High temperatures help mosquitos and the virus reproduce; rain has less effect than thought
From the ashes of a devastating Zika virus outbreak last year, scientists are piecing together how it happened, and they’re using climate variables to get ahead of the next pandemic.
The Zika virus rampaged through the Americas in 2015 and 2016, charging out of Brazil and into neighboring countries inside the Aedes aegypti and Aedes albopictus mosquitoes.
Named for the Zika Forest in Uganda, where it was first isolated in 1947, the disease usually presents with mild symptoms, or none at all. But the outbreak last year caused a surge of microcephaly, a birth defect, and an increase in Guillain-Barré syndrome, which can lead to paralysis.
Pictures of babies with shrunken heads and immobilized victims led to a coordinated effort in afflicted countries and across borders to track and fight the infection.
“This response has been the most complex outbreak response for CDC,” said Ben Beard, deputy director of the Centers for Disease Control and Prevention’s Division of Vector-Borne Diseases, in an email.
“It has required expertise from across the agency, including subject matter experts in pregnancy and birth defects, sexually transmitted infections, mosquito control, laboratory science, travelers’ health, virology, transfusion medicine, and communication science,” he added.
As of July 19, the CDC reported 5,392 cases of Zika in U.S. states since 2015, including 224 infections acquired locally. The CDC also reported 36,986 Zika cases in U.S. territories like Puerto Rico.
There is no vaccine, and there are only limited treatment options. So health officials are concentrating mosquito control measures to prevent new Zika infections. They’re also using travel advisories for pregnant women, blood testing and public information campaigns.
Researchers are using the unprecedented outbreak to study the emerging disease in real time. And they’re uncovering lessons that could buy time for health officials to mobilize a response to mosquito-borne disease before another outbreak occurs.
Beard explained that Zika infections, like almost all diseases spread by mosquitoes, hinge on two variables: infected hosts and capable carriers.
For Zika to spread, a mosquito that’s able to carry the virus has to bite a person already carrying it. The virus then needs enough time to reproduce inside the mosquito. The insect then has to bite an uninfected person. Health officials have also found that the virus can be spread sexually.
This means that people are key to spreading the disease across borders and oceans—not just mosquitoes.
“The virus was introduced by infected travelers returning to the United States from an area with active Zika transmission,” Beard said. “Because this was the first global outbreak of Zika virus, virtually everyone in the United States (and globally) had not been infected and was therefore susceptible to infection.”
Predicting human behavior is difficult, so some scientists focused on the mosquito.
The number of Aedes mosquitoes rises and falls depending on temperature and rainfall, so researchers reasoned that climate variables could portend their spread.
In a paper published earlier this month in the journal Frontiers in Microbiology, researchers asked, “Could the recent zika epidemic have been predicted?”
The short answer is no.
However, they used climate data from afflicted regions to build a model for Aedes populations. That helped researchers highlight potential infection hot spots for mosquito-borne disease transmission one to three months in advance.
“Both the mosquitos that transmit Zika and the virus itself are climate-sensitive,” said co-author Ángel Muñoz, a postdoctoral research associate in atmospheric and oceanic science at Princeton University, in an email.
“High temperatures, like the ones observed during the record-breaking years 2015 and 2016, generally increase the virus replication rates and also the speed of mosquito reproduction,” he explained. “The overall effect of high temperatures is an increase in the potential risk of transmission.”
There are three main time scales to consider when it comes to warming: annual temperature variation from factors like warming in the Pacific Ocean during El Niño years, decadal temperature swings and long-term temperature increases from global warming.
Last year, all three trends were on an upswing, leading to an unusually potent brew for diseases carried by Aedes mosquitoes.
There were also paradoxes. It rained a lot last year, a trait that would seem to drive mosquito populations and the virus. But that didn’t necessarily happen.
“We had very high rates of Zika transmission where it was very warm and very dry,” said co-author Madeleine Thomson, a senior research scientist at the International Research Institute for Climate and Society at Columbia University. “In poor areas, people don’t necessarily have access to good running water, so they store water, and that water storage increases [mosquito] breeding sites.”
On the other hand, periods of heavy rain created stagnant water pools that became ideal breeding sites for Aedes mosquitoes.
With these factors in mind, researchers constructed a computer model and tested it to see how well its projections aligned with how the virus actually spread. They could see climate signals of an outbreak a month ahead of time and, in some geographical areas, as much as three months before it might happen.
There are a few caveats. The scientists said the model could forecast at the country level and for some large regions, but it does not provide useful enough information at smaller scales, like cities or villages.
They also note that Aedes mosquitoes also spread dengue and chikungunya, two other notorious tropical diseases, so the model effectively measures risks of all three diseases rather than any one of them alone.
Since Zika is a new disease in the Americas, it’s still too early to tell whether it will follow in the footsteps of its cousins or forge its own path through the Americas or around the world.
“The model is not going to differentiate whether it’s Zika or dengue or chikungunya,” Thomson said. “It’s just telling you that transmission suitability is up.”
She added that the key to filling in the blanks with infection forecasts is better data, getting more granular information over smaller regions and measuring over different time scales.
As the climate changes and humanity becomes more mobile across the planet, health officials are bracing for viruses like Zika to take root in new environs and for new infections to emerge in its wake.
“We are seeing an accelerated threat from mosquito-borne diseases overall,” said the CDC’s Beard. “Over the past few decades, we have seen a resurgence of dengue and the introduction of West Nile, chikungunya, and now Zika virus into the Western Hemisphere.”
“It is very hard to determine which one might come next—Zika virus wasn’t on anyone’s radar screen before it became an international threat,” he added. “What we do know is that in this age of globalization, more will be coming—we just don’t know from where or when.”