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Scientists discover a pathogen (more dangerous than coronavirus) killing endangered chimpanzees

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Kuldeep Singh
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The disease has a 100% mortality rate in Sierra Leone primates. The researchers worry the bacteria could pass to humans

On a Friday night in mid-January, Jackson, a five-year-old chimpanzee living at the Tacugama Chimpanzee Sanctuary in Sierra Leone, alarmed his keepers by ignoring his dinner. By Saturday, he was lethargic and had seizures. Jackson has since improved – he is eating and appears stable, despite persistent diarrhea – but his survival is by no means guaranteed. 

“The disease is a lot like: you see ups and downs. One day they are very good, the next, they are very bad” says veterinarian Andrea Pizarro, general manager of Tacugama.

Jackson has Epizootic Neurological and Gastroenteric Syndrome (ENGS), a mysterious disease that has killed 59 of the 60 Tacugama chimpanzees since 2005. After struggling to identify the cause of the disease for years, scientists and veterinarians finally have a possible culprit: a newly discovered species of Sarcina, a type of bacteria commonly found in the environment that is occasionally associated with gastrointestinal illnesses in humans. 

As the researchers reported in Nature Communications, the finding suggests that some species of Sarcina may in fact be very virulent but, so far, have not been recognized.

“Perhaps there is this variety of different Sarcina that look the same but have acquired genetic properties that allow them to be more pathogenic,” says study lead author Leah Owens, a veterinarian and PhD candidate at the University of Wisconsin-Madison. “That can have repercussions for human and animal health.”

Danger without disease

Tacugama is the only sanctuary in Sierra Leone for western chimpanzees, a critically endangered subspecies whose range once spanned West Africa but is now limited to eight countries. Located eight miles southeast of Freetown, on the edge of the Western Area National Park, the award-winning and accredited sanctuary also conducts environmental education, ecotourism and community conservation projects. Ninety-nine chimpanzees permanently reside in Tacugama today. Many of them were rescued as babies from the illegal wildlife trade.

Chimpanzees in Tacugama began contracting ENGS in 2005, although it took veterinarians years to realize that the animals they were losing had died from a common cause. The syndrome manifests itself differently in each chimpanzee; some show neurological signs, such as co-ordination disorders and seizures, and while others suffer from gastrointestinal upset or both. Even some animals seem to recover from ENGS, only to succumb weeks or months later, and others simply fall dead without warning signs.

Tacugama vets chased down several red herrings, including a virus that causes neurological problems, so they vaccinated all the chimps at the sanctuary. They also carried out a thorough removal of a poisonous plant found in the chimpanzee enclosure. But cases kept coming. In 2016, the Pan African Sanctuary Alliance, a coordinating organization for primate sanctuaries on the continent, approached epidemiologist Tony Goldberg, Owens’ advisor at the University of Wisconsin-Madison. Goldberg was immediately intrigued.

“This is an unknown infectious disease that poses a serious health and survival risk to an endangered species, which turns out to be our closest relative,” he warned.

“It took two and a half years to get permission to export the chimpanzee samples to the United States (mostly because the Ebola outbreak was underway at the time) and to work out the logistics to ship them safely,” explains the specialist Rachel Nuwer in her research. In the end, the Wisconsin researchers obtained tissue, blood, serum, and stool samples from 19 chimpanzees that had died of the syndrome and 14 healthy ones.

Owens, Goldberg and their colleagues performed a comprehensive analysis of the samples to characterize all viruses, bacteria and parasites present. Several of the samples “had an incredible amount of readings for this bacterium, like 90 percent or more,” explained the specialist. Diagnostic sequencing and statistical analyzes confirmed that the bacteria was not present in any of the healthy chimpanzees, suggesting a link to ENGS.

By appearance, the microbe appeared to be Sarcina ventriculi, which looks a bit like a four-leaf clover and is ubiquitous in water and soil around the world. The species was first discovered in a 19th-century human patient who developed vomiting, but then largely disappeared from the scientific literature related to the disease. Genome sequencing revealed, however, that the team had not found S. ventriculi, but a completely unknown Sarcina species, which they named Sarcina troglodytae. 

“In all the decades this bacterium was known to exist, the medical community never realized that what they had been calling S. ventriculi could actually be a group of related bacteria,” Goldberg said.

Chimpanzees are not the only primates to have recently contracted Sarcina. Since 2010, there has been a growing number of human patients, often in patients who have undergone bariatric surgery, primarily in the United States. Doctors have primarily diagnosed Sarcina ventriculi, based on appearance rather than genetics, however, it makes it impossible to tell which species are actually infecting people. 

But some human cases of Sarcina infection, including a fatal one, have presented with effects eerily, similar to those seen in chimpanzees, Owens suggests.

“The question is: Is this a new emerging pathogen that is different from Sarcina we think we know? Or is there something in the host that is changing, that allows them to get infected and get sick from this?” asks the expert.

New attackers

Owens and Goldberg hypothesize that there is a diversity of unrecognized Sarcina species, some of which are benign and others are opportunistic pathogens. The challenge now will be to untangle those different species, determine how virulent species are causing disease, and discover what environmental triggers inside or outside the body predispose certain primates to infection. Answering these questions could not only help protect an endangered species but also people. As Owens says, “Chimpanzees are basically us, genetically.”

The findings also raise questions – and hopes – about how best to treat Tacugama’s primate residents for ENGS. “This study represents a starting point to guide future research on the unfortunate probability of future cases and offers ideas for tailoring treatment interventions,” says Livia Patrono, veterinarian and postdoctoral researcher in primate infectious diseases at the Robert Koch Institute in Berlin.

Tacugama vets are already changing their approach to treatment. Jackson, unlike previously infected chimpanzees, receives probiotics and a special diet, in addition to specific antibiotics. 

“Before we are lost, trying to concentrate on everything,” concludes Pizarro

“Now we know what we have to protect ourselves against ”.

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