Roundworm infection increases female fertility

Surprising finding among Bolivian ethnic group

Scientists have – slightly improbably – discovered that infection by the roundworm Ascaris lumbricoides increases fertility in women belonging to Bolivia's Tsimane ethnic group.

In their abstract, published in Science, the researchers explain that they studied "nine years of longitudinal data from 986 Bolivian forager-horticulturalists, experiencing natural fertility and 70% helminth [parastic worm] prevalence."

They found that infection with different worm species affected fertility in completely distinct ways. While the roundworm Ascaris lumbricoides "is associated with earlier first births and shortened interbirth intervals," hookworms "delayed first pregnancy and extended interbirth intervals."

The scientists believe these effects are caused by changes to the immune system. Team member professor Aaron Blackwell, of the University of California, explained to the BBC that "women's immune systems naturally changed during pregnancy so they did not reject the foetus."

He continued: "We think the effects we see are probably due to these infections altering women's immune systems, such that they become more or less friendly towards a pregnancy."

The upshot of all this for Tsimane women is that those who have Ascaris lumbricoides aboard can expect to have an average of two more children. Half of those blighted with hookworm, however, will not have fallen pregnant by the age of around 26 to 28 – something that has "a huge effect on life," according to parasitic worm expert professor Rick Maizels.

Naturally, the Ascaris lumbricoides effect might offer the possibility of a future fertility treatment. Fertility boffin professor Allan Pacey, from Sheffield University, told the Beeb (BBC): "It is very surprising and intriguing to find that infection with this particular species of roundworm actually enhances fertility.

"Whilst I wouldn't want to suggest that women try and become infected with roundworms as a way of increasing their fertility, further studies of the immunology of women who do have the parasite could ultimately lead to new and novel fertility enhancing drugs."

Ascaris lumbricoides is the "largest intestinal nematode of man," with females reaching up to 40cm in length. It's estimated that well over a billion people worldwide are infected, around three-quarters of them in Asia.

While it's comparatively uncommon in South America, "some populations have infection rates as high as 95 per cent."

The worst effect of heavy infection is intestinal obstruction, which can prove fatal. ®

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