A common parasite may be influencing the sex composition of the world's human population.
Toxoplasma gondii, which people often pick up from cats, infects between 20% and 80% of societies worldwide. Parasitologist Jaroslav Flegr and his colleagues at Charles University in Prague studied the medical records of 1803 infants born in three maternity clinics in Prague that routinely test for antibodies to toxoplasma, which can cause miscarriages and birth defects. The usual sex ratio at birth is 104 boys for every 100 girls. But the 454 pregnant women who tested positive gave birth to 290 boys and only 187 girls. The women with the highest antibody levels had more than twice as many boys as girls, the team reports this month in Naturwissenschaften.
Flegr says the parasitic infection may suppress the maternal immune system, which sometimes reacts against male embryos and causes more boys to be miscarried. Larger samples are needed, but the data are intriguing, says parasitologist Joanne Webster of Imperial College London. What isn't clear, she adds, is the evolutionary advantage that the parasite might get from skewing the sex ratio.
