Here's the strongest evidence we have so far about Zika's link to a disturbing birth defect

Zika, which has been spreading quickly in the Americas, has been linked with a rise in a birth defect called microcephaly that causes babies to be born with abnormally small heads.

On Wednesday, scientists published a study in the New England Journal of Medicine reporting the case of a pregnant woman with Zika who likely transmitted the virus to her foetus.

As far as we know, it’s the most direct evidence linking the virus to the disorder so far — analysis of the child’s brain tissue revealed evidence of Zika infection, suggesting it had crossed from the mother into the child via the placenta, the organ that nourishes the baby until birth.

The current Zika outbreak prompted the World Health Organisation to declare a global public health emergency.

The virus, which is spread by mosquito bites and likely by sex, commonly causes symptoms such as fever, rash, joint pain or red eyes. There is no vaccine or treatment, and while infections are usually mild, it has been linked to birth defects in pregnant women.

A case of Zika passed from mother to child

In October 2015, a healthy 25-year-old pregnant woman in Ljubljana, Slovenia who had been working in Natal, Brazil was found to have abnormalities in her foetus. She had been working in Brazil near the end of her first trimester when she developed a high fever, severe muscle and eye pain, and an itchy rash. Doctors suspected she had Zika, given the high rates of the virus in the community at the time. Unfortunately, the woman was never tested for the virus, and scans of the foetus at that time appeared normal, according to the study.

When she was 29 weeks pregnant, ultrasound scans revealed problems with the foetus. At 32 weeks, the scans of the foetus revealed it was in the third percentile for weight and below the second percentile for head size. Doctors also observed a buildup of hardened calcium deposits on the foetus’ brain and on the placenta, which suggest damage caused by the virus. Doctors suspected that the developing foetus had microcephaly, and the woman had an abortion.

When scientists examined the baby postmortem, they found more calcium deposits and traces of the Zika virus in its brain, suggesting the mother had passed the virus to the foetus, and may have caused its brain abnormalities.

The link between Zika and microcephaly is still being investigated, but authorities are warning pregnant women to avoid travelling to countries most affected by the virus, and women living in some of these countries to try to delay getting pregnant until 2018.

What is microcephaly?

Microcephaly can cause developmental delays, seizures, or problems with speaking, sitting, walking, balance, feeding, hearing, and vision in babies born with the disorder, according to the CDC. The symptoms, which are often lifelong, range from mild to severe. In some cases, the condition can be life-threatening.

Microcephaly can be diagnosed with an ultrasound late in the second trimester or early in the third trimester of pregnancy, or after the baby is born by measuring its head and comparing it to growth charts.

We still don’t know exactly what causes babies to develop this defect, but it can be related to genetics, infections, severe malnutrition, or exposure to alcohol, drugs, or other toxic chemicals.

Zika and the risk of microcephaly

Ordinarily, microcephaly is pretty rare. About 2 to 12 babies per 10,000 are born with the condition in the US. In Brazil, where the Zika outbreak has been especially severe, microcephaly cases before the outbreak were about 0.5 cases for every 10,000 births. But in the second half of 2015, that number jumped to about 20 cases per 10,000 live births, according to a CDC report.

In the past few months, researchers have found genetic material from the Zika virus in the brains of infants born with microcephaly in Brazil, which suggests the virus can cross over the placenta and infect the baby’s nervous system, STAT reports.

However, a pregnant woman who gets the virus will not necessarily have a child with microcephaly, and experts don’t know the exact risk.

STAT reports that most of the Zika-infected women giving birth to babies with microcephaly contracted the virus during their first trimester of pregnancy, although they may still be at risk into their second trimester. Most congenital infections happen during the second and fourth months of pregnancy, when the most brain cells are being formed.

According to the CDC, Zika usually only remains in the blood of an infected person for a few days to a week, so it cannot infect a baby that is born after it has been cleared. While experts currently don’t think there’s a risk that Zika will cause birth defects in future pregnancies, women who have had the virus should try to consult their doctors before becoming pregnant.

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