Pregnancy: Retirement Notice For The Stork?

25. January 2016

Intestinal worms and other parasites are freeloaders that can harm the health of their host. Yet they may perhaps also have an unexpected effect: they might be able to enhance fertility and offer help so that women are able to become pregnant sooner.

About 2 billion people worldwide are infected by parasitic worms. Quite often such worm infections run their course without symptoms – but they can also lead to severe problems such as malnutrition, abdominal pain and diarrhoea or anaemia. Researchers at the University of California in Santa Barbara (USA) have now discovered a surprising connection: intestinal parasites could also play a part in helping women more quickly and more frequently become pregnant.

From previous research it is known that aside from female sex hormones a number of other factors have an impact on whether a woman becomes pregnant or not. An embryo is then able to settle in the womb, if it is tolerated by the mother’s immune system. At the same time we know that intestinal parasites such as roundworms and hookworms influence the immune system of their host via complex mechanisms so as not to be flushed out, allowing them to be able to settle permanently in his body.

In a recent study [Paywall] Aaron Blackwell and his team examined for the first time how an infestation of intestinal parasites has effects on the fertility of women living in very simple and nature-based conditions. The participants in the study were 986 Bolivian women belonging to the Tsimane culture, who live as gatherers and small-scale farmers in the Amazon region. The researchers over a period of nine years studied the birth rate of the women who used no contraceptive methods.

Parasites as a fertility agent?

70 percent of women were carriers of an intestinal worm infection. On average the Tsimane women brought ten children into the world during their lifetime. However, worm infestation influenced their fertility considerably: women who were infected by hookworm bore on average only seven children. In addition, their first pregnancy occurred later and the intervals between the births were greater than among women without worm infestation. With infestation of roundworms on the other hand the reverse effect was able to be observed: here, the average number of children was 12, in addition the first pregnancy happened earlier and the intervals between the births were longer.

“These effects could be related to the type of immune responses triggered by the various intestinal worms”, the researchers write. Hookworms thus among other things provoke inflammatory reactions, which could lead to the reduction of fertility. “In contrast roundworms initiate a T-helper cell Type 2 reaction – a so-called Th2 response. Regulatory cells are activated here which then weaken the immune response”, says Blackwell. The decreased immune response may help to ensure that a fertilised ovum is not rejected by the body and can easily settle in the womb.

“The results clearly show that fertility is not only influenced by hormones, but by many other factors – including the immune system”, says Gil Mor from the Yale School of Medicine (USA), who is studying the role of the immune system during the implantation of the ovum and during pregnancy.

Intestinal worms – all natural, or in fact harmful?

Intestinal parasites could through their physiological and immunological properties have an important impact on fertility, Blackwell and his team write. Many women in western industrialised countries might also therefore be having problems in getting pregnant because their immune systems overreact, according to Blackwell. Perhaps the human body is accustomed to being infested with parasites, because this has been the rule rather than the exception over the course of evolution. “If they are missing, this can possibly lead to an impaired immune system and overreactions that adversely affect fertility”, the researchers suggest.

Nevertheless, it is questionable whether an infestation of intestinal parasites is really a desirable condition. It’s pregnant women and children in particular who are regarded to be especially at risk of suffering from health problems stemming from a worm infection. Several studies thus show that a worm infestation during pregnancy can adversely affect the health of mother and child: in one study [Paywall] this was associated with a lower birth weight for the children and with the mother having anaemia. In another investigation a connection was demonstrated between pregnant mother’s having hookworm infestation and cognitive and gross motor impairments of their children at the age of twelve months.

The efficacy of vaccinations, which includes those such as cholera, tuberculosis or pneumococcus [Paywall] vaccines as well as a vaccine candidates against malaria [Paywall], may also be reduced by intestinal parasites.

In any case, the results could be helpful for better understanding the role played by the immune system in the implantation of the ovum, and during the course of pregnancy. “This could help in developing treatments for fertility problems resulting from an impaired immune system”, says Blackwell.

Developing new treatment approaches to infertility

The researcher harbours less scepticism with regard to the possible adverse effects of intestinal worms,. Women should indeed not intentionally get infected with intestinal parasites in order to increase their fertility – because the effects have not been adequately studied as yet. In addition, it is not known how worm infections impact upon women from western countries that are usually rarely affected by worm infestations. “However, it is unlikely that women would experience severe side affects from an infection with roundworms, if these come from a reliable source and do not include other pathogens”, says Blackwell.

He does however also admit that worm infections could have advantages and disadvantages for people from western countries. “On the one hand they could reduce allergies and increase fertility”, says Blackwell. “On the other hand they can lead to anaemia and other side effects and increase susceptibility to viral infection. “it would be preferable, therefore, first to better understand the effects of intestinal worms on the body – and then to try to develop a vaccine or other treatment approaches that have similar effects to the parasites.

The idea for Blackwell and his team’s study came by way of an amusing event: a colleague and her husband, who were part of the field studies, had recently decided to have a child. “She then became pregnant much sooner than expected – and responded spontaneously by saying that it may possibly be due to the parasites”, says Blackwell.

Original publication:

Helminth infection, fecundity, and age of first pregnancy in women [Paywall]
Aaron D. Blackwell et al.; Science, doi: 10.1126/science.aac7902; 2015

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