Evidence for Causal Association Between Zika and Microcephaly

This week, the World Health Organization labelled the Zika virus a public health emergency. Here is a look at the evidence of the relationship between the virus and microcephaly.

In September 2015 Brazil announced the discovery of Zika in 2 infants born with microcephaly in Brazil. After the introduction of a microcephaly registration this autumn, the reported incidence of microcephaly was 20-fold higher than in 2010—2014. Earlier in 2015, in this same region, there had been an outbreak of the Zika virus, a new disease in this part of the world. Alarm bells went off.

Prior to this discovery, the Zika virus infection was considered to be a mild disease. This is why, even though there was a noticeable outbreak in Brazil a year ago, it was not brought to international attention. The game-changing factor is the possibility of a causal association between infection during gestation and fetal development. Furthermore, the vectors for Zika are widespread, so locations that haven’t been affected today may be dealing with its impact tomorrow. But let’s back up for a moment.

What evidence do we currently have for this relationship?

  • There was an increase in the incidence in microcephaly the same year there was an outbreak of Zika virus in northeastern Brazil.
  • So far, out of 4180 cases of microcephaly reported since October, 6 fetuses/infants were confirmed to have Zika virus in tissue, and 2 pregnant women with fetuses exhibiting microcephaly were found to have Zika virus in their amniotic fluid.
  • The Brazilian Ministry of Health launched an initial investigation including 35 affected infants born from August through October. Rash during pregnancy was used to represent Zika virus infection in the infants’ mothers. Overall, of the 35 mothers investigated, 26 reported rash during pregnancy. All of them had samples of their cerebral spinal fluid taken and sent for testing at a Brazilian laboratory. The results of these tests have not yet been released.
  • Of all of the cases of microcephaly reported in Brazil since October, 732 have been assessed. More than 60% of these cases have been discarded because they were not considered true cases or they were caused by a factor other than the Zika virus.

It is important to recognize this relationship is possible. There is a long history of diseases that can affect the development of the fetus if the mother becomes infected during gestation. Childhood vaccination has helped us to minimize many of these. However, the small amount of scientific information that has been available on Zika and pregnancy has caused mass panic.

El Salvador announced that women should postpone getting pregnant until 2018, and in certain high-risk areas, they have been advised to wait indefinitely. Airlines are refunding tickets that have been purchased for travel to affected areas. Press sources are reporting regularly and loudly with information that is often misleading and frantic. Confusion and inaccurate reporting led to a similar sense of hysteria during the Ebola outbreak last year. However, Ebola claimed thousands of human lives while the Zika virus is not known to be fatal.

For general recommendations, CDC is still stating that we need case control studies to confirm this relationship. On its Traveler Health page, CDC recommends that women who are pregnant reschedule travel to countries reporting Zika because the information on the association between the virus and microcephaly is “still evolving.” All other travelers are simply recommended to wear insect repellent, which are the same recommendations for nearly all non-vaccine preventable vector transmitted diseases. Though the World Health Organization did not declare the Zika virus a public health emergency until this week, the world was already treating it as one.

It is important to be well informed, especially on issues that can affect our health and our children’s health. However, before we, as healthy, non-pregnant adults, start canceling our trips, let’s take a deep breath and a step back, and let’s look at the facts.

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