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Parasites don’t have many pluses, but women with certain intestinal worms are more fertile — maybe because of how the invaders affect their immune systems.

The finding came from the Amazonian lowlands of Bolivia. Aaron Blackwell, an anthropologist at the University of California, Santa Barbara, said the idea for the study sprang from an experience one of his coauthors had. When Melanie Martin, a UCSB anthropology graduate student, went into the field with her husband to do research, the couple was able to conceive easily. Martin knew that parasitic worms called helminths can affect the human immune system, and pregnancy is also tied to immune changes.


“She sort of offhandedly was like, maybe I got a worm and that helped me conceive,” Blackwell said. “So then we decided to actually look at the question.”

Researchers gathered data as part of an ongoing effort called the Tsimane Health and Life History Project. The Tsimane, an indigenous group, has been participating in the project since 2002. Between 2004 and 2013, doctors who visited the Tsimane villages gave physical exams and checked their patients’ feces for roundworms and hookworms.

Using data on pregnancies and worm infections for 986 Tsimane women, researchers analyzed the timing and number of pregnancies.


They found that hookworm infections seemed to negatively impact reproduction. When women had hookworm infections, they went longer between pregnancies. But women with roundworms got pregnant sooner and more often, researchers report this week in Science.

The difference might have to do with how parasites alter their hosts’ immune systems. Research has shown that roundworms trigger an anti-inflammatory response, while for hookworms the response is both anti-inflammatory and pro-inflammatory. Pregnancy also causes an anti-inflammatory immune response, helping a mother’s body tolerate her fetus.

“It’s possible that the roundworm evolved to mimic those effects of pregnancy,” Blackwell said. In a way, the parasite might avoid detection by pretending it’s a fetus.

Most Tsimane have on-and-off infections, sometimes with both types of worms at once. But a woman infected with roundworms for her whole life, the study estimated, would have two more births than an uninfected woman. A woman with lifetime hookworms, on the other hand, would have three fewer births.

Researchers didn’t measure the severity of parasite infections, but Blackwell said roundworm infections in this population are usually milder. More intense hookworm infections might tax the body more and contribute to their negative effect on fertility.

Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, who wasn’t involved in the study, said these findings add to research over the last decade showing that infections by parasitic worms matter to women’s reproductive health. But all the other effects have been negative ones, he said. In African women, for example, such infections are an important cause of infertility and pregnancy complications.

If some worms can boost pregnancy odds by posing as a fetus, it would fit with other research showing the possible benefits of worms. Because they can alter the immune system, worms have been used as an experimental treatment for Crohn’s disease, multiple sclerosis, and even allergies and asthma.