Although the World Health Organization and the CDC have both accepted that there is enough evidence to assume causality between maternal Zika virus infection and an increase in severe microcephalic births (see NEJM: CDC Concludes Zika Causes Microcephaly & Other Birth Defects), the search for additional evidence continues.
It is still unknown whether there are any co-factors that might contribute to a bad fetal outcome, and while our understanding of how the virus causes these types of congenital defects is growing, it is still limited.
Yesterday The Lancet published a correspondence from researchers in Brazil who tested serum and cerebrospinal fluid from 31 neonates with microcephaly, and found IgM antibodies for Zika in 30 (97%) of their CSF samples.
First a link to the abstract, then I'll be back with a bit more.
Published Online: 18 April 2016
The epidemic of microcephaly in Brazil has been declared a Public Health Emergency of International Concern by WHO.1 The declaration states that a causal relationship between Zika virus infection during pregnancy and microcephaly is strongly suspected, although not yet scientifically proven.1 The hesitancy to accept causation in the presence of much epidemiological circumstantial evidence is due to the paucity of laboratory confirmation of Zika virus in affected neonates. Here, we report the serological confirmation of Zika virus infection in the CNS of 30 neonates with microcephaly.
(Continue . . . )
The detection of IgM in the neonate's CSF is significant because only IgG antibodies are capable of crossing the the placental or brain barriers, indicating these neonates experienced Zika infection of their central nervous systems.
You can follow the link to read the entire report (which is brief), but the author's concluded:
Zika-specific IgM was detected in 30 (97%) of 31 CSF samples and in 28 (90%) of 31 serum samples. One CSF sample tested negative. Monotypic response to Zika virus in the CSF was confirmed by plaque reduction neutralisation test. Since IgM does not cross either the placenta barrier or the blood–brain barrier, the presence of IgM in the CSF indicates that the neonate had the infection in the CNS.3, 4
The finding of Zika-specific IgM in the CSF of those 30 of 31 neonates with brain abnormalities indicates that they had a congenital infection with Zika virus. We believe that this is very strong evidence that the microcephaly was a consequence of Zika virus infection.