Scientists trying to predict the future path of Zika say that 2.6
billion people living in parts of Asia and Africa could be at risk of
infection, based on a new analysis of travel, climate and mosquito
patterns in those regions.
Some of the most vulnerable countries include India, China, the Philippines, Indonesia, Nigeria, Vietnam, Pakistan and Bangladesh, according to the research.
Experts
caution that the study could overestimate the number of people at risk
because they don’t know whether Zika had already landed in some of these
countries in the past and allowed people to develop immunity. More than
two-thirds of people infected with Zika never get sick, and symptoms
are mild for those who do, so surveillance systems may have missed
cases.
Although Zika was first identified in 1947, the virus
wasn’t considered a major health threat until a major outbreak in Brazil
last year revealed that Zika can lead to severe birth defects when
pregnant women are infected.
In February, the World Health
Organization declared the spread of Zika a global emergency, and
epidemics have been sparked in at least 70 countries. In the last few
weeks, it has sickened more than 100 people in Singapore and started
spreading in Florida. Zika is mostly spread by a specific species of
tropical mosquito, but it can also be spread by sex and through blood
transfusions.
Researchers hope their new study will help officials plan ahead to possibly avoid some of the worst effects of Zika.
“For
countries with a finite amount of resources, this may help them use
those resources as efficiently as possible,” said Dr. Kamran Khan, an
infectious diseases physician and scientist at St. Michael’s Hospital in
Toronto, the study’s senior author.
To figure out where Zika
might gain a future foothold, researchers examined patterns of people
traveling from infected regions in the Americas to Africa and Asia and
combined that with an assessment of local conditions, including mosquito
populations. They used the spread of a related virus, dengue, as a
model for Zika since the same mosquito species transmits both diseases.
Dengue is not spread by sex, like Zika, but mosquitoes are responsible
for the vast majority of Zika cases globally.
The study was published online Thursday in the journal, Lancet.
Some
experts pointed out that the expected impact of Zika will depend
largely on whether people have any previous exposure to the virus — and
that is unknown.
“No one has ever looked, so we don’t know if
there is any pre-existing immunity to the virus,” said Dr. Abraham
Goorhuis of the University of Amsterdam, an author of a commentary that
accompanies the Lancet study. The virus in the Americas is an Asian
strain that was responsible for a large outbreak in French Polynesia and
other Pacific Islands in 2013 and 2014.
“If there was broad
circulation of this virus in Asia, then it could be that the risk of
Zika spreading to Asia won’t be as bad as we think,” Goorhuis said. He
said another big unknown was whether people who might have been exposed
to the African strain of Zika might be protected once the slightly
different Asian version arrives. It’s unclear how widely the original
1947 strain found in Africa may have spread.
Goorhuis also noted
it was possible Zika might eventually burn itself out after about a year
or so of circulation, the way a related virus, chikungunya, recently
did.
Others said scientists should seize the chance now to prevent Zika from becoming entrenched elsewhere.
Ben
Neuman, a virology professor at the University of Reading, said that
once the virus has started to spread by mosquitoes locally, and not just
by travelers, it can become entrenched in animals as well as people and
be extremely difficult to eradicate. “We have the opportunity right now
to prevent (Zika’s) spread and by doing so, halting the spread of
unnecessary suffering,” he said.