The Very Bad Touch: Will Koalas Be Destroyed by a Bad Case of Chlamydia?

By Mary Carmichael

Australia's koalas are dying in droves of something newspapers are coyly calling a "stress disease." You have to read down to the fourth paragraph of the Associated Press report to find out what said illness is: chlamydia. Yes, the same sexually transmitted disease you heard about in health class is killing off one of nature's cuddliest creatures.

In humans, chlamydia is caused by a nasty bacterium called . The koala version comes in two varieties: and . Unlike their human cousin, these bacteria often attack the respiratory system and the eyes, causing pinkeye and blindness. And like their human cousin, they damage the sex organs, leading to incontinence, scarring, and infertility in females. Sam the Koala, a YouTube sensation, suffered this fate in August—she died during surgery to remove cysts from her bladder and uterus.

Untreated chlamydia can scar the human female reproductive tract, too, rendering it infertile. But the disease almost never kills people. If it did, the hospitals would overflow: it's the most commonly reported bacterial STD in the U.S., and it's on the rise. The CDC estimates that at least 2,291,000 people have it—and that number doesn't include cases from the prison population, the military, and people younger than 14 or older than 39. In 2007, 1.1 million new cases were reported, and the CDC suspects that 1.7 million other people caught chlamydia that year without knowing it—the disease often causes no obvious symptoms, which means many (perhaps most) cases go undiagnosed and untreated.

Why is this silent disease very loudly driving koalas closer to extinction? An infection rate somewhere between 50 and 90 percent is only part of the problem. Wild koalas have been taking hits from all sides in the last decade. Some 80 percent of their natural habitat has been destroyed. They're overcrowded into what's left, so diseases of all kinds are getting passed around like mono in a dorm. Since 2000 they've been fighting a new HIV-like retrovirus that causes leukemia, other cancers, and bone-marrow failure. And although they have few natural predators, they now have to watch out for cats, dogs, cars, and trucks. In this sense, the articles referring to chlamydia as a "stress disease" are right. Koalas that live in plush habitats with little exposure to other diseases can often carry the chlamydia bacterium with no serious symptoms. But it's getting harder to mount much of a fight against it when their immune systems are weakened by so many other woes.

Conservationists are scrambling to save the koalas before the chlamydia epidemic gets worse. Some animals have been moved into less crowded areas; the hope is that lower stress levels will yield a better natural immune response. The disease can also be treated with antibiotics, though that strategy is problematic. Scientists are still trying to figure out which drugs work best in koalas; the ones used in humans may not be appropriate. And any drug will need to be given daily over the course of weeks—a regimen that can only be administered in zoos and conservation centers.

What wild koalas really need is a chlamydia vaccine. Happily, one is in the works. Last year Australian scientists took a formulation that looked promising in mice and injected it into 18 uninfected female koalas. The animals' T cells mounted an immune response, suggesting that the vaccine might protect them against future infection. But there's a lot of lab research left to do before scientists head for the trees with syringes in hand.

Still, by searching for a koala vaccine, scientists are likely to lay some groundwork for a human one. Here, then, is an example of that rarest type of animal research—one that's good for people and, we can hope, good for the animals, too.

CARMICHAEL is a senior writer for NEWSWEEK.