Was the Zika Outbreak a Product of Climate Change?

Zika’s history as a “tropical” disease plays into the theory that global warming has played a role in the presence of virus-carrying Aedes Aegypti and Aedes Albopictus mosquitoes, and thus the disease itself, in heretofore “temperate” climates such as the southeastern United States.

However, there’s some question as to whether or not that theory holds water, at least when existing evidence is taken into account.

In a study published on August 26 by the journal PLOS Neglected Tropical Diseases, researchers in the Department of Environmental Science, Policy, and Management at the University of California, Berkeley have concluded that Zika-carrying mosquitoes will not be “as serious a problem in temperate climates as in the tropics,” according to study lead author Colin Carlson, a PhD candidate and researcher at the school. The study findings are based on bioclimatic models developed by the research team and designed to establish the “outer bound on where the virus is likely to persist,” using a “comprehensive geospatial dataset of Zika occurrences” in the virus’ “native range,” since it was first identified in 1947.

“It’s difficult to say whether climate change has been responsible [for any Zika outbreaks in temperate climates to date], but our models don’t provide strong evidence that it has,” Carlson told Contagion. “We know that climate change is helping mosquitoes spread to new places, and our models suggest most of those expansions won’t be accompanied by Zika because Zika already doesn’t go everywhere mosquitoes can.”

Despite some “doomsday” projections about the future of Zika in the mainstream press, the analysis by Carlson et al suggests that the outbreaks of Zika in Mexico and North America—particularly in Florida—may be an anomaly, given that these regions are “outside the native ecological niche of the disease.” They write that the cases identified to date “may be linked to either genetic shifts between strains, or El Nino, or similar climatic events,” but that there is no evidence suggesting that these outbreaks may become regular occurrences in the future.

In fact, compared to another mosquito-borne infection, Dengue fever, Zika may be “more constrained by the seasonality of precipitation and diurnal temperature fluctuations,” indicating that the latter virus will likely be confined to the tropics “without significant evolutionary change,” they write. “Public fear of a vector-borne Zika epidemic in the mainland United States is potentially informed by biased or limited scientific knowledge.”

Which, of course, is not to say that climate does not and has not played a role in Zika’s prevalence in certain parts of the world. In fact, in a study published online on September 1 by the journal The Lancet Infectious Diseases, scientists identified the ecological niche for Zika virus in the Americas and analyzed travel data over the course of a year (December 2014 to November 2015). They determined that many countries across Africa and Asia-Pacific may be vulnerable to Zika virus outbreaks, with India, China, the Philippines, Indonesia, Nigeria, Vietnam, Pakistan, and Bangladesh expected to be at greatest risk of Zika virus transmission due to a combination of high travel volumes from Zika affected areas in the Americas, local presence of mosquitoes capable of transmitting Zika virus, suitable climatic conditions, large populations and/or limited health resources.

“An estimated 2.6 billion people live in areas of Africa and Asia-Pacific where… local Zika virus transmission is theoretically possible,” study co-author Kamran Khan, MD, MPH, a scientist at the Li Ka Shing Knowledge Institute at St. Michael’s Hospital in Toronto said in a statement released with The Lancet paper. “Warmer temperatures in the northern hemisphere (when mosquitoes are more active) increase the risk of new outbreaks appearing outside of the Americas. The potential for epidemics to occur in parts of Africa and the Asia-Pacific region is particularly concerning given that the vast numbers of people who could be exposed to Zika virus are living in environments where health and human resources to prevent, detect, and respond to epidemics are limited.”

In addition, Carlson and his co-authors note that because Zika infection is often asymptomatic and can be transmitted via sexual contact, the possible locations of future outbreaks are a bit harder to predict. “Sexual transmission changes everything, as do some of the other contact-based transmission routes that are being conjectured now,” Carlson said. “Sexual transmission of Zika could already be a significant problem in the United States and the rest of the temperate world because so many cases are asymptomatic. [It] might be hard to monitor.”

Meanwhile, their research also suggests that climate change “will make Zika worse in the tropics, and that it could gain a lot of ground in the next 35 years,” he added. Indeed, a commentary published in June in the journal PLOS Neglected Tropical Diseases, cites climate change, along with other forces such as economic downturn and human migration, as a potential catalyst for the emergence or re-emergence of other vector-borne diseases in countries along the Mediterranean Sea, including Portugal, Spain, and the Balkans. “Of particular concern,” according to the author of the commentary, Peter Hotez, MD, PhD, dean of the National School of Tropical Medicine at Baylor College of Medicine, “is the recent emergence or re-emergence of arbovirus infections,” such as Chikungunya (first identified in Italy in 2007 and later found in France and Spain), Dengue fever (in Madeira, Portugal, where more than 1,000 cases have been reported), and West Nile virus (primarily in Bulgaria, Hungary, and Romania as well as in Italy). As Dr. Hotez writes, in Europe, both Dengue and Chikungunya are transmitted by Aedes Albopictus mosquitoes, which were first detected in Spain in 2004. Aedes Albopictus has also recently been linked with Zika. To date, no locally transmitted cases of Zika have been reported in Europe—although Dr. Hotez and others believe that this could change.

Dr. Hotez writes, “Southern Europe has become ‘ground zero’ for the vector-borne neglected tropical diseases… Indeed, there is now a startling similarity between the vector-borne diseases patterns of North Africa and Southern Europe so that all the nations surrounding the Mediterranean Sea have become almost unified with respect to their endemic neglected tropical diseases.”

In an email exchange, Dr. Hotez told Contagion, “I think the major impact of climate change on disease so far is in Southern Europe, especially among vector-borne diseases, and now with the rise of vector-borne diseases in the Americas. But we cannot rule out other factors that include poverty, urbanization and human migrations. The problem is we need a new generation of scientists able to solve complex problems that can be conversant with biomedical scientists, political scientists, economists, environmental and earth scientists. We don’t have that yet.”

Carlson agrees to a point, noting that while “theory suggests climate change drives vector-borne diseases into emergence, that’s no guarantee that’s what happened here.” However, “tropical diseases aren’t, by and large, staying in the tropics anymore. A number of ‘tropical diseases,’ even malaria, used to have much greater ranges before mosquito control and modern medicine. But those diseases are coming back to some of those places because of climate change. That relationship is pretty unequivocal. That doesn’t mean every single emerging disease is the consequence of climate change—it’s sort of the same principle as, not every single hurricane or storm is the direct consequence of climate change, but the trend itself is still getting much worse. Long story short: climate change means vector-borne diseases, plural, will get worse but individual diseases are variable.”

All of which means that, like the weather, the role of climate in the spread of viruses such as Zika may be unpredictable.

Brian P. Dunleavy is a medical writer and editor based in New York. His work has appeared in numerous healthcare-related publications. He is the former editor of Infectious Disease Special Edition.