A nine-year-long study on indigenous Bolivian women revealed that certain types of parasites affect the likelihood of pregnancy. Researchers from the University of California, Santa Barbara (UCSB) found out that two species of helminths—a family of parasitic worms that lodge themselves in a person’s intestines—can also lengthen birth intervals.
The study observed 986 Tsimane’ women, an indigenous group from the lowlands of Bolivia, for nine years. Their average family size consists of nine children. Families with fewer children are considered to be of a lesser status.
About 70 percent of Tsimane’s population is infected with parasitic worms. The study focused on hookworms and giant roundworms, the two most common parasites.
Lead author Aaron Blackwell and his team from the UCSB discovered that women infected with roundworms were more likely to conceive and would have two additional children, while those infected with hookworms were less like to become pregnant and have three fewer children.
“Hookworm infection tended to increase the length of the intervals between births and that was consistent across all ages. But younger women infected with roundworm had shorter birth intervals,” Blackwell said. He added that these effects are likely due to the worms affecting the immune system.
“Although we don’t know the precise mechanism behind these results, our findings are still compelling and suggest that immune modulation — via our ‘old friends’ the intestinal worms — can have far-reaching effects on the body, even though the findings may be less applicable in developed populations where women only have a few children over their lifetime,” said Michael Gurven, co-author of the study and a UCSB anthropology professor.
An estimated two billion people are currently infected with intestinal helminths. Because of the Tsiname’s living conditions, most of them are infected with helminths and the others are very vulnerable.
“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,” Blackwell said.
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