Actor and prolific social media maven George Takei took some heat last week for promoting a post that claimed that the numerous cases of microcephaly seen in Brazil may have been caused by an insect growth inhibitor called Pyriproxyfen, rather than by the Zika virus. Scientists swear that that particular chemical is not dangerous to humans. Given the lack of oversight of the chemical industry, there is very little trust in such pronouncements, but there is no clear evidence linking Pyriproxyfen to microcephaly.
The evidence in favor of a connection between Zika and microcephaly is stronger, but not yet proven: Some dead microcephalic babies have been found with the virus in their brains, or in their amniotic fluid. Some mothers of microcephalic babies have been infected with Zika. But there are also sound reasons to doubt whether Zika is related to the microcephaly in Northeast Brazil.
First of all, despite heart-rending pictures of babies with almost no foreheads, it isn’t that clear that there has actually been an increase in microcephaly. Definitions of microcephaly vary between doctors. Medical statistics kept in that area are not that thorough or reliable. Also, some attribute the epidemic to the awareness effect, and suspect that microcephaly hasn’t attracted much attention before.
Second, one bit of research may indicate that an increase in microcephaly began in 2012, two years before the recognition of the increase in Zika virus. It may be that Zika began to increase earlier than noticed, or it may be that the two are unrelated.
Third, Zika outbreaks in other regions have not been shown to correspond to more cases of microcephaly. Again, there may be bad reporting and varying definitions of microcephaly in those regions, or there may be no connection.
And fourth, as noted in FiveThirtyEight’s, Why It’s So Hard To Prove Zika Is Causing Birth Defects:
Zika would not be the first virus to lead to microcephaly — rubella can also cause the condition — but it would be the first virus of its kind known to trigger it. Other flaviviruses, such as dengue and West Nile, are not known to cause microcephaly, and that’s perplexing, said Moore, the RAND pediatrician. “There are plenty of other viruses in this family, and none of them cause this.”