A pregnant woman infected by zika virus does not face a greater risk of giving birth to a baby with microcephaly if she has previously been exposed to dengue virus, according to a Brazilian study that compared data for pregnant women in Rio de Janeiro and Manaus.
A zika epidemic broke out in Brazil in 2015-16 in areas where dengue is endemic. Both viruses are transmitted by the mosquito Aedes aegypti. Some of the states affected by the zika epidemic reported a rise in cases of microcephaly, a rare neurological disorder in which the baby's brain fails to develop completely. Others saw no such rise.
According to this new study by Brazilian researchers, two factors explain the rise in microcephaly in only some areas: the high zika attack rate, and the mother's having contracted the virus in the first trimester of pregnancy. The term attack rate in epidemiology refers to the number of cases divided by the total population.
The study was supported by FAPESP via two projects (16/15021-1 and 13/21719-3), and was conducted under the aegis of the Network for Research on Zika Virus in São Paulo (Rede Zika). The results are described in an article in Viruses, a peer-reviewed open-access journal published by the Multidisciplinary Digital Publishing Institute (MDPI). The article appeared in late April in a special issue on zika and pregnancy.
"The discrepancies between regions in terms of the numbers of reported cases of microcephaly during the zika epidemic were puzzling. One of the hypotheses was that prior exposure to dengue might make zika more severe, but in the state of São Paulo there weren't many adverse effects of zika even though the region is highly endemic for dengue, so we decided to try to find out what could explain the differences," said virologist Maurício Lacerda Nogueira, a professor at the São José do Rio Preto Medical School (FAMERP) in São Paulo, and co-principal investigator for the study alongside Patrícia Brasil, a researcher at Oswaldo Cruz Foundation (FIOCRUZ) in Rio de Janeiro.
According to Nogueira, their attention was drawn to the attack rate when they analyzed the data for the two state capitals. In Rio de Janeiro, where many cases of microcephaly were reported, the number of people infected by zika…