Pregnant woman waits for a routine general checkup, which includes Zika screening, at the maternity ward of the Hospital Escuela in Tegucigalpa, Honduras, Jan. 27, 2016.
Pregnant woman waits for a routine general checkup, which includes Zika screening, at the maternity ward of the Hospital Escuela in Tegucigalpa, Honduras, Jan. 27, 2016.

Scientists undertaking two different studies on mice have learned more about how the Zika virus could effect the fetuses of pregnant women.

The studies, published in the journals Cell and Nature, are the result of research designed to help develop vaccines for the virus. The virus has only a mild effect on adult humans, but can be devastating to unborn children whose mothers are infected.

Both studies looked at how the virus is transmitted from mother to child through the placenta, an organ that nourishes the fetus before the baby is born. While the placenta is usually resistant to infection, scientists found that the Zika virus was able to penetrate the mouse placenta and flourish inside it.

The levels of Zika virus in mouse placentas were 1,000 times greater than in the blood of the pregnant mice carrying them. 

The infection of the placenta gave the virus access to the brain of the mouse fetus, where the virus could disrupt normal development by attacking and killing nerve cells.

FILE - A worker sprays insecticide for mosquitos a
FILE - A worker sprays insecticide for mosquitos at a village in Bangkok, Thailand, Jan. 13, 2016.

Such a process may occur in human fetuses, where Zika has been linked to microcephaly — which causes a baby to be born with an abnormally small head and possible developmental problems.

'Tip of the iceberg'

Alysson Muotri of the school of medicine at the University of California, San Diego, and senior author of the Nature study, told reporters Wednesday that the death of nerve cells indicates Zika could cause a range of problems beyond microcephaly.

"This is the tip of the iceberg," he said. "We should probably rename this 'congenital Zika syndrome,'" rather than expect microcephaly to be the only effect on human fetuses.

Zika also is believed to contribute to the development of Guillain-Barre syndrome in adults — which affects the nervous system, can cause respiratory problems and, in severe cases, can lead to paralysis.

Muotri’s statements were based on experiments done on mice, but also involved exposing human brain cells in lab dishes to the strain of virus linked to 1,200 cases of microcephaly in Brazil.

Mice are not normally affected by the Zika virus, so scientists tested their theories on mice whose immunity to the virus was blocked — in one case, by injection of antibodies that blocked the immune response and, in another, by using mice with genetically weakened immune systems.

Severina Raimunda holds her granddaughter Melisa V
FILE - Severina Raimunda holds her granddaughter Melisa Vitoria, left, who was born with microcephaly and her twin brother, Edison Junior, at the IMIP hospital in Recife, Pernambuco state, Brazil, Feb. 3, 2016.

Michael Diamond — who led the study conducted by a team of Chinese researchers, as well as by scholars at Washington University in St. Louis — said the new information about Zika's transmission to the placenta and fetus will help in the development of preventive measures.

The studies, Diamond said, show the first animal model of in utero transmission of the Zika virus — with outcomes similar to those found in women and their babies.

"Now, we can begin to see whether vaccines can prevent transmission of the Zika virus to the fetus," he said.

Fear of infection

Fear of Zika infection is growing — especially with Brazil, the center of the outbreak, holding this year's Olympic Games in Rio de Janeiro in September.

While the virus has only a mild effect on adult humans, experts believe it can be transmitted sexually, putting women who are pregnant or trying to get pregnant at risk. And while the virus is spread by a mosquito with a small flight range, the World Health Organization says the virus is thought to have spread from one part of the world to another through human travelers, or mosquitoes accidentally transported with humans.

South Korea Zika Virus
FILE - South Korean Olympic athletes and models present the South Korean Olympic team uniforms for the 2016 Rio de Janeiro Olympic Games.

WHO suspects the 2014 World Sprint Championship brought Zika to Brazil from French Polynesia.

With a large influx of international travelers expected in Brazil this year, experts fear the virus could travel to a number of locations and spark new outbreaks. Some officials are cautioning pregnant women not to travel to Brazil, or have sex with potentially infected partners.

Olympic athletes are encouraged to wear clothing that protects against mosquito bites while in Brazil. South Korea has gone so far as to develop leisure wear for its athletes that includes long sleeves and mosquito repellent. And some public health experts this week called for the Games to be postponed, moved or both.