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The death of the Oregon Zoo's youngest elephant was sudden, but not unusual: The virus that killed her is the leading cause death for young Asian elephants like Lily worldwide, affecting captive animals and animals in the wild.

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Lily had EEHV, or elephant endotheliotropic herpesvirus. Like herpesvirus in humans, it most often presents as small bumps on the skin. But sometimes, the virus causes severe internal bleeding.

The virus was first identified in 1995, when an elephant at the Smithsonian National Zoo died. Today, that zoo runs the National Elephant Herpesvirus Laboratory. Their lab manager Erin Latimer says that for a long time, they were the only people studying the elephant virus.

It’s thought that almost all captive and wild elephants worldwide carry the virus, but it’s mostly only older calves who get sick. Scientists think that when they’re very young they’re protected by the antibodies in their mother’s milk. Calves are particularly susceptible once they’re weaned: Their immune system isn’t fully developed, but they’re no longer protected by their mother. By the time they’re about 8 years old, the risk starts to decrease.

Lily was the youngest elephant at the Oregon Zoo. She died on Nov. 29, the day before her sixth birthday. Oregon Zoo Marketing

There is no way to tell if an elephant is going to get sick from EEHV. It hides in nerve cells where it can’t be detected. Veterinarians can only find the virus once it’s circling through the animal’s blood, at which point it may be too late to treat. No one knows why the virus stays dormant in some elephants, but kills others.

At least 40 young elephants in the U.S. have succumbed to the virus since it was first identified, Latimer said. It’s unknown how many have died in the wild, because they’re too hard to track.

Oregon Zoo veterinarian Kelly Flaminio says that about 1 in 5 young Asian elephants dies from the disease in captivity. It’s reasonable to suspect that the same number die in the wild.

Because the disease is so common, zookeepers perform weekly blood tests and send samples off to the National Elephant Herpesvirus Laboratory at the Smithsonian. It’s partly for research (the Smithsonian is trying to develop better treatments and a vaccine) and partly for diagnostics.

Lily the elephant, not long after she was born. Oregon Zoo Marketing

That’s how they found out Lily was sick: Even though she wasn’t showing symptoms, the Smithsonian lab told zookeepers on Wednesday that the virus was active at low levels in her blood.

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By Thursday morning, Lily was acting lethargic, and staff began to treat her with human antivirals — the only known treatment. Later that night, she was dead. She’s the only elephant at the Oregon Zoo to have succumbed to the virus.

For zoo staff, it’s particularly hard. Elephants are intelligent animals with strong personalities, and Lily was, by all accounts, vivacious. Friday would have been her sixth birthday.

“I was here when she was born,” said Flaminio, who was there when Lily’s brother Samudra was born, too. “Her brother was subdued, but from the moment [Lily] was born, she knew exactly what she wanted: She wanted get up and she wanted to run around and she wanted to roar and she wanted to trumpet.”

And that was what she did, every day, until she got sick.

Lily, the Oregon Zoo's youngest elephant, plays in the water Oregon Zoo Marketing

There’s not a lot veterinarians can do to help elephants once symptoms appear. According to Latimer, if the virus is caught early it has a roughly 40 percent mortality rate with treatment. But once there are symptoms, the mortality rate goes up to 70 percent or higher.

The sick elephants are treated with human anti-herpes drugs, because there hasn’t been funding to develop drugs that treat elephant herpes. Veterinarians aren’t even positive the treatments work, but they do know that the only elephants that survive EEHV receive human antivirals. No one wants to take the risk of not administering the drugs.

The Oregon Zoo has no plans to close the elephant exhibit, though that could change: They want the elephants to have access to all of their habitat.

“We want them to know that Lily is gone and have space to grieve together as a family. Elephants do that,” Flaminio said. It’s not just a hard time for the humans at the zoo.

Because the virus is so hard to treat and detect, the best way to stop it is to prevent it. But it’s so prevalent there’s no easy way to prevent transmission. The National Elephant Herpesvirus Laboratory is working to develop a vaccine, to stop elephants from getting the virus in the first place.

It’s not easy, Latimer said. They haven’t found a way to get the virus to reproduce in the lab — which you need to do before you can make a vaccine the traditional way — so they’re looking into other methods. They’ve partnered with universities and medical schools. Research is still early, but they’re optimistic.

There is some good news: When Latimer first began her work at the Smithsonian, her lab was the only one in the world testing for EEHV. That’s no longer the case.

More labs have sprung up. Some zoos can even test for the virus on their own. And Latimer has traveled to Thailand and Cambodia and Borneo to help others start their own laboratories to test for the virus and contribute to research. They hope that, one day, imperiled wild elephants won’t die from the disease that took Lily.

There are at least 15 of these labs today.