Infections with the giant roundworm (Ascaris lumbricoides) increase female fertility, according to a nine-year study.
Yet the same study found that infections with hookworm (Ancylostoma duodenale or Necator americanus) lead to a decrease in fertility.
The study involved 986 women from an isolated, indigenous Amazonian population with a 70 percent prevalence of intestinal worm infections. Roundworm and hookworm are the most common intestinal worms within the population, with a prevalence of up to 20 percent, and 56 percent, respectively.
The researchers found that women infected with roundworm had, on average, two more children than women free of intestinal parasites. They tended to have their first baby earlier, conceive more frequently, and have their last baby at a later age than uninfected women.
By contrast, hookworm infections were associated with three fewer pregnancies on average. Compared with uninfected women, they had their first pregnancies later, became pregnant less often, and had their last baby at an earlier age.
Women who were hosts to both parasites, the roundworm and the hookworm, appeared to experience a compound effect of both infections, resulting in fertility outcomes similar to those observed in uninfected women.
On average each woman in the study gave birth to nine children over her lifespan.
'These opposing effects [of hookworm vs roundworm infection] are likely due to helminth infection affecting the immune system, which in turn affects the likelihood of conception,' said Professor Aaron Blackwell, a lead author of the study.
Roundworms are truly large, often reaching the length of 35 centimetres, and they rely on their host's food for their own nutrition. Hookworms are tiny in comparison, at only about a centimetre long; however, they puncture the lining of their host's bowel to feed on blood.
As a result, the host reacts differently to both parasites. For example, the researchers found that women infected with hookworm had a lower body-mass index (BMI), and lower haemoglobin levels, in comparison with uninfected members of the population, whereas women infected with roundworm did not.
Notably, the immunological responses to roundworm infections closely resemble the immunological state during pregnancy, unlike the responses occurring during hookworm infections. Roundworms cause a strong T cell type 2 (TH2) response whereas hookworms trigger a mixture of responses of the TH1 and TH2 types.
'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 Professor Michael Gurven, a co-author of the work.
Professor Blackwell added: '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.'
The study, conducted by anthropologists from the University of California Santa Barbara (UCSB), was published in the journal Science.