How TB Jumps From Humans to Wildlife -- Vet Seeks Clues

Helen Fields, National Geographic magazine 20 Feb 09;

In ordinary circumstances, Kathleen Alexander would gladly have let her two kids run around the front yard on their own. But during her stint as senior wildlife veterinary officer for Botswana's Department of Wildlife and National Parks from 1995 to 2001, things were not exactly normal. Their front yard was Chobe National Park, a 4,000-square-mile (10,360 square-kilometer) park in northern Botswana with lots of free-ranging wildlife. Alexander always went outside with her children, so they wouldn't get "smushed by an elephant."

But one sunny day in June 2000, she encountered a different problem: two banded mongooses, so thin their ribs stuck out, wandering around the sand pit where the children liked to play. These groundhog-sized animals are common through sub-Saharan Africa, but they run away from humans. Alarmingly, these mongooses weren't afraid of her. "It was clear they were sick," she recalled.

Alexander trapped one of the animals and tested it. Her tests revealed it was sick with tuberculosis—the human version. For the first time, free-range wild animals were confirmed to have contracted a human disease.

(For more on non-human strains of tuberculosis read "Lions in South Africa Pressured by TB Outbreak, Hunters.")

It's well known that diseases can—and do—move from animals to people. Avian influenza, which comes from birds, was first confirmed in humans in 1997. But the opposite had never been proven for wild animals, although animals in captivity were sometimes known to get human illnesses.

Tracking TB in Mongooses

Tuberculosis (TB), a highly contagious disease that spreads when an infected person coughs, sneezes, or spits, is a serious problem across southern Africa. Most people in Botswana are infected by the time they are adults. Tuberculosis and HIV often go together; many people with AIDS actually die of TB. The towns of Kasane and Kazungula, next to Chobe National Park, are no exception. Those towns are also infused with wildlife. Warthogs wander the streets and mongooses are common.

Alexander believes mongooses probably pick up the bacteria that cause tuberculosis by nosing around human waste. They like to investigate possible food sources by sticking their noses straight into garbage piles, septic tanks, and sputum. Alexander thinks bacteria get into tiny cuts on their noses, then spread through their bodies. Unlike humans, who can be infected with tuberculosis for years, mongooses appear to sicken and die immediately.

Since that first discovery in 2000, Alexander, who now teaches at Virginia Tech, has been tracking tuberculosis in Chobe National Park's mongooses with support from the National Geographic Society's Committee for Research and Exploration. She hopes to learn more about how the mongooses get infected with tuberculosis and how they fit into the park ecosystem. (National Geographic News is owned by the National Geographic Society.)

Most of the mongooses in the park now sport GPS collars that record their movements. "Following them on foot is just not possible because of all the elephants and lions," Alexander said. She hopes to figure out what they do all day and who their major predators are. Maybe domestic dogs hang out with mongooses in the wild, which would mean they could carry deadly bacteria back to their owners. Or perhaps scavengers like hyenas are getting tuberculosis from mongoose carcasses.

In the meantime, tuberculosis outbreaks among mongooses continue. Alexander has documented five since 2000, and dozens of animals have died. One troop that hangs around a tourist lodge had more than 20 healthy animals eight years ago; after a 2008 outbreak, only seven remain. In 1996, scabies, a parasitic skin disease, was found in rare mountain gorillas that live along the border of the Democratic Republic of the Congo and Rwanda. The gorillas are a popular ecotourism attraction and get a lot of visits from researchers and rangers, too. Veterinarians who studied the outbreak concluded that the gorillas probably caught the parasites from humans (although they couldn't rule out the possibility that other animals were the source). One baby gorilla died. A more deadly human disease could wipe out an entire family of gorillas.

Animal diseases in humans have been well documented. Avian flu is a constant worry. Severe acute respiratory syndrome, or SARS, which in 2003 made people around the world flinch at every cough, probably originated in bats. HIV started as a primate virus and, according to the most commonly accepted theory, probably jumped to hunters when they killed, butchered, and ate infected chimpanzees.

It makes perfect sense that diseases would also go the other way, said University of Georgia epidemiologist David Stallknecht: "Humans are really putting a lot of infectious material on the ground, so it's just a no-brainer" that wild animals would sometimes get human illnesses. And not just in Africa—it could happen anywhere animals and people come into contact.

So why had it never been confirmed before Alexander encountered two sick mongooses in Botswana? Probably, Stallknecht said, because no one was looking.

In fact, there are plenty of examples of animals in captivity getting diseases from the humans they spend their days with. Zookeepers who work with primates know to stay home—or wear a face mask—when they have a cold, so their charges don't catch the sniffles.

Contagious diseases are a problem with less closely related animals, too. In 1996, three circus elephants from an Illinois farm died of tuberculosis; the bacteria that infected them was identical to that in the sputum of one of their human handlers who had the disease. Now elephant farmers in the U.S. are required to test the animals every year, checking for tuberculosis bacteria with a procedure called a "trunk wash."

Could Risks Boomerang Back to Humans?

As humans keep moving into wildlife territory for tourism or because of population pressures, they'll probably bring more disease to wild animals, warns Kristine Smith, a wildlife veterinarian who works on global health at the Wildlife Conservation Society. "More and more, some of the last wild places in the planet are heavily bordered by dense communities," she said—in many cases, desperately poor communities with little access to health care, and often no running water or sewers.

Researchers are also concerned that a human disease in wildlife might provide bacteria or viruses with another haven in which to evolve and combine with other pathogens, which could make them more dangerous later. For example, said Alexander, it's possible that tuberculosis could evolve to be more virulent when it enters a new host.

Human disease transmission to wildlife could come back around to hurt humans, too, Smith said. "If they're contracting diseases that we can get, they're going to be spreaders. Someone can give an animal a disease, and someone else can get it."

At this point, no infectious diseases have been shown to jump from humans to wild animals and back again; it may not happen often. Some of the diseases that jump may be relatively harmless to animals, or may not get back to humans at all—but scientists just don't know enough yet about how diseases move around.

Alexander hopes new genetic tests that can detect a long list of pathogens at once will give her and other scientists a an easy way to find human diseases in wildlife and determine their prevalence, leading to a clearer picture. "It seems improbable that every other animal is linked up in terms of pathogen transmission, but we don't give [diseases] to anybody else," she said. She hopes her work will help other wildlife experts and epidemiologists understand how to better protect free-ranging animals from human disease—and ultimately stop them from bringing it into our front yards.

Conservation Concerns

Banded mongooses aren't in danger of going extinct. They live across southern Africa in large numbers. But if a disease can jump from humans to one wild animal, it could do the same with others. A new human disease could be disastrous for an endangered species. That includes a lot of primates. Since they're so closely related to humans, it's not hard for them to get our diseases.