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Can we make gorillas sick? Study reveals much about them — and us.

In Africa, researchers gather saliva samples from partially chewed leaves that the primates discard after eating. They then extract the DNA and analyze it for viruses.

A mountain gorilla in the Pablo group feeds during a routine health check by Gorilla Doctors in the Volcanoes National Park in Rwanda. (Skyler Bishop/Gorilla Doctors)

African mountain gorillas are picky eaters. They strip off the most delicious bits of plants and spit out the rest, leaving a trail of partially chewed leaves drenched in saliva.

It turns out these slimy specimens can tell scientists a lot about not only the health of gorillas but also diseases that may afflict humans as well — and the interaction between the two.

“Humans and gorillas share more than 98 percent of their DNA, meaning that much of our physiology and the way that we respond to pathogens are similar,” says Tierra Smiley Evans, research faculty member at the One Health Institute of the UC Davis School of Veterinary Medicine. “If we can collect information about viruses that infect gorillas, we can learn about similar viruses that infect humans.”

More than 1,000 wild endangered gorillas live in protected areas of Rwanda, Uganda and the Democratic Republic of Congo. They are a big tourist draw. That has made them susceptible to diseases these visitors may carry, says Evans, chief veterinary and scientific officer for Gorilla Doctors, a nonprofit group that helps care for the primates.

Because mountain gorillas are so genetically similar to humans, they can easily exchange infections with them, including respiratory ailments that can be mild in people but potentially more serious in gorillas, Evans says. For this reason, tourists and personnel who visit the parks must wear masks when they are near gorillas and must keep a distance of nearly 33 feet from the animals.

These precautions already have made a difference. In Rwanda’s Volcanoes National Park, for example, respiratory infections fell from an annual average of 5.4 outbreaks in gorilla family groups in the five years before the pandemic to 1.6 outbreaks after a pandemic was declared, experts say. “The decline documented during this time period correlated with fewer people coming into close proximity with the gorillas since the start of the pandemic, and with the use of face masks and safe distancing,” Evans says.

“The increase and interest in eco-tourism brings people from all over the world close to regular contact with the gorillas,” says Benard Ssebide, head veterinarian in Uganda for Gorilla Doctors. “Mountain gorillas are actually vulnerable to many human diseases such as influenza, covid, Ebola, tuberculosis, measles, polio, among others. The main threat we see now are infectious diseases ranging from respiratory illnesses to intestinal parasitic infections.”

Evans says that collecting food scraps soaked in saliva from an endangered species offers a noninvasive way of monitoring the gorillas for infectious diseases.

It “can be an important way to understand what may be happening in a population that you would not typically have access to through more traditional, labor intensive sample collection methods requiring darting, trapping, and anesthetizing individuals,” she says.

Gorilla Doctors also works with the Karen C. Drayer Wildlife Health Center at Davis, Calif., to monitor the gorillas’ health and treat them if they become sick or injured, as well as with the National Institutes of Health’s Center for Research in Emerging Infectious Diseases (CREID) network, a group of international research centers in regions where emerging — and re-emerging — infectious-disease outbreaks will probably occur. One goal is to better understand what happens to emerging viruses as a result of climate change, especially the transition from forest to urban landscapes.

“We know that climate change will continue to impact the distribution and epidemic patterns of infectious diseases,” Evans says. “Primates are often the first to be impacted by infectious diseases moving into new locations and can be indicators of impending outbreaks of disease in humans. A good example of this is yellow fever virus, where acute outbreaks and die-offs in primate populations in South America often are the first indicator of an impending human outbreak.”

As changing land-use patterns result in more human contact with wildlife, the opportunity for pathogens to spill over from animals to people increases, she says, citing as an example the often-fatal disease of Ebola.

“As we’ve seen in western lowland gorillas in other parts of Africa, great apes are impacted by Ebola virus in very similar ways to humans,” Evans says. “Understanding what is circulated in great apes that are overlapping with other wildlife reservoir populations, such as bats, can provide very important information for both great ape and human health.”

Both humans and gorillas can become infected with Ebola through contact with the same wildlife reservoir host and become sick and die, but this research is ongoing.

She stresses, however, that there has never been a case of Ebola among mountain gorillas — “an important point that must be made clear” — although outbreaks have occurred among western gorillas, a different species. But “we have shown that human communities surrounding mountain gorilla habitat have antibodies against Ebola virus, showing that the virus is in the region and, therefore, we must remain vigilant,” Evans adds.

The researchers conceived the idea in 2012 of gathering saliva samples in the field to extract gorilla DNA and analyze it for viruses. The project involved scouting the animals with the help of trackers, watching and waiting for them to finish eating and leave, and retrieving the discarded wild celery stalks. The researcher would record each gorilla’s name, matching it to its leftovers. Veterinarians know each gorilla by name and “nose print” — distinct wrinkles above their noses, which are similar to human fingerprints.

Initially the scientists wanted to determine whether any wild gorillas were infected with herpes simplex 1 virus, which causes cold sores in people. In adult gorillas, “it likely causes fever, malaise and oral lesions as we’ve seen in the few eastern gorillas that have been infected in captivity,” Evans says. “What we don’t know is what the impact could be on [gorilla] infants or immune compromised individuals, and what impacts this could have at a population level.”

“Any new virus in a new species is cause for concern,” Ssebide says. “Once a latent virus — that is, a virus that infects its host for life but is then shed intermittently, such as a herpes virus — enters a population, there is very little you can do to control it.”

To their relief, they did not find herpes simplex 1. But their research, detailed in a recently published study, did detect other types of herpes viruses, although ones specific to gorillas and not passed from humans. These include a gorilla version of cytomegalovirus, which in humans can harm a developing fetus; lymphocryptovirus, similar to human Epstein-Barr virus, which causes mononucleosis; and a rhadinovirus — which Evans described as “a novel finding” — that is much like the virus associated with human kaposi sarcoma, a cancer of cells lining lymphatic or blood vessels.

Their effects in gorillas are unknown. It is also unknown — although probably unlikely — whether they could jump into humans. The scientists say they hope to conduct further studies on them, Evans says. The finding that the animals did not have human herpes simplex 1 suggests that procedures in place to protect the animals from human exposure have been effective, although it would be risky to become complacent, as cross-species transmission is always possible, the scientists say.

“This [possibility] is very worrisome because more dangerous viruses can cross as well,” says Ssebide, referring to transmission between species. “Whenever a virus crosses the species barrier into a new species, you can never know what effect it would have.” He cited the coronavirus, which causes covid-19 and is widely thought to have originated in animals.

“You can see the effect [the coronavirus] caused once it crossed to humans,” he says. “Because we don’t know the impact that a human herpes virus could have on the wild mountain gorilla population, we need to remain vigilant.”

Evans began working with Gorilla Doctors while in veterinary school, taking part in a summer research project in Rwanda. “It’s when I started thinking about saliva projects,” she recalls. “At that point, we weren’t looking for viruses and pathogens, just looking to see if we could detect salivary enzymes — trying to figure out if we could get enough saliva to detect something. It really piqued my interest to realize that we could.”

Gorilla personalities, as well as the years-long comedy and drama of their lives, has become a big part of the experience of studying them, the researchers say. They celebrate their joy — the birth of a new baby, for example — and also worry about their health and safety, particularly as it applies to the work they do.

“It’s natural for us — as scientists and doctors — to be concerned about their exposure to some of the pathogens we study,” Evans says. “We worry about the gorillas just as a physician would for their human patients. “Over time, you get to know who they are, especially the really charismatic ones. It’s amazing how quickly you can recognize gorilla faces. Like human faces, their faces — and their movements — are different and easily recognizable.”

Ssebide says he loves seeing silverbacks — adult male gorillas — and adult females today who were infants 20 years ago when he first rescued them from wire snares set by hunters to trap other wildlife.

“Seeing a baby gorilla grow to becoming a silverback and starting its own gorilla group and remembering all the interventions that had to be done on such a particular animal, brings lots of memories,” he says.

Similarly, Evans says she remembers her encounters a decade ago with Kabukojo, a young, inquisitive male gorilla and her favorite who at the time was “a rowdy teenager, with high energy — a showoff.”

“He loved to surprise people,” she says. “You would go up to the group he was in, and you would see the other gorillas, but not him. He was waiting to surprise you, running down the mountain and beating his chest. He enjoyed intimidating people.”

Several years later, he was unrecognizable to her.

“That’s Kabukojo? You’re kidding me,” she says. “He was this full-grown chill silverback, lying on his back taking a nap, with babies crawling all over him. A gentle dad. Those were all of his babies, and he was the head of his group. We’d all gotten older. We’d all changed. As they get older, we get older too.”