The Tsimane of Bolivia are an isolated Amazonian indigenous community inhabiting a vast area of lowland forests. They make a living mostly through swidden agriculture, hunting, fishing, and gathering. Their average birth rate, which is relatively high — nine children per woman — has inspired a new study, published in the journal Science (November 20, 2015). The study results show that infection with parasitic worms could help or harm a woman’s fertility, depending on the type of worm.
Parasitic worms — also known as soil-transmitted helminths or intestinal worms — cause some of the most common parasitic infections worldwide. According to the World Health Organization (WHO), approximately 2 billion people are infected with soil-transmitted helminths globally, mostly in the poorest and most deprived communities. They are transmitted by eggs present in human feces, which in turn contaminate soil in areas where sanitation is poor.
The main species of soil-transmitted helminths that infect people are the roundworm (Ascaris lumbricoides), the whipworm (Trichuris trichiura), and hookworms (Necator americanus and Ancylostoma duodenale). Adult worms live in the intestine where they produce thousands of eggs each day. In areas that lack adequate sanitation, these eggs contaminate the soil. Once infected, individuals usually have no symptoms. However, infection can contribute to anemia, vitamin A deficiency, malnutrition, impaired growth, delayed development, and intestinal blockages.
The new study shows that the Tismane women infected with roundworm were more likely to become pregnant, and had up to two more children than those without infection, whereas women infected with hookworm were less likely to become pregnant, and had up to three fewer children than uninfected women.
The researchers think that these opposing effects are likely due to the different interactions of roundworms and hookworms with the immune system, which in turn affects the likelihood of conception. Healthy pregnancy is associated with shifts in immune responses, with an increase in regulatory and helper T cell type 2 (TH2) activity. This activity helps to suppress helper T cell type 1 (TH1) responses, thus increasing maternal tolerance of the fetus. Indeed, the mother’s immune system sees the fetus as a foreign entity, and it could otherwise reject it using the TH1 responses.
Parasitic infections that induce systemic immunological changes affect fertility by altering the immune response of the infected women — helper T cell populations generally shift from TH1 toward TH2 responses, and the suppressive activity of regulatory T cells increases to modulate both TH1 and TH2 responses.
The immune response to the roundworm could be more favorable to conception and embryo implantation because it more closely resembles the immunological state in pregnancy, and less closely the one that suppresses fertility.
Michael Gurven, one of the study authors, said in a press release: “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, 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.”