Researchers at the Institut Pasteur estimate that the risk of microcephaly is 1% when the infection occurs early in pregnancy.
Considered harmless until very recently, the Zika virus is now accused of causing Guillain-Barré syndrome in adults and microcephaly in the fetus. And the evidence is mounting against him. A study led by the Institut Pasteur quantified the risk of microcephaly for a fetus whose mother was infected with this virus transmitted by the mosquito Aedes. This work was published on Wednesday in the journal The Lancet.
Since spring 2015, an unprecedented epidemic has hit South America and the Caribbean. This outbreak coincides with an abnormal increase in cases of microcephaly. But many uncertainties remain. “We do not know how many pregnant women have been infected and the number of microcephaly. This is the reason why it is complicated to estimate the risk of this malformation in real time ”, explains to Why actor Dr Simon Cauchemez, first author of this study and head of the Mathematical Modeling of Infectious Diseases unit. According to the latest data from the Brazilian authorities, nearly 5,000 children are said to be affected by this congenital malformation.
In this context, French scientists analyzed data from the 2013 epidemic in French Polynesia. In total, 8 cases of microcephaly in fetuses or babies were reported between September 2013 and July 2015. Their retrospective analysis establishes that of these 8 cases, 7 occurred 4 months after the end of the epidemic, indicating a strong temporal association. between the Zika outbreak and this birth defect.
A 1% risk in the first trimester of pregnancy
Using mathematical models, the researchers then tried to determine the period of pregnancy most at risk by simulating different scenarios. “We estimate that the risk of microcephaly is 1% when the pregnant woman is infected with the Zika virus during the first trimester of pregnancy,” says Dr. Simon Cauchemez, adding that the risk during the second or third trimester is not excluded either.
Compared to other viral infections like herpes or rubella, the risk associated with Zika virus is very low. “For rubella, this risk is greater than 40%. But what must be taken into account is the number of women involved. Because every year in France, less than 10 pregnant women are infected with rubella, while during the Zika epidemic in Polynesia, 66% of the population were affected, which means that a large number of women pregnant were contaminated ”, notes the researcher.
Simon Nightmare, Head of the Mathematical Modeling of Infectious Diseases Unit at the Institut Pasteur: ” When we look at the Zika epidemic in Polynesia, we observe a very significant increase in cases of microcephaly … “
Furthermore, this analysis reinforces the evidence reported in recent weeks. These have demonstrated the presence of the virus in the amniotic fluid of mothers as well as in the brain tissues of stillbirths. Other work has also shown that the Zika virus is capable of infecting a particular type of neuronal stem cells at the origin of the cerebral cortex leading to their destruction, or an abnormal evolution.
Lots of damage
Recent work further shows that Zika’s devastation in the fetus is not limited to microcephaly. Cohort studies will therefore be carried out in the West Indies and Latin America to assess the proportion of pregnant women infected with Zika. Pregnancy monitoring will also be implemented in order to quantify the risk according to the term and identify all these malformations. In addition to these cohorts, studies on animal models will be implemented to better understand how the infection takes place and the biological mechanisms that can explain the occurrence of these malformations. This work is crucial for finding therapeutic solutions and developing vaccines.
However, these treatments will not see the light of day for 3 years according to the World Health Organization (WHO). Protection against mosquito bites remains the only effective way to escape Zika fever. For women who are pregnant or planning to become pregnant, the WHO and health authorities in many countries, including France, strongly advise against going to affected countries. They also recommend having safe sex.
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