Kimberley Hockings works at Cantanhez National Park in Guinea Bissau near the west coast of Africa. She saw four chimpanzees with lesions on their face and hands that were unlike any she’d seen before.
She works as a conservationist and sent the photos to a wildlife veterinarian, Fabian Leendert, who was immediately concerned it was a human disease. Upon testing one of the chimps, who’d recently died, they confirmed it was leprosy.
There have been leprosy cases in captive chimps, mostly in the early 20th century, but never in the wild:
Given our shared DNA, it shouldn’t surprise you that chimps are highly susceptible to human viruses and pandemics.
This is part of why ecotourism is causing a massive decline in great apes. My friend works in ape conservation and it isn’t uncommon, even before coronavirus, for them to wear face masks when working with gorillas and chimps. In many cases, it’s illegal not to.
The risk goes both ways. Humans have likely caught several major diseases from chimps throughout history. The leading theory on the origins of HIV is that it transferred over from chimps in the 1920s in Subsaharan Africa, originally as the Simian Immunodeficiency Virus (SIV). It entered humans after locals ate chimps and later mutated into HIV.
When a disease transfers from human to chimp, it can easily incubate and mutate into something we don’t have treatments for. It’s already happened with a strain of antibiotic-resistant staph infections which bounced back from gorillas to humans.
This is called zoonotic disease (animal to human). These risks are particularly pronounced in wet markets, where humans are in close contact with food animals. Recent discoveries of a severe acute respiratory syndrome, a coronavirus-like virus, were found in horseshoe bats.
Do not ever feed a chimp in the wild, or approach them in any manner. Getting so much as scratched by a wild chimp can be extremely dangerous. They carry diseases that can ravage your body. They have the ability to maul you. Leprosy is the least of your concerns.
There’s significant debate on how the chimps caught leprosy. Some scientists believe this strand came from a non-human species. Though human transfer is very possible, the chimps are isolated from human contact within a nature preserve. The origin may remain unknown, indefinitely.
But what does leprosy mean for these chimps?
There are a handful of possible outcomes.
Western Chimpanzees are the most sophisticated sub-species chimps, demonstrating the use of tools, including spear-like branches to hunt other small primates. Many biologists argue they have entered an early stone age.
They are so smart that if you introduce one chimp who has an advanced hunting technique to a group, the rest will abandon their old method and copy his or her version.
Importantly, these chimps have also demonstrated ousting behavior. When they see or smell a sick member of the group, they will threaten and chase the member away. It could save the species from the ravages of leprosy.
What happens next?
The good news is that chimps tend to isolate themselves within groups. They don’t converge at music festivals or political rallies.
There are still 200,000 leprosy cases a year in humans (most are easily treated). And perhaps it is grimly symbolic that just as chimpanzees appear to enter a minor stone age, they are afflicted with a disease that has haunted humans since the origins of civilization. Leprosy led to social ostracization from its earliest inception and could save chimps from further infection if they mimic human behavior.
The problem? Ousted chimps often seek new communities, where they could spread the disease. Additionally, chimp communities occasionally engage in hand-to-hand warfare with each other which would cause a major outbreak.
The situation is complicated
Human intervention is a possible solution. Western chimpanzees are critically endangered and need protection, with threats from poaching and land encroachment continuing to press inwards.
Wildlife vets can typically put the anti-biotics in bananas and other types of food (this treats leprosy) but it would be challenging to implement at scale with entire communities, as anti-biotics require consistent treatment. Even further, chimps can go symptomless for months so it would be difficult to know which ones to treat.
It’s a bad situation — not dire — but bad. It’s never good when a news story resembles the plot of a Michael Creighton novel.