Researchers conducting a nine-year study on the indigenous Tsimane people of Bolivia have stumbled upon a startling discovery: the infection of parasitic worms in women affected their fertility.
The researchers, who were from the University of California Santa Barbara, found that women infected with the giant roundworm (Ascaris lumbricoides) were more likely to become pregnant. On the other hand, women infected with hookworm (either Necator americanus or Ancylostoma duodenale) were less likely to become pregnant.
"We found that different species of helminths - a family of parasitic intestinal worms - could have either positive or negative effects on the timing of a Tsimane woman's next pregnancy," lead study author Aaron Blackwell, assistant professor of anthropology, said in a press release.
Tsimane women have an average of 10 children in their lifetime, but those who had parasitic roundworms had shorter birth intervals and had two more additional children, giving birth to as many as 12 children in their lifetime. However, those who had hookworm infection had longer birth intervals and had three less children compared to women who were not infected with either roundworm or hookworm, giving birth to only seven children on average in their lifetime.
The researchers attributed the effect of parasitc worms in women's fertility to "the balance of immune responses that the different worms induce, rather than to the physiological costs of parasitism."
"These opposing effects are likely due to helminth infection affecting the immune system, which in turn affects the likelihood of conception," Blackwell explained. "Our findings suggest that helminth infections may have substantial effects on demographic patterns in developing populations. Further, these results may also have implications for fertility in developed populations, where many fertility problems are connected to autoimmune disorders."
The researchers also noted that Tsimane women infected with parasitic worms typically showed no symptoms and had "extremely elevated" immunoglobulin E levels. Previous studies have shown that infection with such parasitic worms affected the likelihood of being infected with other parasites or diseases.
"This study examines yet another domain where helminths and their regulatory effect on the immune system may be relevant to broader aspects of health and well-being," study co-author Michael Gurven, anthropology professor and co-director of the Tsimane Life History and Health Project, said in the press release.
The study was published in the Nov. 20 issue of the journal Science.
