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EVIDENCE IS MOUNTING

Scientists in the hot zone of the epidemic explain why Zika’s link to birth defects is getting stronger

By Akshat Rathi

The Zika outbreak continues to spread explosively. The latest status report from the World Health Organization says that local transmission has been reported in 52 countries and territories around the world.

Though mostly benign in adults, it is Zika’s link to serious birth defects that has raised the most questions. Researchers from Brazil and their collaborators took to Reddit to answer some of those questions. Quartz has edited and condensed a selection of their answers for clarity.

My wife and I are leaving for a cruise on Sunday to Mexico, Honduras, and Belize. I’ve been told if she gets Zika, we shouldn’t even try for a baby for two years. Is that true?

We don’t have all of the evidence needed to provide firm recommendations, but based on what we do know Zika does not cause a chronic infection and the period that the virus persists is likely short. It seems like the virus may persist for weeks in certain secretions like urine and semen, but not for periods over many months or years. We also haven’t seen cases of women exposed prior to pregnancy who have transmitted Zika virus to fetus after conception.

We don’t know exactly how long the virus remains in the body. Sufficient numbers of patients have not been studied to get good estimates and we don’t know if the virus in bodily fluids remains infectious.

How certain are you of the link between Zika virus and fetal complications such as microcephaly? What type of evidence is needed to confirm the link?

Several lines of study need to be pursued to evaluate whether there is a causal association. The possibility is based on: first, the association between the Zika virus transmission in Brazil in early 2015 affecting the northeastern region; second, the appearance of microcephaly cases detection of viruses in several cases of spontaneous abortions, still births and live born infants. More recently a study followed pregnant women who had a documented infection and found that defects, including microcephaly, developed in 29% of the fetuses during gestation, providing additional but not absolute evidence between cause and effect.

Additional evidence will need to evaluate the strength and generalizability of the association, which is being done by several groups in different sites in Brazil in case-control investigations. Further, experimental animal studies will be important in providing clear evidence that the virus indeed causes disease manifestations similar to seen in the cases observed in the outbreak in Brazil.

It also seemed as though of the babies with microcephaly only a very small percent had the Zika virus. What could be the significance of that?

The small percentage that the Zika virus has been detected in microcephaly cases is due to many factors, but one prominent reason is the lack of access to laboratory diagnosis. Other reasons include that a proportion of cases identified initially as suspected microcephaly may not have microcephaly given the case definitions used (false positives) or may be due to other causes than Zika.

What do you think about the possible link between the anti-larval chemicals used in Brazil and the rise in microcephaly?

We didn’t see any relation between agrotoxic exposure and fetal microcephaly in our cases. Our group is studying cases of birth defects since July 2015, and the link with the Zika virus infection is very strong.

What would explain the recent association with birth defects and exposure to the Zika virus?

Probably, the low birth rates in countries affected in French Polynesia was not able to detect the association between microcephaly and the Zika virus. Also, the virus strain circulating in Brazil is not the same as the one in Africa, where Zika cases have sporadically occurred for decades.

Would Zika follow the same outbreak patterns as dengue in continental US?

We can not predict with confidence how local transmission of Zika will evolve, but it seems likely that it will parallel that of dengue where there may be a few clusters of transmission in south Texas and south Florida. We need to understand in more detail the transmission determinants of the Zika virus in the mosquito host to be able to project the future risk. At this point we do not know if Zika will be “worse” or “better” than dengue.

Do we know how the Zika virus crosses the placental/umbilical boundary?

No, we still have no answer for this question.

Are there possible animal models for studying the effects of the Zika virus?

Researchers at the Federal University of Rio de Janeiro are working on the effect of the virus in contact with experimental brains. Also, researchers at the University of Wisconsin are currently conducting non-human primate studies and releasing their data in real-time.

Do your observations of birth defects linked to the Zika virus look similar to other viral infections of fetuses? 

The presentation of the Zika virus in fetuses does parallel those seen for other congenital infections such as cytomegalovirus, toxoplasmosis, etc with respect to microcephaly, cerebral calcifications, cerebral atrophy and ventriculomegaly. However there does seem to be important differences. First, the unusually severe presentations of the cases we are seeing in Brazil. Second, although you can see such severe central nervous system manifestations with other infections such as CMV, they usually are accompanied by manifestations that affect other parts of the fetus. Third, there are specific lesions such as loss of the size of cerebella which seem somewhat unique in cases which we believe are due to Zika.

I am heading to Colombia this weekend, do I have anything to worry about (such as Guillain Barre) and what I can I do to stay safe?

The use of icaridin-based insect repellents and use of long sleeves shirts should give you good personal protection.

Microcephaly incidents predate the Zika outbreak. Are they more closely linked to other factors such as poor nutrition or environmental pollutants than Zika?

You are correct that there were cases of microcephaly prior to the outbreak. Microcephaly is causes by a large list of diseases including other congenital infections of the fetus as well as genetic disorders. These were likely underreported in Brazil prior to the recent Zika outbreak. However, there does appear to be an increase in microcephaly cases and the severity of disease in these cases which makes us highly suspect a possible association.

Akshat Rathi
Senior reporter
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