A common parasite may be influencing the sex ratio of the world's population. According to a new study, pregnant women who have latent infections of Toxoplasma gondii are significantly more likely to give birth to boys than girls.
T. gondii infects between 20% and 80% of human populations worldwide. Initial exposure usually causes only mild flulike symptoms, but the infection is for life: The parasite hunkers down into a cyst that stays resident in muscles and other tissues. Humans are most often infected by eating undercooked meat or through contact with feces from infected cats, which are the only animals that harbor the parasite during the sexual stage of its life cycle.
Parasitologist Jaroslav Flegr and his colleagues at Charles University in Prague, who have studied the effects of Toxoplasma in humans, had found preliminary evidence that latent infections might correspond with longer pregnancies. But when they examined the medical records for 1803 births in three maternity clinics in the city that routinely test for Toxoplasma antibodies, they found a much more striking difference between infected and uninfected women: Infected women have more boys. The usual sex ratio at birth is .51, meaning 104 boys are born for every 100 girls. But in the 454 women who tested positive for antibodies to Toxoplasma the ratio was .60--which translates to about 150 boys per 100 girls. For women with the highest levels of antibodies against the parasite, the ratio was .72, equivalent to 260 boys for every 100 girls, the team reports online this week in the journal Naturwissenschaften.
Flegr says one explanation could be that the latent infection suppresses the immune system. The maternal immune system sometimes reacts against male embryos, leading to a higher miscarriage rate for boys in early pregnancy. Followup studies in his lab suggest that infection in rats has a similar effect, he says.
The data are preliminary, but intriguing, says parasitologist Joanne Webster of Imperial College London, who has studied the effects of Toxoplasma on rat behavior. And it will not be hard to see whether the effect holds true for larger samples: If a woman is infected with Toxoplasma for the first time while she is pregnant, the parasite can travel across the placenta and cause miscarriage and birth defects, so testing for previous exposure to the parasite is standard in many maternity clinics. What isn't clear, she says, is the evolutionary advantage that the parasite might get from skewing the sex ratio.
The latest find adds another twist to Toxoplasma. The parasite is already infamous for its effects on the behavior of infected rats--it seems to prompt more daring behavior towards cats (ScienceNOW, 26 July 2000). And some researchers have speculated that infection might account for cultural differences between nations as well, perhaps making some societies more open and risk-taking.