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How Parasitic Worms Affect Reproduction

3806 How Parasitic Worms Affect Reproduction
A Tsimane family. Michael Gurven

Parasitic worms called helminths infect hundreds of millions of people around the world, and can impact both conception and pregnancy. According to a new Science study with Amazonian women, hookworm infections can decrease reproduction rate, whereas giant roundworm infections can increase it. 

In humans, helminths can directly infect the fetus or the genitals and other reproductive organs. However, little is known about the effects of intestinal helminths on our fecundity (the ability to reproduce), fertility, and birth spacing, or the amount of time in between babies. Hookworms (Ancylostoma duodenale or Necator americanus) and giant roundworms (Ascaris lumbricoides) each infect 500 million to 800 million people, and they’ve been known to change our immune responses.


So, a team led by Aaron Blackwell from the University of California Santa Barbara analyzed data collected over nine years from 986 Tsimane forager-horticulturalist women living in the lowlands of Bolivia. This population rarely uses pharmaceutical contraceptives, and they have an average birth rate of nine children for each woman. Meanwhile, helminths infect 70 percent of the population. 

The researchers found that different helminth species can have contrasting effects on fecundity. A hookworm infection delays the first pregnancy and extends inter-birth intervals. Women repeatedly infected were likely to have up to three fewer children in their lifetime than uninfected women. Meanwhile, infection with A. lumbricoides roundworm is associated with earlier first births and shortened inter-birth intervals. These women have up to two more children in their lifespan than women who have never been infected. These effects are likely because the two parasites invoke different immunological changes. 

A healthy pregnancy is also associated with changes in immune responses. During the latter phase of the menstrual cycle, the responses of immune cells called regulatory and type 2 (TH2) T-cells increase. If conception occurs, these increases continue through the pregnancy in order to suppress the response of type 1 (TH1) T-cells. This increases the maternal tolerance of the fetus, which are immunologically distinct from the mother. 

Roundworms are known to increase TH2 levels, while hookworms evoke a mixed TH1 and TH2 response. So these parasites might be indirectly affecting reproduction rates by altering the host’s T-cell balances. The immune response to roundworms may be more favorable to conception since it more closely resembles the immunological state in pregnancy – and less closely resembles the pro-inflammatory states that suppress it. 


However, does this (unexpected positive) association between roundworms and reproduction mean that the parasites actually increase the reproduction rate or do they suppress responses that would otherwise decrease it? Or is it something else? It’s hard to say right now using data from just one population. "In the population we studied, other kinds of infections might suppress fertility, and A. lumbricoides might just be counteracting this suppression, rather than increasing fertility," Blackwell explains to IFLScience. If that’s the case, these effects might not be found in populations without those other infections, like in the U.S. 

He adds, however: "We actually think it's more likely that the immune function effects of infection, and the changes in fertility, would apply across all women, and possibly would be even more pronounced in women in the U.S. This is because we think humans probably evolved in an environment with constant helminth infections, and that in the absence of this, women may have ‘dysregulated’ immunity that impacts fertility." They’ll need to see the effects in other populations to say for sure.

Image in the text: A Tsimane child crawls on the ground. Michael Gurven


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