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 It also presented a mystery: why had a virus that had been little
more than a footnote in the annals of infectious diseases taken such
a devastating turn in the Americas? How had Africa and Asia, where
Zika had quietly circulated for decades, escaped with no reports of
major outbreaks or serious complications?
Scientists initially theorized that Zika’s long tenure in Africa and
Asia may have conferred widespread immunity. Or, perhaps older
strains were less virulent than the one linked in Brazil to more
than 2,100 cases of microcephaly, a birth defect characterized by
arrested brain development.
Now, amid outbreaks in Singapore, Thailand, Vietnam and other parts
of Southeast Asia, a much graver explanation is taking shape:
perhaps the menace has been there all along but neurological
complications simply escaped official notice.
The question is driving several research teams, according to leading
infectious disease experts and public health officials.
The answer is immediately important for Asia, the region most
affected by Zika after the Americas. Thailand has been hardest hit
with more than 680 reported Zika infections this year, followed by
Singapore with more than 450 and Vietnam with as many as 60.
Much of the population lives in the so-called "dengue belt," where
mosquito-borne diseases are prevalent. And vulnerable countries –
including Vietnam, the Philippines, Pakistan and Bangladesh - are
ill-prepared to handle an outbreak with any serious consequences,
experts said.
Lacking evidence of varying degrees of virulence, public health
officials have warned Asia leaders to prepare for the worst. The
scientific community is following similar assumptions.
"Zika is Zika until proven otherwise. We assume that all Zikas are
equally dangerous," said Dr. Derek Gatherer, a biomedical expert at
Lancaster University in Britain.
WHICH ZIKA?
The World Health Organization recognizes two major lineages of Zika.
The first originated in Africa, where it was discovered in 1947 and
has not been identified outside that continent. The Asian lineage
includes strains that have been reported in Asia, the Western
Pacific, Cabo Verde and, notably, the Americas, including Brazil.
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.
The first record of a widespread outbreak was in 2007 on
Micronesia's Island of Yap.
Experts began to suspect a link to birth defects during a 2013
outbreak in French Polynesia when doctors reported eight cases of
microcephaly and 11 other cases of fetal malformation.
In 2015, it hit Brazil, causing spikes in an array of neurological
birth defects now called congenital Zika virus syndrome, as well as
Guillain-Barre syndrome, a neurological disorder that can lead to
temporary paralysis.
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.
At least three microcephaly cases have been identified in Asia, but
the verdict is still out.
For two microcephaly cases in Thailand, public health officials
could not determine whether the mothers had an older Asian strain of
Zika or a newer one that returned from the Americas, said Dr Boris
Pavlin, WHO's acting Zika incident manager at a recent briefing.
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In Vietnam, where there have been no reports of imported Zika
infection, officials are investigating a third case of microcephaly.
If it is linked to Zika, Pavlin said it would suggest the older
strains there could cause microcephaly and, perhaps, Guillain-Barre.
In Malaysia, where at least six cases of Zika infection have been
reported, authorities have identified both an older Southeast Asian
strain and one similar to the strain in the Americas, suggesting the
possibility that strains from both regions could be circulating in
some countries.
The hunt is on in Africa as well. In Guinea-Bissau, five
microcephaly cases are under investigation to determine whether the
African lineage of Zika can cause microcephaly.
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 Tuesday.
"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."
HERD IMMUNITY
Scientists also are trying to learn whether people in places where
Zika is endemic are protected by "herd immunity." The phenomenon
limits the spread of virus when enough of a population is inoculated
against infection through vaccination, prior exposure or both.
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 – and, importantly, 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 of health sciences at Northeastern University in Boston.
That's far below the 80 percent population immunity one mosquito
borne virus expert estimated in the journal Science would be
necessary to block Zika.
Researchers also believe it's possible that microcephaly went
undetected in parts of Asia and Africa where birth defects weren't
well tracked.
That too, is under investigation, said Dr David Heymann, Chair of
the WHO Emergency Committee, at a press briefing last week.
"Now," he said, "countries are beginning to look back into their
records to see on their registries what the levels of microcephaly
have been."
(Reporting by Julie Steenhuysen in Chicago and Amy Sawitta Lefevre
in Bangkok; Additional reporting by Mai Nguyen and My Pham in Hanoi
and Susan Heavey in Washington; Editing by Michele Gershberg and
Lisa Girion)
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