The Mysterious Cause of a Deadly Illness in Sanctuary Chimps Revealed

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In 2005, an elusive illness began to strike chimps at the Tacugama Chimpanzee Sanctuary in Sierra Leone. The disease presented itself differently in the infected chimpanzees and seemed only to affect chimps at this particular sanctuary. While some chimps exhibited signs of a digestive disorder, others had seizures and lack of coordination—and sometimes the chimps showed all of these symptoms, reports Rachel Nuwer for Scientific American. Infected chimps appeared to recover from the disease only to succumb months later, even with medical care, alarming veterinarians at the sanctuary.

Now, over a decade later, researchers have linked the mysterious illness to a new species of bacterium within the genus Sarcina, reports Asher Jones for The Scientist. The study was published this week in Nature Communications.

“It was not subtle—the chimpanzees would stagger and stumble, vomit, and have diarrhea, sometimes they’d go to bed healthy and be dead in the morning,” says Tony Goldberg, a disease ecologist at the University of Wisconsin-Madison, to Ann Gibbons for Science.

The sanctuary's biologists and veterinarians previously concluded that this disease did not infect humans and wasn’t contagious. When living chimps were examined, researchers recognized a pattern of neurological and gastrointestinal symptoms. After chimps died from the disease, researchers observed intestinal damage, reports James Gorman for the New York Times.

Frustrating attempts at pinpointing the cause of the illness—from looking at potential viral diseases to toxic plants that grew near the sanctuary—inspired the Pan African Sanctuary Alliance to collaborate with researchers at the University of Wisconsin-Madison in 2016, reports the New York Times. After a year-long application process for transportation permits, samples of blood, serum, fecal, and tissue from 19 dead chimps and 14 healthy ones arrived in Wisconsin, reports Scientific American.

The eureka moment occurred in 2018 when Leah Owens, a veterinary science Ph.D. candidate in Goldberg’s lab, found an abnormal-looking bacterium in the brain tissue of an infected chimp, reports Science. Using genetic sequencing, the bacterium was seen as a likely culprit after it showed up in 68 percent of samples from infected chimps. It was only after Owens grew the bacteria in a piece of brain tissue that she could see its shape under the microscope and identify it as part of the genus Sarcina, known for its clover-like shape, reports the New York Times. But the bacteria found in the ill chimps, while belonging to the same genus as Sarcina is different.

The Sarcina genus is not fully understood in both humans and animals. One version of the bacteria, S. ventriculi, can cause gastrointestinal distress in humans and usually occurs after surgical procedures, reports the New York Times. After sequencing the bacterium’s genome, researchers saw that while the genome closely resembled the previously known S. ventriculi, this bacterium contained genes that made it more deadly.

“Maybe there’s this range of different Sarcina that look the same but have gained genetic properties that allow them to be more pathogenic that can have repercussions for human and animal health,” Owens tells Scientific American.

The newly discovered bacterium was classified as, S. troglodytae, reports Science. Researchers named the bacterial illness the chimps were experiencing “epizootic neurologic and gastroenteric syndrome,” or ENGS.

While researchers deciphered what caused the chimps' illness, the reasons why it occurred or where it came from remain a mystery. ENGS peaks in March during the dry season when chimps are given more food, and researchers suspect that something within the chimps’ biology or in their sanctuary environment is prompting the disease, reports Science.

Owens and her team are now applying for grants to research what the source of the bacterium might be. Meanwhile, the staff at the Tacugama Chimpanzee Sanctuary are using the findings to treat their infected chimps with antibiotics and antacids, Science reports.

“Before, we were lost, trying to focus on everything, now we know what we have to protect against,” says, general manager at Tacugama and veterinarian, Andrea Pizarro to Scientific American.

Elizabeth Gamillo | | READ MORE

Elizabeth Gamillo is a correspondent for Smithsonian and a science journalist based in Milwaukee, Wisconsin. She has written for Science magazine as their 2018 AAAS Diverse Voices in Science Journalism Intern.