Women who carry a common parasite are more likely to give birth to boys, Czech researchers report.

In most populations, 51 percent of babies born are male, Dr. S. Kankova of Charles University in Prague and colleagues note. But among a group of women infected with the bug Toxoplasma, 61 percent of babies born were boys.

“In light of the high prevalence of latent toxoplasmosis in most countries, the impact of toxoplasmosis on the human population might be considerable,” the researchers write in the journal Naturwissenschaften.

Toxoplasmosis prevalence ranges from 20 percent to 80 percent worldwide, they note. People generally contract the infection by eating undercooked meat or being exposed to cat feces, which is why pregnant women are advised to avoid cleaning out cat litter boxes.

Babies of women who contract the infection during pregnancy are at risk of birth defects including mental retardation, deafness, and blindness. Children of women infected at least six months before pregnancy are not thought to be at risk.

While most people with normal immune systems aren’t sickened by the parasite, once they are infected most will carry it for a lifetime.

The researchers sought to determine whether Toxoplasma might influence gender ratios by looking at 1,803 infants born at 3 different clinics between 1996 and 2004. About 55 percent were boys.

But among the 454 women who tested positive for Toxoplasma, 61 percent of babies were boys. And the higher the amount of antibodies to the parasite a woman carried, the more likely she was to give birth to a boy; 72 percent of women with the highest levels of Toxoplasma antibodies delivered boys.

Kankova and colleagues suggest that toxoplasmosis may influence a woman’s immune system in a way that allows more male embryos to survive.

SOURCE: Naturwissenschaften, Online October 7, 2006.