antiviral
properties and quickly mounted defenses, the team says. Although the viruses
spread more slowly in the monkey cells, the cells were swiftly killed.

Pathogens
can only spread so fast internally before they kill their host, Brook says. But
if the host has an immune system that can defend against rapidly spreading viruses,
a virus might evolve to infect new cells even faster than it would in a
different environment, in a sort of arms race. And if a quick-spreading virus
from bats were to infect another species that lacked batlike defenses? “It
would probably cause extreme virulence,” Brook says.

There are
more than 1,400 bat species in the world, Olival says, and the current study
focused on only two. “It’s important to remember that all other bat species
might have totally different responses as well,” he says.

Olival is also curious how the findings might apply to other animals that can carry deadly viruses, such as rodents. “Bats are not the only mammal that are reservoirs for human zoonotic viruses,” he says. “The question is not only how do bats cope with viruses, but how do other mammal species that are reservoirs cope with the viruses they carry?”

While bats do carry lots of deadly viruses, “I don’t want people to walk away wanting to kill all the bats,” Brook says. Closely related animals are more likely to transmit viruses to one another, and bats and humans are not close relatives. “Bat viruses are not likely to spill over to human populations. It’s just that when they do, they are virulent.”

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