The global spread of the Zika virus has been expected to mirror certain other diseases also transmitted by the Aedes mosquito. This mosquito, present throughout the tropical and sub-tropical globe, is responsible for spreading a range of illnesses, from dengue and chikungunya to yellow fever and the ‘newer’ Zika virus.
Following its emergence in South America, Zika spread to South-East Asia in 2016. As India has recently suffered large epidemics of both dengue fever and chikungunya, many thought it was just a matter of time before Zika would affect this country of 1.3 billion people. The recent report from the WHO confirms that Zika has indeed now spread to India as 3 cases have been identified, all from a single medical facility in Ahmadabad.
There are several interesting features of this report, however. Firstly, these infections occurred some time ago: 2 cases from November 2016 and 1 from January of this year, leading to some speculation as to the cause of the delay in notification and the possibility of a cover-up. Secondly, two of the Zika-positive samples were from pregnant women: one positive result came from a routine screening and the other from a recent mother who had serendipitously developed symptoms of a viral infection in hospital - and so was tested. Taking all these factors together, it seems very likely that there have been many more Zika infections in the area, and the true number of Zika infections remains unknown.
It is not certain at present why some countries have had higher complication rates from Zika; Brazil for example has reported thousands of cases of microcephaly, but some countries have reported few such complications. One suggestion is that another factor may be required in addition to the Zika virus to produce these devastating complications. This so-called Zika-plus theory has several candidates for the extra factor required, but the most likely factor needed may be prior infection with the dengue virus. As India has had large recent dengue epidemics, there is a chance that the population may already be primed for complications following a Zika outbreak. This is the fear that has concerned many who have seen the transmission of Zika to India as somewhat inevitable. Let’s hope we are wrong.