THURSDAY, Dec. 14 (HealthDay News) -- Scientists say a single gene holds the key to the cat-borne toxoplasmosis parasite, which can cause birth defects if caught by pregnant women.

The gene, called ROP18, is 90 percent responsible for making Toxoplasma gondii dangerous to humans, says a study in this week's issue of the journal Science.

Cats are commonly infected with this parasite, along with some livestock and wildlife. It's estimated that about 25 percent of humans are infected with the parasite, but infections usually produce no symptoms in healthy people.

However, the parasite can cause congenital defects or the death of fetuses in pregnant women. That's why pregnant women are warned not to change cat litter. T. gondii can also lead to serious disease in people with weakened immune systems and, in rare cases, cause serious eye or central nervous system disease in healthy people.

This study found that 90 percent of harmful strains of T. gondii have a different form of the ROP18 virulence gene than more benign strains. This finding should make it easier to identify dangerous strains and treat them, the researchers said.

"Clinically, it may be helpful to be able to test the form of the parasite causing the infection to determine if a case requires aggressive management and treatment or is unlikely to be a cause of serious disease. This finding will advance us toward that goal," senior author L. David Sibley, professor of molecular microbiology at Washington University School of Medicine in St. Louis, said in a prepared statement.

ROP18 produces a protein that belongs to a class of signaling factors called kinases.

"Kinases are active in cancers and autoimmune disorders, so pharmaceutical companies already have libraries of inhibitors they've developed to block the activity of these proteins," Sibley said.

"Some patients can't tolerate the antibiotics we currently use to treat T. gondii infection, so, in future studies, we will want to screen these inhibitor libraries to see if one can selectively block ROP18 and serve as a more effective treatment," he added.