IMMUNITY BONUS

A parasitic worm could be making some women more fertile

Tiny worms already have a huge impact on humans. About 800 million people around the world are infected by roundworm or hookworm, with symptoms that vary from negligible to lethal. But it seems there’s another impact: A surprising new study claims that certain worm infections can make women more fertile.

The conclusion comes from a study, published in Science, of Tsimane people in Bolivia. Researchers from the University of California, Santa Barbara were involved in studying other aspects of the tribe’s life, but then accidentally hit upon the fertility hypothesis, and collected data from nearly 1,000 women over nine years.

Both roundworm and hookworm are parasites spread through contaminated food and water. Most often, roundworm causes malnutrition by disrupting how the body absorbs all its nutrients. Hookworm does the same but only for iron and proteins.

The study found that women with roundworm had their first child earlier than normal, and those with hookworm had them later. Also, those with roundworm had a smaller gap between each child and those with hookworm had longer gaps—even after controlling for other factors, such as age, health, and season.

What shocked the researchers was the size of the effect. Projected over a lifetime, Tsimane women with roundworm would have two more children than those never infected, and those with hookworm would have three fewer. (A Tsimane woman averages 10 children.)

So what’s going on? One would expect an infection to make women less fertile; infections trigger the immune system and a woman’s fertility is very sensitive to those sorts of changes in the body. But why should some infections increase fertility? The researchers don’t know, but they have a hypothesis.

After women ovulate, their immune system starts producing fewer type-1 cells (those that directly attack foreign bodies such as viruses) and more type-2 cells (those that produce antibodies rather than attacking directly). This is thought to happen so that the body’s immune system won’t attack the embryo as a foreign body.

It’s been known for quite some time a roundworm infection triggers the immune system to respond in a similar way—by producing fewer type-1 cells and more type-2 cells. A hookworm infection, on the other hand, provokes the body to produce both type-1 and type-2 cells. So it may be that when women have roundworm, their immune systems are already less inclined to attack an embryo, and so their embryos have a higher likelihood of surviving the first stage of pregnancy. That might explain the difference in fertility rates.

Not that any woman with fertility problems should seek to get a roundworm infection; it can be life-threatening. But given how widespread roundworm and hookworm infections are, these tiny parasites could be having quite an effect on global demography.

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