CHICAGO, Feb. 1 (Xinhua) -- Two viruses closely related to Zika, West Nile and Powassan, can spread from an infected pregnant mouse to her fetuses, causing brain damage and fetal death, according to a study from Washington University School of Medicine in St. Louis.

The findings suggest that Zika may not be unique in its ability to cause miscarriages and birth defects, viruses related to Zika also can cross the placenta in mice and cause fetal brain damage and death.

As part of the study and for comparison, researchers at Washington University also studied the effects of two mosquito-borne viruses distantly related to Zika: chikungunya and Mayaro. Both are found in Brazil and can cause arthritis.

They injected female mice at day six of their pregnancies with one of the four viruses, then examined the placentas and fetuses a week later.

All four viruses infected the placentas and fetuses, but levels of West Nile virus were 23- to 1,500-fold higher than those of the other three viruses in the placentas, and 3,000- to 16,000-fold higher in the heads of the fetal mice.

In addition, brain tissue from West Nile-infected fetuses showed severe damage under the microscope, while brain tissue from chikungunya-infected fetuses appeared healthy.

Researchers also found that about half of the fetuses whose mothers were infected with West Nile or Powassan virus died within 12 days of infection, whereas no fetuses from mothers infected with chikungunya or Mayaro died.

The researchers then infected human placentas with one of the four viruses or Zika virus, and found that the three flaviviruses, Zika, West Nile and Powassan, multiplied in human placentas while chikungunya and Mayaro did not.

"Millions of people were infected with Zika in a short time, and I think that made it easier to see, in people, that Zika virus could infect and cross the placenta and cause fetal damage," said Jonathan Miner, an assistant professor of medicine at Washington University and the study's senior author. "But our data show that other flaviviruses have the same capacity, at least in mice."

Additional epidemiologic studies to determine whether West Nile infection can cause miscarriage and brain damage in people may be prompted by this discovery.

"I don't want people to think that we're saying West Nile is definitely a threat to pregnant women and their babies," Miner said. "We're saying it's possible. But until we know for sure, it's always a good idea to wear bug repellant."

West Nile infects thousands of people every year in the United States, and about 1,000 people a year develop life-threatening brain infections that can cause persistent neurological problems. Powassan is a rare virus spread by ticks. There are only a few dozen documented cases of disease caused by the virus in the U.S. over the past decade, mostly in the Great Lakes region. Like Zika, West Nile and Powassan also belong to the flavivirus family and target neural tissues.

The study was published on January 31 in Science Translational Medicine.