As fear of the Zika virus spreads nearly as quickly as the pathogen itself, two new laboratory studies offer the first solid evidence for how it could cause brain defects in babies: The virus appears to preferentially kill developing brain cells. The observation bolsters the growing case for a connection between the virus, which is spreading rapidly across Latin America, and an increase in the number of cases of microcephaly, a birth defect in which the brain fails to grow properly. The new work, done independently by two groups, shows that the virus readily infects neural stem cells—the precursors of neurons and other brain cells—whether they are grown on cell culture plates or coaxed to form 3D minibrains called cerebral organoids.

The work “is going to be very important,” says Madeline Lancaster, a developmental biologist who studies human brain development at the Medical Research Council Laboratory of Molecular Biology in Cambridge, U.K. The results “are quite consistent with what you’re seeing in the babies with microcephaly.”

Zika virus, named after a forest in Uganda where it was first isolated decades ago, usually causes only mild symptoms in people, including fever and rash. But after the virus started spreading across northeastern Brazil last year, doctors there noticed a striking increase in the number of babies born with microcephaly. Many of the mothers reported having symptoms consistent with Zika infection during their pregnancies. But it has been difficult to prove a link between the virus and the birth defects because blood tests for Zika virus are only accurate for about a week after infection.

Nevertheless, circumstantial evidence has accumulated. Researchers have identified the virus in amniotic fluid of pregnant women whose fetuses were diagnosed with microcephaly and also in the brain tissue of a fetus diagnosed with the disorder. But because researchers had conducted scant research on the virus before this year, they had little data to suggest how the virus could cause such damage.

To gauge the virus’s possible effects on the developing brain, researchers at Johns Hopkins University in Baltimore, Maryland, and Florida State University in Tallahassee used induced pluripotent stem (iPS) cells to grow, in lab dishes, immature brain cells called human cortical neural progenitor cells. (iPS cells are adult cells that have been reprogrammed into stem cells that can grow into most of the tissues in the body.) They then exposed the neural progenitor cells to a lab strain of Zika virus.

The virus readily infected the neural stem cells, neuroscientists Hongjun Song and Guo-li Ming, virologist Hengli Tang, and their colleagues report today in Cell Stem Cell. Three days after the virus was applied, 85% of the cells in the culture dishes were infected. In contrast, when the virus was applied to cultures of fetal kidney cells, embryonic stem cells, and undifferentiated iPS cells, it infected fewer than 10% of the cells by day 3. Immature neurons derived from the neural progenitor cells were also less susceptible to the virus; 3 days after receiving a dose of the virus, fewer than 20% of those cells were infected.

The researchers noticed that the infected progenitor cells were not killed right away. Instead, the virus “hijacked the cells,” using the cellular machinery to replicate themselves, Song says. That helped the virus to spread quickly through the cell population, he says. His team also reports that infected cells grew more slowly and had interrupted cell division cycles, which could also contribute to microcephaly.

In a separate set of experiments, other researchers found that the virus can hamper the growth of another type of neural stem cell. In a preprint posted online on 2 March, neuroscientist Patricia Garcez and stem cell researcher Stevens Rehen at the D'Or Institute for Research and Education in Rio de Janiero, Brazil, report growing human iPS cells into clusters of neural stem cells called neurospheres, as well as into 3D organoids that in some ways resemble a miniature version of the human brain. When they infected the growing cells with Zika virus isolated from a Brazilian patient, the virus quickly killed most of the neurospheres and left the few survivors small and misshapen. Infected organoids grew to less than half their normal size.

Lancaster says the results echo earlier studies of gene mutations that cause microcephaly, which also affect neural progenitor cells. “You have two very different causes of microcephaly, but you see something very similar happening: a depletion of neural stem cells, and that would lead to fewer neurons” in the developing brain, she says.

Plenty of questions about the Zika virus and its apparent link to birth defects remain unanswered. Both Garcez and Song say they are now repeating their experiments with other viruses, including dengue, a virus closely related to Zika that is prevalent in the regions currently affected by the outbreak. (Some scientists suspect that previous exposure to other viruses could affect the outcome of Zika infections.) Researchers also still need to figure out how the virus crosses the placenta and infects the fetus directly, something most viruses can’t do.