Toxoplasmosis could be one of most important influences on human offspring sex ratio
The latest Parasitology Paper of the Month is Do differences in Toxoplasma prevalence influence global variation in secondary sex ratio? Preliminary ecological regression study by Madhukar S. Dama, Lenka Martinec Nováková and Jaroslav Flegr.
Sex of the fetus is genetically determined so that an equal number of sons and daughters are born in large populations. However, the ratio of female to male births across human populations varies significantly. Many factors have been implicated in this. The theory that natural selection should favour female offspring under suboptimal environmental conditions implies that pathogens may affect secondary sex ratio (ratio of male to female births). Could this mean Toxoplasma-infected women have fewer sons? The seems to be that it depends on the longevity of the infection.
According to a 10 years old study by Kankova et al. women with recent infection have more male newborns, while in those with an older infection the number of sons decreased. To our surprise, nobody has tried to repeat the study despite all the necessary human data being available in medical record databases. In 2014, we published an ecological study showing that disease burdens in particular countries correlate with local prevalence of toxoplasmosis. A side-product of this study was currently the largest database of toxoplasmosis-prevalence data, which we then used to search for correlation between prevalence of toxoplasmosis and secondary sexual ratio (SSR) – the representation of boys in newborns.
The results showed toxoplasmosis to be in fact the third most important of 15 analyzed factors, after the son preference and fertility. The association was stronger in non-European countries, where the prevalence is mostly high and women are usually infected during childhood. In Europe, many women are infected after the age of 25 and therefore the negative influence of old infections on the SSR is probably neutralized by the positive influence of recent infections.
Positive effects of recent toxoplasmosis on SSR is probably caused by subdued embryo quality control in infected women, which results in lower probability of miscarriage for both embryos with various defects, and more immunogenic male embryos. While the mechanism of the opposite effect in older infections is unknown, it might be a manifestation of the Trivers-Willard effect – the increased probability of birth of daughters in women in poor physical condition. Our results as well as the result of the 2014 study, showing that differences in prevalences of toxoplasmosis could explain about 23% of between-countries variability in total disease burden, suggest that Toxoplasma gondii infection could be one of the most important environmental factors influencing the global variation of the offspring sex ratio in humans.
What should be done next? First of all, the study should be repeated, on regions of large countries, such as the USA or France, for which the toxoplasmosis prevalence data and the SSR data are available. It is also important to search for the empirical support for Trivers-Willard model-based explanation, namely for signs of deteriorated health in people with an old infection. Studies have already showed an association between toxoplasmosis and increased risks of certain neuropsychiatric disorders, including schizophrenia and epilepsy. Growing number of studies suggest that this could be just a tip of an iceberg and that toxoplasmosis could have strong effects on the incidence and progress of many common disorders, including cardiovascular diseases and cancer.
Parasitology, originally called Medical zoology, is usually treated as a poor relative of “the big medicine” as parasites seemingly cause far fewer deaths than bacteria, viruses, smoking, or obesity. However further research may reveal that successful control of just one parasite, Toxoplasma gondii, could enormously reduce other-factor associated disease burdens.
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