Can a parasite be good for you? Although some types of infection seem to lower a woman’s fertility, there’s evidence that women infected with roundworm have more children.

This seems to be the case for the Tsimane people, who live in the Bolivian Amazon. They use little contraception, and the average woman gives birth to nine children. Parasitic worms called helminths are very common, infecting 70 per cent of the population.

To see how these parasitic worms might affect fertility, Aaron Blackwell of the University of California, Santa Barbara, and his colleagues collected data from 984 women over nine years. Their project was inspired by a team member who had begun trying to conceive with her husband while conducting fieldwork in Bolivia. When they were quickly successful, she wondered if her helminth infection may have had an effect.


The team found that women infected with the most common helminth, a type of hookworm, tended to have their first child later, and to have longer gaps between each child.

But women infected with roundworm, a different kind of helminth, seemed to experience the opposite, with an earlier first pregnancy and shorter gaps between children. Projected over a lifetime, women infected with roundworm would be expected to have two more children than uninfected women, and those with hookworm three fewer.

“What surprised us was the size of the effect,” says Blackwell. “We hadn’t predicted we’d see such large or robust changes in fertility.” They were careful to include other possible explanations in their analysis, before accepting that the fertility effects really were linked to the parasites.

Immune interaction

The team does not yet know what is responsible for the effect, but it could be to do with the women’s immune systems. Hookworm infections seem to provoke a mix of two different immune cells: type 1 T cells, which instruct other cells to directly attack pathogens, and type 2 T cells, which tell cells to produce antibodies. But roundworms seem to shift the balance of the immune system towards producing more type 2 T cells.

A similar shift happens in pregnant women, perhaps to avoid type 1 T cells from harming a developing embryo. If this is the case, immune reactions might prevent successful pregnancies in women who have hookworm, whereas the immunological shift provoked by roundworm may make embryos more likely to survive.

Together, roundworm and hookworm may affect as many as 800 million people worldwide, so could be having large effects on global demography, says Blackwell.

But parasitic infections can also cause life-threatening complications during pregnancy, says Peter Hotez of Baylor College of Medicine in Houston, Texas. “It is not often appreciated that helminth infections are the most common afflictions of girls and women living in poverty,” he says.

Journal reference: Science, DOI: 10.1126/science.aac7902

Image credit (top): Michael Gurven

Image information (middle): A hookworm, which may reduce fertility in women (credit: Thierry Berrod/Mona Lisa Production/Science Photo Library)

Image information (bottom): A roundworm, which may increase fertility in women (credit: Eye of Science/Science Photo Library)

Read more: “Parasitic worms: just what the doctor ordered?“