Having parasites can boost fertility

Parasitic worms may cause baby booms.

Amazonian women infected with giant roundworms could bear up to two more children during their lifetime compared with uninfected women, an analysis of nearly a decade of medical data suggests. Hookworms, on the other hand, might act as birth control. Women with these parasites could have three fewer children than uninfected women, researchers report in the Nov. 20 Science.

No one knows how exactly worms tweak fertility. The parasites could tinker with immune cell numbers, making conditions ripe (or wrong) for pregnancy, speculate behavioral ecologist Aaron Blackwell of the University of California, Santa Barbara and colleagues. 

Blackwell’s team studied worm infections in Tsimane women in Bolivia. Hookworms — bloodsucking parasites about the length of a grain of rice — seemed to postpone women’s first pregnancies. But women infected with giant roundworms — sleek, whitish parasites that can be thicker than a straw and longer than two pencils — started having babies earlier with shorter breaks between pregnancies.

The results are the latest to offer an upside to worm infections. Some intestinal interlopers may fend off other parasites (SN:10/5/13) and curb allergies (SN:1/29/11).