The Healthy Human Gets the Worm

Image Gallery David Mack / Photo Researchers, Inc.

Back in the Stone Age, humans had to put up with all sorts of creepy crawlies. Parasites ran amok in people’s innards, freeloading on nutrient supplies. The parasites took a toll, but over the millennia, those that killed off their meal ticket too quickly didn’t make it. The survivors of this evolutionary shakeout include parasitic roundworms and flatworms, hitchhikers that allow their human hosts to live.

While this scenario might appear to be win-lose, with humans the clear losers, research now suggests that may not be the whole story. In their drive to make humans hospitable hosts, parasites have developed the ability to suppress inflammation aimed against them. And this, it turns out, isn’t necessarily a bad thing.

“They have evolutionarily adapted to this long-standing inter­action with their hosts—that’s us—and developed strategies to help the host dampen its immune response,” says Helmut Haas, an immunologist at the Research Center Borstel in Germany.

These strategies are not subtle. But humans have survived the effects and even adapted well to them: A toned-down immunity is, perhaps, the norm. A sober immune system might still defend against enemies while not overreacting to everyday substances in the environment, or otherwise going awry. Suddenly those prehistoric times don’t sound so bad—no Crohn’s disease, no multiple sclerosis, no asthma.

In a stroke of medical inspiration as bold as it is counterintuitive, scientists are now testing this theory by treating patients with live microscopic eggs or larvae of parasitic worms designed to quell these very afflictions. Several clinical research trials are under way and more are planned. Whether promising early results will lead to treatments for these known or suspected autoimmune conditions—and extend to allergies, type 1 diabetes, and other cases of immune revolt—remains to be seen.

Parasitic worms, or helminths, elicit a visceral response from people—in every sense. Worms don’t make for polite dinner conversation and, in industrialized countries, are considered a relic of days thankfully in the past. The notion that hel­minths are bad guys has been drummed into children of Western nations, as well as their doctors, for several generations. (The Latin name for one hookworm is Necator americanus, meaning “American killer.”)

But helminths have thrived in mammals for millions of years, fine-tuning the parasite act along the way. “These parasites do some harm to the host,” says immunophysiologist Derek McKay of the University of Calgary in Canada. “But if I’m [a parasite] inside a host, at the very least I want the host to reproduce and make more host babies.”