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How A Virus In Snakes Could Offer Clues To Ebola In Humans

Taryn Hook was so worried about her sick boa constrictor, Larry, that she appealed to a virologist at the University of California, San Francisco, for help.
Courtesy of Taryn Hook
Taryn Hook was so worried about her sick boa constrictor, Larry, that she appealed to a virologist at the University of California, San Francisco, for help.

Scientists have found a surprising link between deadly Ebola virus and a disease that's been killing boa constrictors in zoos and aquariums.

A team at the University of California, San Francisco, has found evidence that a previously undiscovered virus is responsible for something called inclusion body disease in boas. And this virus, described in the journal mBio, appears to be related to both Ebola and another deadly class of viruses called arenaviruses.

The discovery should make it possible to contain outbreaks by testing snakes for inclusion body disease before putting them in a collection. It also may help researchers figure out how some dangerous viruses in animals end up infecting people.

"We know a lot about viruses in pigs and bats and mice," says Joseph DeRisi, a virologist at UCSF. "No one suspected snakes might be a repository of information about these hemorrhagic viruses."

DeRisi and his team might never have discovered the virus that's killing snakes without the efforts of a woman named Taryn Hook.

Hook, who lives in the San Francisco Bay Area, had become worried about her snake, a 7-foot-long boa constrictor named Larry.

"Larry is a member of the family in every respect," she says. "He gets in bed with us and watches television. He likes American Idol."

He is also a therapy animal for Hook, who says she suffers from anxiety and obsessive-compulsive disorder. But Larry had been sick for months, Hook says. And veterinarians couldn't figure out why, though they suspected a virus.

Hook was desperate. Then she heard about DeRisi at UCSF. DeRisi became famous in animal circles several years ago when he helped identify a virus killing parrots and other exotic birds.

Hook thought maybe he could save her snake.

"So I wrote him a letter with a picture of myself and Larry in our backyard," she says. "And I pled with him, explaining that he was my last hope."

Hook's letter wasn't just about Larry, but about lots of sick boa constrictors, DeRisi says. It described how inclusion body disease, which Hook suspected Larry might have, had become a major problem in aquariums and zoos, and for people who keep snakes as pets.

The first symptoms of the disease are often that a snake stops eating or begins regurgitating its meals. But eventually, it attacks the snake's brain and nervous system.

"Some of these snakes tie themselves into knots," DeRisi says. "They roll on their back, and they exhibit behaviors like stargazing, where they wave their heads in the sky sort of uncontrollably."

DeRisi thought he might be able to find the cause of inclusion body disease if he looked at genetic material from snakes that had died of it. So he asked around. And he learned that an outbreak had just been discovered in boas at an aquarium a few minutes from his lab.

DeRisi's lab extracted lots of genetic material from the sick boas. And DeRisi was pretty sure that somewhere in all that genetic material was code for the virus causing inclusion body disease.

The trick was to figure out which genetic material belonged to the snakes and which belonged to the virus. A researcher in DeRisi's lab named Mark Stenglein did that by comparing all the genes found in the sick boas with the complete genetic code, or genome, of a healthy red-tailed boa named Balthazar.

"With the boa genome in hand, Stenglein could filter away what was boa, leaving behind what was not boa, presumably the thing making them sick," DeRisi says.

The approach worked. Stenglein soon found genetic code for a virus related to so-called arenaviruses, which can cause deadly infections in people. "But they were different from all previously described arenaviruses," Stenglein says

No one had ever found an arenavirus in a reptile before, so that was pretty big news in itself. But then Stenglein found something even more surprising about this particular arenavirus.

"One of its genes is actually most closely related to the same gene in Ebola virus," he says. "So this virus is actually a mashup, or a genetic mix of arenaviruses and Ebola virus."

The virus kills snakes but appears harmless to people, DeRisi says.

The finding raises two possibilities, DeRisi says. One is that at some point snakes carried both arenaviruses and Ebola viruses, allowing them to swap genes. Another possibility, he says, is that "Ebola and arenavirus as we know them today evolved from this."

Either way, the finding suggests that reptiles can harbor versions of some of the world's most deadly viruses. That information means scientists need to expand the range of animals they study when trying to explain outbreaks of these viruses in people.

As for Larry, the sick boa that started this line of research, tests have shown that he's not infected with the newly discovered virus.

Larry still gets sick from time to time, but his symptoms are being controlled with medication, Hook says.

Copyright 2021 NPR. To see more, visit https://www.npr.org.

Jon Hamilton is a correspondent for NPR's Science Desk. Currently he focuses on neuroscience and health risks.