Zika is Zika Until Proven Otherwise
The World Health Organization (WHO) recognizes two major lineages of Zika, one from Africa, and another from Asia.
“We assume that all Zikas are equally dangerous," said Dr. Derek Gatherer, a biomedical expert at Lancaster University in Britain.
The first Zika originated in Africa, where it was discovered in 1947, has not been identified outside of that continent.
The second version, the Asian lineage includes strains that have been reported in Asia, the Western Pacific, Brazil and, notably, North America.
The Asian lineage was first isolated in the 1960s in mosquitoes in Malaysia. But some studies suggest the virus has been infecting people there since the 1950s. In the late 1970s, seven cases of human infection in Indonesia were reported.
In 2015, Zika arrived in Brazil, causing spikes in an array of neurological birth defects now called congenital Zika virus syndrome. Additionally, some Guillain-Barre syndrome is Zika related, and is a neurological disorder that can lead to temporary paralysis.
Zika is spread mostly by the bite of an infected Aedes species mosquito (Ae. aegypti and Ae. albopictus). These mosquitoes bite during the day and night. Zika can be passed from a pregnant woman to her fetus. Infection during pregnancy can cause certain birth defects.
There is no vaccine or medicine for Zika.
Viruses mutate rapidly, which can lead to strains that are more contagious and more virulent. Many researchers theorized early on that the devastation in Brazil was caused by an Asian strain that had mutated dramatically.
That theory relies, among other things, on the absence of Zika-related microcephaly in Asia.
So when Zika broke out in parts of Asia earlier this year, researchers were on the lookout.
If researchers were to connect a case of microcephaly to an older Asian strain – and not one that boomeranged back from Brazil -- it would debunk the early theory.
It would mean Zika "did not mutate into a microcephaly-causing variant as it crossed the Pacific," Gatherer said.
“It is a top research priority at WHO”, said Dr Peter Salama, executive director of the agency's health emergencies program, in a press briefing.
"That is a critical question because it has real public health implication for African or Asian countries that already have Zika virus transmission," Salama said.
"We are all following this extremely closely."
WHO experts believe Zika moved explosively in the Americas because there was no prior exposure. It's not clear how widely Zika has circulated in Africa and Asia, whether there could be pockets of natural immunity.
Moreover, whether immunity to one strain would confer immunity to another.
“One recent review of studies suggests 15 to 40 percent of the population in some African and Asian countries may have been previously infected with Zika,” said Alessandro Vespignani, a professor at Northeastern University.
WHO researchers also believe it's possible that microcephaly went undetected in parts of Asia and Africa where birth defects weren't well tracked.
Microcephaly is a condition where a baby’s head is much smaller than expected. During pregnancy, a baby’s head grows because the baby’s brain grows. Microcephaly can occur because a baby’s brain has not developed properly during pregnancy or has stopped growing after birth, which results in a smaller head size.
Microcephaly can be an isolated condition, meaning that it can occur with no other major birth defects, or it can occur in combination with other major birth defects.