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.

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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.”