This is a common question that many have used to question the link between Zika and microcephaly. It is a valid point. After all, viruses have been around forever so if Zika caused microcephaly wouldn’t we already know that?
The answer is pretty simple – Zika is actually a relatively new virus for humans.
Zika was first isolated from primates in Uganda in 1947. The first reported case in humans was in 1952.
Between 1952 and 2007 Zika seemed to grumble along a narrow equatorial zone -mostly affecting primates. For humans it was an infrequent, obscure virus.
Before 2007 there were 14 known cases of Zika affected humans
Yes, 14 known cases. It is fair to say we knew nothing about Zika and humans before 2007 except 14 people didn’t get too sick and they all lived.
The first big outbreak was in 2007 on Yap Island. There were 49 confirmed cases and 59 suspected (meaning people who got sick and it was either proven or very likely their illness was due to Zika). However, many more people were infected than became ill as 74% of the population aged three and over had evidence in their blood of a recent Zika infection. Yap has a relatively small population (the census at the time was just over 7,000 people). The birth rate is approximately 25/1,000, so 175 births/year. As the infection rate with Zika is about 75%, the outbreak only lasted a few months, and if Zika is the cause of microcephaly the attack rate is clearly not 100% it is unlikely there would have been enough pregnancies exposed at the right time.
The second outbreak of Zika was in 2013 in French Polynesia. Because a test for Zika was developed in 2006 and the outbreak on Yap was only 5,000 miles away the medical personnel were prepared for the possibility of Zika testing wise. Of the 270,000 residents about 20,000 were infected with Zika. The outbreak ended almost as abruptly as it started and no one died or became very ill, however, there were signs that the virus may have triggered Guillain-Barre, a devastating neurological illness that can be triggered by a viral infection, as the number of cases/year jumped from an average of two to 42. There were also 17 cases of fetuses with serious birth defects involving the nervous system. Those cases are still under investigation.
Which brings us to Brazil where an estimated 440,000 -1.3 million people were infected with Zika in 2015. Brazil and El Salvador and Colombia have seen a spike in Guillain-Barre and the incidence of microcephaly has jumped 20-fold in Brazil. Now it is possible that this spike in microcephaly is over reporting or there is another cause, however, the Zika virus has been identified in miscarried specimens and, as reported in the New England Journal of Medicine, in the brain of a fetus aborted at 32 weeks because of severe microcephaly.
More work is needed to draw the line from Zika to microcephaly, however, the lack of definitive previous cases of Zika-related birth defects shouldn’t be a reason to disprove the hypothesis. Before 2013 there were unlikely enough people who had ever been infected to know anything about the impact of Zika on a developing fetus and there are 17 cases from the 2013 outbreak under investigation.