The growing presence of Zika virus in Brazil has captured international attention in the past week, as one report after another has come in of its apparent link to birth defects in the babies of infected mothers. The news has sparked travel advisories, new guidelines for the care of pregnant women during an outbreak and intense speculation on where the mosquito-borne disease — which had rarely ever been seen in the western hemisphere until last spring — might turn up next.
And scientists are starting to ponder whether climate change may play a role in its future spread.
Although Zika has not yet been detected extensively in the United States, except in a few isolated travel-related incidents, its new presence in South America — and, recently, Puerto Rico — has scientists guessing that it’s only a matter of time. The Centers for Disease Control and Prevention (CDC) notes that “because the Aedes species mosquitoes that spread Zika virus are found throughout the world, it is likely that outbreaks will spread to new countries.”
Predictions about the spread of infectious diseases like Zika often revolve around the movements of the hosts that carry them — in this case, mosquitoes belonging to the genus Aedes, which are known to carry a variety of notable infectious diseases, including yellow fever, dengue fever, Chikungunya and now Zika. Aedes aegypti, also known as the “yellow fever” mosquito, and Aedes albopictus, or the Asian tiger mosquito, are two common species in the Americas. A. aegypti seems to be the preferred host for Zika, although it’s likely that A. albopictus is also able to carry it.
The thing about these mosquitoes is that they tend to thrive best in warm, wet climates, giving rise to concerns that future climate change may help them multiply and even spread into new parts of the North American continent. This would be more than a mere annoyance to humans — scientists have warned that it could pose a significant public health risk with the potential for the increased transmission of mosquito-borne diseases.
In recent years, researchers have increasingly devoted themselves to the investigation of how future climate scenarios might affect these mosquito populations. And many have concluded that a warmer world is likely to be a boon to the bugs, allowing them reproduce faster, emerge earlier in the season, survive longer and even spread northward.
The appearance of Zika in the Americas, where it was likely carried by travelers from the eastern hemisphere, thus adds one more disease to the list of potential public health concerns under a warming scenario.
But University of Arizona professor and epidemiologist Heidi Brown cautions that the issue is not as simple as it looks — for Zika or for any other mosquito-borne disease. While a warmer climate may be a plus for mosquitoes, that doesn’t necessarily mean it’s a plus in all ways for the transmission of pathogens they carry.
The thing to remember, according to Brown, is that mosquitoes are not just “flying syringes.” In order to spread disease, they must first consume the blood of an infected person — and survive the encounter. They must then live long enough for the virus to make it out of their gut and up into their salivary glands. And then they must bite another, uninfected human and infect them.
So access to humans is key for mosquitoes to spread disease. And as Brown pointed out, the changing climate is also likely to have effects on human behavior, in a way that may or may not make it easier for mosquitoes to get to them.
“With warming, we change our behavior,” she said. “We might go out earlier, we might go out at different times of the day. People working outdoors, people not working outdoors, whether we’re running air conditioning and staying indoors — it’s a combination of these things.” Factors such as the type of clothing we choose to wear or whether we apply insect repellent when we go out also plays a role in whether mosquitoes are able to get to us.
So merely looking at the behavior of mosquitoes may be a simplistic way of modeling future disease transmission, Brown said. Without better knowledge of how human populations react to different climate scenarios, it would be difficult to come up a truly accurate projection for how a disease will spread.
And while studies of mosquitoes have revealed fairly consistent patterns of their relationship with climate, studies of the association between climate and infectious disease burdens — dengue fever, most commonly — have produced inconsistent results.
A 2013 paper in Environmental Health Perspectives noted, “Climate influences disease ecology at many levels, and the many nonlinearities and feedbacks present in the system create complex dynamics that are not easily modeled or understood. In addition, human factors, including behavior, immunity, and socioeconomic influences, also contribute to the complexity of these relations.”
But all of those caveats aside, the proliferation of mosquito populations is undoubtedly a plus for the viruses they carry, and researchers are fairly confident that climate will play an important role for the insects. As Brown put it, all other conditions held constant, “if I put one mosquito in a cold temperature [and one in a warm temperature] and I give both of them an infectious blood meal, the one in the warmer temperature should be able to then get the virus out sooner than the one that was in the cold temperature.”
And one of the biggest concerns among scientists is not merely how well mosquito populations survive in their current locations, but what new areas they may spread into. A. aegypti, for instance, is more common in the southern United States, but some scientists have suggested that it may begin to spread northward as the climate warms, potentially bringing the viruses it carries into new human populations. A. albopictus has already spread much farther north, but could continue marching up into Canada if temperatures continue to heat up.
Of course, aggressive eradication campaigns could outweigh even the influence of a favorably — to the mosquito, anyway — changing climate. In Brazil, where Zika is currently wreaking the most havoc, relief may eventually come in the form of a genetically modified sterile version of the A. aegypti mosquito that biotechnology company Oxitec has been testing in the country. The GM insect has already shown high rates of success at reducing mosquito populations by, in effect, blocking their ability to reproduce, and the company announced on Tuesday that it would be expanding the project.
Otherwise, humans can attempt to combat the mosquito-friendly effects of a warmer climate with more traditional approaches, Brown said, such as limiting the amount of standing water in communities and using mosquito screens and insect repellent.
For the time being, all of these considerations mean it would be a stretch to say the U.S. should prepare for a rash of Zika epidemics just because of climate change. It’s still unclear whether the virus will gain a foothold in North America at all, where reports of its presence have been few and isolated.
However, the disease’s growing presence in areas farther south does mean it wouldn’t be a bad idea to keep it on the radar — along with the handful of other infectious diseases that have been creeping through human populations with the movements of their mosquito hosts.
In the meantime, scientists are hoping to continue improving their predictions on the future movements of these pathogens by honing their models and adding in factors such as the interactions between insects and humans.
“We’re doing our best to understand it,” Brown said.