A couple of months ago, a young elephant died in a zoo in Oregon. The cause of her death was a viral disease known as elephant endotheliotropic herpesvirus (EEHV) hemorrhagic disease, which is a common cause of young Asian elephant mortality. Undeniably tragic for everyone involved, this high profile death triggered activist demonstrations, who protested the confinement of elephants and in some cases claimed that captivity caused her disease or made her more susceptible to it. In fact, this enigmatic disease has claimed the lives of wild and captive elephants, and it is largely from the captive population of elephants that we know as much about it as we do.

Let’s talk about herpes

The causative agent of EEHV hemorrhagic disease is a herpesvirus. I can hear you snickering in the back, so just be aware that it’s not lack of elephant sex education that causes this, and in fact herpesviruses are extremely common in animals, usually not sexually transmitted, and usually don’t cause significant disease in their hosts. EEHV is an exception to the “harmonious pathogen” rule, however, as it has been documented to cause a lot of elephant deaths.

Herpes viruses in general tend to be strongly associated with a single host species. They may cause mild symptoms in the host (e.g. cold sores in humans are caused by a herpesvirus) but very rarely cause serious disease. But there are some examples of a virus “jumping” into a closely related species and causing disaster. Herpes B virus, for example, is a common host-adapted infection in macaque monkeys that causes minimal symptoms. But if a human is infected through a bite or a scratch, it causes encephalitis and has one of the highest case fatality rates of any known infection. Fortunately it’s relatively rare, but I still cringe when I see people conducting monkey business without protection.

There are an extraordinary number of herpesviruses, and given non-native host disease is so uncommon, when researchers were first trying to figure out why Asian elephants were dying from this virus, they reasonably thought that it was a host-adapted virus from African elephants* housed in the same zoos. Although this was a sound hypothesis at the time, it had yet to be tested, and has been misrepresented ever since. The speculation was enough to be used as “evidence” that housing elephants in artificial conditions (if it’s not clear, African and Asian elephants’ ranges do not overlap in the wild) directly led to the deaths of these young animals.

The Oregon Zoo

Lily’s death was the most recent case of EEHV hemorrhagic disease and the first to occur at the Oregon Zoo, in Portland, Oregon. This zoo is unfortunately no stranger to controversy and has attracted severe criticism from animal rights activists over the years. And yes, it is Portland, which may have a lower protest threshold than say Des Moines, and seems out-there enough to have been a plausible setting for the infamous hoax paper equating dog park behavior to rape culture. All the same, there is a history of controversial incidents at the Oregon Zoo, and this was not the first organized protest in front of its gates. The most prominent group is specifically opposed to the elephant program.

I’d like to briefly dab the distal millimeter of my toe into the firestorm of controversy surrounding elephants in captivity and say this: most modern zoos are deeply dedicated to the humane care of their animals as well as conservation and endangered species survival, while at the same time undeniably the descendants of an exploitative system. Elephants are not endemic to Oregon (for at least the last 15,000 years), and it was not high-minded conservation efforts that brought the original population of most zoo animals to where they have ended up today. That’s not the case nowadays, but it was not that long ago when we practiced some pretty shameful methods of “obtaining” exotic animals. That doesn’t change our responsibility to take care of the animals in modern zoos, or make it any easier to return captive species to the wild. And it doesn’t mean species in the wild can’t benefit from knowledge gained from their captive relatives.

After Lily’s death there were protests at the zoo, and although they were opposing elephant captivity in general, there were some suggestions that the zoo was responsible for the death. Of course, the activists are against captivity, so they’re motivated to assign the blame to the zoo for any negative outcome. According to the activist website, “Elephants in captivity are susceptible to life-threatening health problems, like … endotheliotropic herpesvirus”. EEHV, however, has been documented in wild elephants and there is no evidence to suggest that it’s more common in captivity. That’s not to say there aren’t important differences between captive and wild animals (for example, there are a number of diseases in captive black rhinoceros that are virtually unknown in their wild counterparts), but that doesn’t mean that every problem a zoo animal gets is because it’s in a zoo.

One of the main claims still made by activists is that EEHV hemorrhagic disease is the result of unnatural zoo conditions, in which different species are often in close proximity. As stated above, this was the original hypothesis for why a herpesvirus would cause fatal disease in what was suspected to be a non-native host. Research in the last 20 years has since shown that there are more than a dozen subtypes of EEHV and that not only are the most common types responsible for deaths (EEHV1A and EEHV1B) not found in African elephants, but that wild Asian elephants who had never had been near African elephants have died from the disease. It appears that the most fatal subtypes are actually host-adapted to Asian elephants, although no one knows why.

Gaps in knowledge about EEHV

“We don’t know, but we’re working on it” would probably be an appropriate motto for EEHV researchers. There are so many things we don’t know about this significant killer of elephants. That’s not going to satisfy the grieving public or passionate activists after an elephant calf death, but it is the truth. A ton (I’m going to give myself credit for an elephant pun here) of work has been done on this disease over the last 20 or so years since the first cases were described, but the picture appears to be getting more confusing, not less.

Here are some depressing facts we’ve learned about EEHV: It has killed about 20-30% of the captive-born elephant calves born in North American zoos in the last 50 or so years. There have been about 100 cases worldwide, the vast majority in Asian elephant calves (only four cases are known in African elephants), and there are about eight documented survivors. All of the survivors were treated with expensive and sometimes difficult-to-access antiviral drugs, although many of the animals that died were also treated with these drugs, so it’s hard to say if the treatment actually helped or not. Most of the deaths are in animals between 1 and 4 years of age. The epidemiology of this disease has been a tough nut to crack, with almost every individual case caused by a different strain, and no clear reason as to why some individuals are affected and some aren’t. One of the craziest parts of the problem is that this virus is endemic in almost every herd of elephants that’s been examined, so it’s extremely unlikely that we’d be able to eradicate it. It’s been killing young elephants left and right for decades and we don’t understand how it’s transmitted, why it’s so virulent, or how to prevent it. It may have even been a cause of elephant mortality for hundreds if not thousands of years, but we have no idea.

One of the major reasons our knowledge is limited is that we haven’t been able to grow the virus in the lab yet, despite apparently a great deal of effort in doing so. If this hurdle could be crossed, we could make a lot more progress in our understanding of the pathology and treatment of the disease. There have been recent developments in elephant cell culture that might get us closer, and there are a lot of very smart people working very hard on solving the problem.** It’s indisputable that research on captive elephants has shed the most light on the disease. It’s also reasonable to suspect that the only animals who have survived the disease are ones in captive settings that received veterinary care.

Conclusion: Everyone loves elephants, this virus sucks

The trunk and tail of it is that EEHV hemorrhagic disease is one of the most serious threats facing the global Asian elephant population and we know frustratingly little about it. The passions ignited by a high profile animal death in a zoo are understandable but probably misplacing blame in this case. I happen know people in both the “liberate the pachyderms” camp, as well as the “hands-on daily care of individual elephants in zoos” group. The thing they share in common is a deep love of these animals and a desire to protect them. The truth is we know so little about EEHV and what to do about it that it might be a long time before we get any good answers. Captivity has its faults, and there is reason to suspect that many of the “charismatic” species such as elephants and orcas that you can currently see in a zoo will not be on display forever, but there are undeniable scientific benefits that can be gained from non-invasively studying these creatures while they’re in our care.

Pictured: DAAAAWWWWW!!!!!

And after that bummer, allow me to present:

*There are three extant elephant species, two of which are commonly housed in zoos: the African bush elephant (Loxodonta africana), and the Asian elephant (Elephas maximus). The third species, the African forest elephant (Loxodonta cyclotis), is extremely rare to nonexistent in captivity. For super-nerd points, there are 185 known species of extinct elephants.

**Many of whom are at the National Elephant Herpesvirus Laboratory run by the Smithsonian’s National Zoo, although there are a lot of zoo veterinarians and researchers working on this problem.



  • Greg Bishop is a veterinarian in Oregon who works mainly with dogs and cats. As a fan of the SBM blog, he sees the enormous amount of bad science and information in the field of veterinary medicine as an opportunity, not a problem! But also a problem.

Posted by Greg Bishop

Greg Bishop is a veterinarian in Oregon who works mainly with dogs and cats. As a fan of the SBM blog, he sees the enormous amount of bad science and information in the field of veterinary medicine as an opportunity, not a problem! But also a problem.