Scientists just found another critical piece of evidence that links Zika to a disturbing birth defect

Scientists just found another piece of evidence that connects the untreatable Zika virus with a birth defect called microcephaly.

On Wednesday, researchers reported finding the virus in the amniotic fluid of two pregnant women whose fetuses were diagnosed with microcephaly.

In a study in the Lancet Infectious Diseases journal, the scientists said their finding suggests that the Zika virus can cross the placental barrier.

But importantly, this evidence does not prove Zika causes microcephaly, the condition in which babies are born with abnormally small heads. More research is still needed to understand the link.

"This study cannot determine whether the Zika virus identified in these two cases was the cause of microcephaly in the babies," said Ana de Filippis, the doctor who led the study at the Oswaldo Cruz Institute in Rio de Janeiro, Brazil. "Until we understand the biological mechanism linking Zika virus to microcephaly, we cannot be certain that one causes the other."

For the most part, only about 1 in 5 people with Zika ever shows symptoms, which most commonly include fever, rash, joint pain, and red eyes, though there have been cases of a temporary neurological disorder called Guillain-Barre Syndrome associated with Zika. Zika is a mosquito-borne illness, meaning it passes from person to person via mosquito bite.

The World Health Organization has declared the Zika epidemic spreading from Brazil a global public health emergency and called for urgent studies to establish with its association with rising number of cases of suspected birth defects can be proven.

For this study, de Filippis' team investigated the cases of two women, aged 27 and 35, from Paraiba in northeastern Brazil.

The women had symptoms of Zika infection - including fever, muscle pain and a rash - during their first trimester of pregnancy. Ultrasounds taken at approximately 22 weeks of pregnancy confirmed the fetuses had microcephaly.

The researchers took and analyzed samples of amniotic fluid at 28 weeks of pregnancy. While the women's blood and urine samples tested negative for Zika, their amniotic fluid tested positive for the virus genome and for Zika antibodies. That suggests that the virus could cross the placental barrier and possibly infect the fetus.

Babies born with microcephaly are at risk of incomplete brain development, which can lead to other complications.

The condition has previously been linked to a range of factors including genetic disorders, drug or chemical intoxication, maternal malnutrition and infections with viruses or bacteria that can cross the placental barrier such as herpes, HIV, or other mosquito-borne viruses such as chikungunya.

Jimmy Whitworth, a Zika expert and professor of international public health at the London School of Hygiene & Tropical Medicine, said the findings "strengthen the body of evidence" pointing to Zika as a cause of microcephaly in Brazil, though it only shows association, not causation.

The best way to prevent infection is to avoid being bitten by the mosquitoes that transmit the disease, by either avoiding travel to areas where the virus is being transmitted or wearing long clothes and using mosquito repellent.

(Reuters reporting by Kate Kelland)