Washington: The number of missing cases of microcephaly epidemic in several countries raises serious questions about the assumed connection between Zika virus and microcephaly, according to a new study.
In Brazil, the rate of microcephaly, a birth defect where a baby's head is smaller than expected, soared with more than 1,500 confirmed cases.
However in Colombia, a recent study of nearly 12,000 pregnant women infected with Zika found zero microcephaly cases, researchers said.
According to a new report by the New England Complex Systems Institute (NECSI) in the US, the number of missing cases in Colombia and elsewhere raises serious questions about the assumed connection between Zika and microcephaly.
The results of a large study of pregnant Colombian women infected with Zika show that of the nearly 12,000 pregnant women with clinical symptoms of Zika infections until March 28, no cases of microcephaly were reported as of May 2.
At the same time, four cases of Zika and microcephaly were reported for women who were symptomless for Zika infections and therefore not included in the study itself.
The Zika and microcephaly cases that were not part of the study show that there are many more pregnancies affected by Zika without symptoms.
Because there are four cases of microcephaly with Zika, and none in the study, there should be about four times as many cases of Zika that are unreported. This means there are at least 60,000 Zika-infected pregnancies in Colombia.
The NECSI report analysed the data and showed that the four cases of Zika and microcephaly that were observed till April 28 are just what would be expected due to the background rate - of the 60,000 pregnancies about 20,000 births would already be expected.
The expected microcephaly rate for countries with no reported infections of 2-in-10,000 births gives exactly four cases, researchers said.
The study published in the New England Journal of Medicine also noted that until April 28 there has been a total of about 50 microcephaly cases in Colombia, of which only four have been connected with Zika.
The four cases are expected for the coincidence of Zika and microcephaly in the same pregnancies even if Zika is not the cause.
NECSI said the cause of microcephaly in Brazil should be reconsidered. One possibility that has been raised is the pesticide pyriproxyfen, which is applied to drinking water in some parts of Brazil to kill the larvae of the mosquitoes that transmit Zika.
Pyriproxyfen is an analogue for insect juvenile hormone which is cross reactive with retinoic acid, which is known to cause microcephaly, researchers said.
