A study of 986 Bolivian women found that on average, a lifetime infection  with a type of roundworm named Ascarius lumbricoides led to an extra two children in the family. Their paper, published in the journal Science, suggests that the worm is altering the host’s immune system, making it easier to become pregnant — in effect, the parasite increases female fertility. The researchers hope this discovery will lead to “novel fertility enhancing drugs.”

“The effects are unexpectedly large,” said Prof Aaron Blackwell, one of the researchers for the BBC News website.

For the Tsimane population in Bolivia, the average family has nine children, and about 70% of the population lives with a parasitic worm infection. The paper suggests that an infected woman’s immune system changes during pregnancy, making their body less likely to reject the fetus. On average, these women had two more children during their lifetime.

“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,” said Prof Blackwell.

Blackwell added that while using the worms as a fertility treatment was an “intriguing possibility,” there is much more work to be done before “we would recommend anyone try this.” But it’s not all roses with parasitic worms. The nine year long study also found that while Ascaris lumbricoides increases fertility in infected women, hookworms had the exact opposite effect, with families showing an average of three less children.

Prof. Rick Maizels, specialized in the workings between parasitic worms and the immune system said: “It’s horrifying that the hookworm effects are so profound, half of women by 26 or 28 have yet to fall pregnant and that’s a huge effect on life.”

Prof Maizels suggested the hookworm may also be causing anaemia and leading to infertility that way.

Bacteria and viruses try to overwhelm the immune system by multiplying rapidly. But parasites have a different strategy, growing slowly and suppressing the immune system, which is why they make vaccines less effective in the host and lighten allergies. This also makes the mother’s drowsier immune system less likely to attack fetal tissue, increasing fertility.

However, the mechanism is yet to be fully understood. Prof Allan Pacey reported that drugs had been administered to the women in an attempt to alter their immune system to boost IVF, but without success.

“It is very surprising and intriguing to find that infection with this particular species of roundworm actually enhances fertility,” said Prof Allan Pacey, a fertility scientist at the University of Sheffield.

He added: “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.”

Currently, one third of global population is believed to be infected with similar parasites.

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