The giant roundworm looks a lot like an earthworm. It's pinkish, up to a foot long and can live for two years.
Right now, the worms are living comfortably in the small intestines of about a billion people worldwide. They're the most common worm infection in humans. The worms typically don't cause many symptoms, aside from diarrhea and abdominal pain. But according to a study published Thursday in the journal Science, they may be capable of affecting a woman's birth rate.
A group of anthropologists collected data on about 1,000 women living in the Bolivian Amazon in a community of subsistence farmers.
It was known that parasites can weaken their human host, making it less likely for an infected woman to conceive. But this study found the giant roundworm to be linked with the opposite effect.
Women with the giant roundworm had a shorter interval between births, says Aaron Blackwell, a biological anthropologist at the University of California Santa Barbara and the paper's lead author. Those women also started giving birth at an earlier age. Over a lifetime, the group writes, a woman chronically infected with the giant roundworm "would expect to have two children more than a woman who was never infected."
The women in the study had nine children on average. Fewer than 5 percent of them used birth control.
The investigation sprouted from an offhand comment that Melanie Martin, also an anthropologist and author on the paper, made a few years ago. Martin had been in Bolivia doing fieldwork for a few months when she and her husband decided to try for kids. The 36-year-old expected it to take a while.
"I think I was pregnant in a week," she says. She joked with a colleague that maybe the "constant deluge" of parasites had a hand in it. (It didn't, because she didn't have worms.)
But the joke turned into a serious study. At the time the group was focused on the "hygiene hypothesis." The gist of the hypothesis is that humans evolved to have parasites in their intestines, and in some cases parasites can actually be beneficial by dampening the host's immune system. Some researchers argue that the high rate of allergies and autoimmune disorders in modern populations stems from our lack of squirmy guests.
As Martin explains, it would be hard on both human and worm if the person's immune system was constantly fighting parasites living long-term in the gut. The person would be sick all the time; the worm would be constantly barraged by an army of immune cells. Instead, the body eventually learns to tolerate the things. "It's kind of like a compromise between the host and the parasite," she says.
That compromise also happens when a woman becomes pregnant with a fetus — which, like a bloodsucking worm, is also a foreign body that the woman's immune system wants to attack. In fact, women's bodies probably spontaneously abort most pregnancies. It's still not completely understood why a mother's body doesn't reject every fetus.
Blackwell and Martin thought that maybe the parasite would dampen the mom's immune system in a way that would make it easier for her body to tolerate both worm and baby.
"This is a sensible hypothesis," Marijke Faas wrote in an email. Faas studies the interaction between hormones and the immune system at the University Medical Center in Groningen, Netherlands. She was not involved in the study.
The finding, Faas says, is "unexpected." But she sounds a cautionary note. While the data does show that the giant roundworm is linked to higher birth rates, she points out, it does not prove that the parasite was responsible. The researchers controlled for other factors that could play a role, like education, village location and other illnesses. But it would require more work to find out if the relationship between worms and babies is more than coincidence.
The group also investigated the giant roundworm's occasional neighbor, the hookworm, a creamy white, half-inch long worm outfitted with hooks and teeth to help latch onto the host's intestine and suck its blood. Women with repeated hookworm infections experienced the opposite effect as those infected with roundworms — they started giving birth at a later age and were overall less likely to get pregnant. Blackwell and Martin found that the women who had hookworms had lower body mass index and lower hemoglobin, indicating the bloodsucker could be taking a toll on its host. The hookworm's impact on a woman's body may make it less likely that she will conceive, Martin explains.
The message is not that there's a parasite that can solve fertility concerns, promoting some pregnancies and halting others. It's that there is a very complicated host-parasite interaction going on, Martin says, "and one byproduct of that is maybe that you are a little bit more or less likely to conceive at certain times depending on your infected state."
One thing is certain: Parasites are not the latest fertility treatment or alternative birth control.
"No. I would definitely not recommend women who want to conceive run out and stomp through latrines and swallow a bunch of dirty water," says Martin.
The group will continue the study, and in a follow-up investigation, hopes to directly measure the effects of parasitic worms on the human immune system.