A mysterious disease is killing chimps in West Africa. Scientists may now know the culprit

Posted in 26/02/2021

Young chimpanzees are put in small enclosures to minimize exposure to a lethal disease at the Tacugama Chimpanzee Sanctuary in Sierra Leone. LEAH OWENS/Science Magazine

By Ann Gibbons (Science magazine)

Disease ecologist Tony Goldberg was stunned in 2016 when he learned that a mysterious infection was swiftly killing chimpanzees at a lush sanctuary in Sierra Leone’s rainforest. “It was not subtle—the chimpanzees would stagger and stumble, vomit, and have diarrhea,” recalls Goldberg of the University of Wisconsin, Madison. “Sometimes they’d go to bed healthy and be dead in the morning.”

Even when veterinarians gave ill chimps antibiotics and fluids, wrapped them in warm blankets, and isolated them in smaller enclosures to try to prevent the spread of infection, they died. At least 53 perished at the Tacugama Chimpanzee Sanctuary between 2005 and 2018.

The refuge is home to nearly 100 chimps rescued from illegal trade, hunting, or abandonment as pets. “It was really upsetting for the staff because there was no end in sight,” says biologist Gregg Tully, executive director of the Pan African Sanctuary Alliance. He sought Goldberg’s help to identify the disease, which is 100% fatal.

Now, after studying tissue samples and DNA from chimpanzees at the sanctuary, Goldberg and his colleagues have identified the likely culprit. In Nature Communications today, they report that a new species of clover-shaped bacterium infected tissue samples from 13 chimps that died, but not samples from 14 healthy chimps.

The mysterious gastrointestinal and neurological disease has not infected veterinarians or other humans. Its closest relative is Sarcina ventriculi, however, a rare cause of gastrointestinal disease that does infect people, as well as cattle, cats, and horses. Although researchers worry about any new disease that might jump between apes and humans, their biggest concern is that it will spread to chimpanzees in other sanctuaries and the wild. “Wildlife in sanctuaries are always the most vulnerable to pathogens that are transmissible by air,” says veterinary epidemiologist Sharon Deem of the St. Louis Zoo, not part of the team.

The big break came in 2018, when Goldberg’s graduate student Leah Owens spotted a strange-looking bacterium in the brain tissue of one of the deceased chimpanzees. “Late at night, I was looking through the microscope and I saw this really weird-looking cubic structure,” she recalls. The team had spent several years screening tissues, feces, and blood samples from the sanctuary chimps for pathogens, finding no smoking gun. Owens realized the bacteria on her slide looked like the clover-shaped Sarcina—a finding confirmed by pathologists.

The researchers then sequenced the genome from the bacteria in the sample, finding it most closely matched that of S. ventriculi. Yet it was distinct enough to classify it as a new species, which they propose to call Sarcina troglodytae, after the species of chimpanzee it infects—Pan troglodytes.

Further studies of the DNA from the new species of bacterium show it has genes that make it more virulent than S. ventriculi. The team also wonders whether cases in other animal species that were classified as S. ventriculi might belong to this new species—or other unidentified types of Sarcina.

Owens is applying for grants to try to identify the source of the bacterium by testing samples of water, air, food, and vegetation she and Goldberg gathered at the sanctuary in 2019. One possibility is that the bacterium is ubiquitous, but something in the environment at the sanctuary or in the apes’ physiology is triggering disease. Most cases occur every March during the hot, dry season, when the animals are provisioned with more food.

Veterinarians at the Tacugama sanctuary are already using the new findings: They’re treating a sick chimp with antacids, anticonvulsive, and antibiotics—similar to the treatment in humans—in hopes of saving its life. In the meantime, other researchers hope to test chimpanzees in other sanctuaries for the infection, as well.