As I noted when I first wrote about Zika virus in January, researchers haven’t definititively established the link between the virus and microcephaly  — abornormally small brains now seen in thousands of infants whose mothers had (confirmed or suspected) Zika infections during pregnancy. Over the past few weeks, though, new published research has provided more evidence linking Zika virus to poor health outcomes. Here are a few highlights:

Zika and microcephaly: Patricia Brasil of Fundação Oswaldo Cruz, Karin Nelson-Saines of UCLA, and their colleagues published their findings in the New England Journal of Medicine, and Donald McNeil Jr and Catherine Saint Louis reported on them in the New York Times. The researchers prospectively enrolled 88 pregnant women who attented a Rio de Janeiro hospital from September 2015 through February 2016 and reported Zika symptoms within the previous five days. Of the 88 women, 72 had positive blood and/or urine tests for Zika. Ultrasounds were performed for 42 women who tested positive and all 16 who tested negative, and found fetal abnormalities in 12 of the 42 women with positive results, and in none of those with negative results. Two of the 42 women for whom no fetal abnormalities were detected by ultrasound experienced fetal deaths. “This is going to have a chilling effect,” said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told McNeil and Saint Louis. “Now there’s almost no doubt that Zika is the cause.”

Zika’s effects on stem cells: Hengli Tang of Florida State University, Guo-li Ming of Johns Hopkins University, and their colleagues published their findings in Cell Stem Cell; Lena Sun and Brady Dennis wrote about them in the Washington Post. The researchers exposed lab-grown human stem cells to Zika virus, and found that the virus quickly infected most of the cortical neuron progenitors, which form the brain’s cortex. Infections spread less quickly through other kinds of stem cells and immature cortical neurons, they found. Joseph Gleeson of the Howard Hughes Medical Institute, who wasn’t involved with the study, told Sun and Dennis that this research “strengthens the case that Zika is a culprit behind microcephaly.”

Zika and Guillain-Barré: Van-Mai Cao-Lormeau of Institut Louis Malardé in French Polynesia, Alexandre Blake of Institut Pasteur in France, and their colleagues published their findings in The Lancet, and Karen Weintraub covered the study in STAT. French Polynesia experienced a Zika outbreak from October 2013 to April 2014, and 42 people were diagnosed with Guillain-Barré syndrome, in which the immune system attacks the nervous system, causing paralysis and sometimes death. One question has been whether Zika is responsible or whether cases of Guillain-Barré in Zika-affected countries could be due to other illnesses; dengue, which is carried by the same type of mosquito that spreads Zika, is one candidate. The researchers found that of the 42 patients with Guillain-Barré, 41 (98%) had evidence of a prior Zika infection. The researchers also created two groups of controls: one included people who presented at hospitals with other febrile illnesses, matched on age, sex, and residence with the Guillain-Barré cases, and another with Zika symptoms but no neurological symptoms, matched on age. Evidence of dengue infection did not differ significantly between the cases and controls. “This is the first study providing evidence for Zika virus infection causing Guillain-Barré syndrome,” its authors conclude. “Because Zika virus is spreading rapidly across the Americas, at risk countries need to prepare for adequate intensive care beds capacity to manage patients with Guillain-Barré syndrome.”

Meanwhile, CDC has reported on nine US women infected with Zika while pregnant (see Maryn McKenna’s writeup at Germination), and advises pregnant women to consider postponing travel to any area where Zika virus transmission is ongoing — which currently includes the Caribbean and South America.