The epidemic of microcephaly -- a birth defect where a baby's head is smaller -- in Brazil has raised serious questions about its assumed connection with Zika virus, a new study explains.
Prenatal Zika virus infection has been linked to adverse pregnancy and birth outcomes, most notably microcephaly and other serious brain anomalies.
In Brazil, the microcephaly rate soared with more than 1,500 confirmed cases.
A new study in Colombia found that the total number of pregnancies with Zika infections is much larger, with 11,944 cases with Zika symptoms being observed in clinical settings. But, no cases of microcephaly occurred in all of these pregnancies.
However, the report cites four cases of microcephaly with Zika in the general population that did not report any Zika symptoms.
This implies that there are many more cases of Zika infection that are not reported, said Yaneer Bar-Yam from the New England Complex Systems Institute in the US.
The Zika and microcephaly cases that are not part of the study show that there are many more pregnancies affected by Zika without symptoms.
"If Zika is to blame for microcephaly, where are the missing cases?" researchers asked.
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 researchers said.
In light of this evidence, the cause of microcephaly in Brazil should be reconsidered, said the paper published in the New England Journal of Medicine.
An alternative cause of microcephaly in Brazil could be the pesticide pyriproxyfen, which is cross-reactive with retinoic acid and is being used in drinking water, the researchers concluded.