Roundworm infection increases fertility among women in the Amazon

Parasitic worms can live inside humans gaining nourishment from the food we ingest (roundworms) or from our blood (hook worms), whilst being safe from predators. Although roundworm infections are generally symptomless they are not something any of us would choose to have. However, it has recently been found that infection with roundworm increases a woman's fertility.

C. elegans roundworm

Intestinal worm infections are common, affecting more than 1 billion people, and are particularly prevalent in tropical areas with poor sanitation. The giant roundworm, Ascaris lumbricoides, is one of the most common and can be 36 cm long.

Despite the repulsion often felt towards parasites, they have a lot in common with a developing human foetus. Both the parasite and the foetus would be deemed by the host to be foreign. Consequently, in order to prevent their destruction, parasites and foetuses need to fool the host's immune system into accepting them.

Indeed, parasites have been shown to trigger some of the same immune changes that occur during pregnancy, eg, stimulating regulatory T cells, which quell immune attacks.

A study was therefore conducted among people living in the Amazon rainforest of Bolivia to investigate the association between intestinal worms and fertility. Researchers analysed data collected from 986 Bolivian women from a forager-horticulturalist settlement over 9 years.

Among this sample 70% of the women were infected with parasitic worms. The infected women in the study were usually unaware they were playing host to the parasites, but those infected with hook worms had a slightly smaller body mass index and lower haemoglobin levels than the other women.

The results indicated that women infected with certain intestinal worms give birth to more children than women who are not. Different types of worm infection were linked with different effects on fertility.

Infection with roundworm (Ascaris lumbricoides) tended to be associated with earlier first births and shorter intervals between births. In contrast, infection with hookworm was associated with a later first pregnancy and extended intervals between births. A woman infected with Ascaris would bear on average two more children in her lifetime than would a woman free of the parasites.

This sounds counter-intuitive since the worms are stealing nutrients that may otherwise be used by the women. The researchers propose that it a consequence of the Ascaris worms reducing inflammation through their effects on the immune system, which might promote conception and implantation of the embryo in the uterus. Hookworms, in contrast, do not have such an impact on the immune system and any reduction in inflammation would be outweighed by the amount of nutrients they steal.

Reproductive immunologist Norbert Gleicher at The Rockefeller University in New York City explained that the results highlight that “the state of the immune system is of crucial importance to successful reproduction”.

It is unlikely that women will be intentionally infected with Ascaris to improve their fertility, but research into the mechanisms by which the roundworm modulates the immune system may lead to the development of new infertility treatments.


Kate Bass

Written by

Kate Bass

Kate graduated from the University of Newcastle upon Tyne with a biochemistry B.Sc. degree. She also has a natural flair for writing and enthusiasm for scientific communication, which made medical writing an obvious career choice. In her spare time, Kate enjoys walking in the hills with friends and travelling to learn more about different cultures around the world.


Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    Bass, Kate. (2018, August 23). Roundworm infection increases fertility among women in the Amazon. News-Medical. Retrieved on September 27, 2021 from

  • MLA

    Bass, Kate. "Roundworm infection increases fertility among women in the Amazon". News-Medical. 27 September 2021. <>.

  • Chicago

    Bass, Kate. "Roundworm infection increases fertility among women in the Amazon". News-Medical. (accessed September 27, 2021).

  • Harvard

    Bass, Kate. 2018. Roundworm infection increases fertility among women in the Amazon. News-Medical, viewed 27 September 2021,


  1. Metin Gunduz Metin Gunduz Norway says:

    Well, let me make an informative and helpful scientific comment  on this rather “unusual and obvious  statistical  correlation” in between roundworms and Fertility rates. It may indeed  help  certain category of “infertile women” in the future. Roundworms have separate sexes; with  immense reproductive capacity. Helminthes exhibit a sublime co-evolution with the host's immune system that has enabled them to successfully colonize almost all multicellular species present in every geographical environment, including over two billion humans on our planet . Among the most important cell types during helminthic invasion are granulocytes: eosinophils , neutrophils and basophils. Depending on the specific context, these leukocytes may have pivotal roles in host protection, immunopathology, or facilitation of helminthic establishment.
       Human  bodies response  to helminthic(roundworm) establishment into the lumen of the intestines  and its effect to the protective Immune system of the host body   are  fundamentally  -not  so  different  for the Immune system’s adaptation  in order to  differentiate  SELF from  NON SELF(Foreign)  during the   Implantation of fertilized human embryo to the mothers  uterine mucosa  from its very early stages(Blastocyst) to  eventual placental tissue  formation and the maintenance of pregnancy  until the term   ;  both(roundworm and human embryo )  both result in    “facilitated immune  balance and adaptation ” in between the Immune system of the mother and the “ foreign- NON SELF” tissues of the embryo  and Roundworm . And the most important factor for the reproductive fertility of women is –miscarriages-  as we know of , they are the result of Chromosomal abnormalities, toxic factors ,trauma ,metabolic abnormalities  etc. OR simply  a “rejection of implantation of (embryo) fetus by the host mother’s immune system” . This category of causes the “rejection of implantation of embryo) fetus by the mother’s immune system  –though exact frequency out of all miscarriages is  not known-  MAY be responsible  for the large majority of miscarriages that occur early or late stages of pregnancies .
    Punch line is : The Roundworm establishment into the intestines of reproductive age women MAY facilitate and optimize the “adaptation” of mother’s immune system for NON SELF –co existence- like future pregnancy  and  statistically decrease the rate of miscarriages dramatically that ultimately results in higher fertility rate .  This issue was investigated indeed in 2012 by an excellent research ( * ) at the link .
    ( * )  Granulocytes in Helminth Infection - Who is Calling the Shots?

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
You might also like... ×
Researchers use gene editing to render male mosquitoes infertile and slow disease spread