In the face of a growing number of cases, and increasing evidence that the Zika virus is responsible for a spike in birth defects, last month the World Health Organization declared the current outbreak of the virus to be a global emergency. Mostly spread by mosquitoes, the WHO have now announced that sexual transmission of the disease is more common than previously thought, and that the virus is indeed “neurotrophic,” preferentially affecting the tissue of the brain and brain stem.

In an address to the media after a Zika emergency committee meeting, WHO Director-General Dr. Margaret Chan said that “all of this news is alarming.” She explained how the virus is now spreading to new countries, on top of the 31 countries and territories in Latin America and the Caribbean where local transmission has already been documented. She also stated that there is a growing body of evidence linking the virus to microcephaly, which so far has been reported in both Brazil and French Polynesia, though they expect more cases to occur in the next few months, particularly in Colombia.

“Reports and investigations from several countries strongly suggest that sexual transmission of the virus is more common than previously assumed,” said Chan. “Women who are pregnant in affected countries or travel to these countries are understandably deeply worried.”

It has been difficult for scientists to definitively state whether or not the virus is causing babies to be born with microcephaly, in which their developing infants’ heads do not grow to the correct size. Before the outbreak in Brazil in 2014, there were 147 cases of the developmental condition reported, yet a year later that number spiked to 2,782 cases.

The Zika virus has been found to preferentially infect brain tissue, and has been detected in the blood and cerebrospinal fluid of fetuses following miscarriages, still births, and abortions from mothers infected with the virus. It has also been found in the amniotic fluid, and “evidence shows it can cross the placental barrier and infect the fetus,” says Chan. While this is not conclusive, it strongly points to the disease's role in several documented birth abnormalities associated with Zika, including microcephaly. 

“The Committee underscored the increasing strength of evidence showing a likely association between Zika infection and fetal malformations and neurological disorders,” concludes Chan. With scientists around the world working to stop the virus, it is hoped that a vaccine could be ready for human trials in the United States later this year.