Women infected with dormant toxoplasmosis are more likely to give birth to boys than women who are Toxoplasma negative, according to research by S. Kankova and colleagues from the Departments of Parasitology, Microbiology and Zoology, Charles University; the Centre of Reproductive Medicine; and GynCentrum, in the Czech Republic.

They found that the presence of the parasite Toxoplasma gondii in the mothers’ blood, one of the most common parasites in humans with a worldwide prevalence of 20-80%, increased the likelihood that these women would give birth to a boy. This is the first study [1], published in Springer’s journal Naturwissenschaften this week, to suggest an effect of parasitic infection on the sex of a baby.

Kankova and colleagues analysed the effect of latent (or dormant) toxoplasmosis [2] on the probability of the birth of a boy in humans. Latent toxoplasmosis is asymptomatic but is usually a life-long infection characterised by the presence of anti-Toxoplasma antibodies in the blood.

They analysed over 1800 clinical records of babies born between 1996-2004 in private maternity clinics in the Czech Republic. Women attending these private clinics were routinely tested for toxoplasmosis. The records contained information on the mother’s age, the concentration of anti-Toxoplasma antibodies in the mother’s blood, previous deliveries and abortions, and the sex of the newborn.

On average worldwide, for every three children born, only one is a boy. Kankova’s team found that Toxoplasma positive mothers gave birth to more boys than did Toxoplasma negative women. The probability of a male birth also increased, up to two boys in three children, with increasing levels of anti-Toxoplasma antibodies. According to the researchers, the increased survival of male embryos in infected women may be explained by toxoplasmosis’ modulating and suppressing effects on the immune system.

The authors caution that this observational study suggests that toxoplasmosis may be the cause of this increase in male births, but it cannot establish cause and effect. They conclude that “an independent confirmation of this tentative conclusion by a manipulative experiment (by experimental infection of animals) is necessary.”

1. Kankova S et al (2006). Women infected with parasite Toxoplasma have more sons. (Naturwissenschaften, DOI 10.1007/s00114-006-0166-2)

2. Toxoplasmosis is an infection caused by parasites transmitted to humans from consumption of raw or undercooked meat of an intermediate host (such as pig, sheep and rabbit) and food or water contaminated with soil containing cat feces. If a pregnant woman is infected with toxoplasmosis, this has potentially fatal consequences for the fetus.