Don’t Believe the Fever Pitch

Climate Change Plays Limited Role in Spread of Dengue Virus

In the summer of 2012, I spent a week in a Cambodian hospital recovering from dengue fever, which I had contracted in Laos. When my bout with dengue comes up in conversation, a standard response is a knowing look accompanied by the obvious explanation: climate change – as in, “any fool knows that dengue is spreading around the world because of climate change.”

After all, report after report links the spread of dengue — a mosquito-borne virus infecting 100 million people annually – to climate change. The causal relationship is now common wisdom. A 2013 World Health Organization (WHO) fact sheet, for example, asserts, "Many of the major killers such as diarrheal diseases, malnutrition, malaria, and dengue are highly climate-sensitive and are expected to worsen as the climate changes." Like malaria’s vector, dengue’s vector, the Aedes mosquito, “is [also] highly sensitive to climate conditions. Studies suggest that climate change could expose an additional 2 billion people to dengue transmission by the 2080s.” Reporting on a dengue outbreak in Florida in September 2013, both NPR and Al Jazeera also included references to climate; Reuters points to climate change as the culprit in the spread of dengue in Thailand. And in November 2013, TIME announced that global warming is aiding the alarming spread of the disease.

Climate change as the obvious culprit seems emblematic of the monomaniacal way climate change is linked to all scourges – be they, tsunamis, hurricanes, famine, floods, or forest fire. But the history of dengue suggests that climate change as a cause of its spread is greatly eclipsed by a myriad of socio-economic and environmental factors.

The earliest account of a dengue-like disease comes from around 400 in China. Evidence of epidemics resembling dengue also occurred in the 17th century. But three epidemics in the late 18th century mark the arrival of what is now recognized as dengue fever. Two of these outbreaks—one in Cairo and one in Jakarta – occurred in 1779. The third epidemic occurred in 1780 – in Philadelphia. Hardly the tropics. Because all three 18th-century epidemics occurred in port cities, dengue likely spread from one continent to another via ships. Some speculate dengue originated in Africa and that slave ships, and concomitant globalization, were the culprits. More specifically, the water storage containers that served as breeding grounds were likely the source.

From the 1820s to the 1940s, dengue epidemics occurred virtually every 10 years in a range of US cities, including Austin, Charleston, and Savannah. During World War II, dengue emerged in Southeast Asia and rapidly spread to other parts of the world, inciting a pandemic. Boston experienced an epidemic in the 1940s, most likely thanks to returning troops.

In the 1950s, hemorrhagic dengue (a type that can develop when antibodies to one of the four strains are present) appeared in Southeast Asia, where it became a common cause of death among children in the 1970s. The serotypes continued to spread on a pandemic level, eventually reaching South and Central America, Cuba, and Puerto Rico. Since 1960, the incidence of dengue fever has increased 30 fold. In 2008, the WHO reported that approximately 2.5 billion people worldwide — 70% around the equator – were at risk of dengue and that the disease was endemic in more than 100 countries.

Given the poor correlation with historic outbreaks and their locations, climate plays a limited role at best in dengue spread. In reality, a multitude of other factors —urbanization, mobility, and globalization – overshadows any role that climate change may play.

As the CDC explains:

As recently as the 1940s, large dengue outbreaks were documented in the United States reaching places as far north as Boston. Today, the situation has changed significantly. Reasonable climate, competent mosquito vectors, and susceptible human hosts are all still present in the continental United States, and dengue viruses are frequently reintroduced by infected travelers. Transmission in the U.S. is rare, however, because there is insufficient contact between infected humans, vector mosquito species, and susceptible humans to sustain transmission …

In countries where transmission does routinely occur, short-term changes in weather, particularly temperature, precipitation, and humidity, are often correlated with dengue incidence. These associations, however, do not describe the occurrence every few years of major epidemics in these areas, suggesting that long-term climate variability does not regulate long-term patterns in transmission. A more important regulator of epidemics might be the interplay of the four different dengue serotypes. The level of prior exposure of a human population to each of the dengue serotypes may be a more critical determinant of whether a large epidemic occurs than climatic cycles.

Rather than climate change affecting dengue spread and incidence, CDC points to “population growth, urbanization, lack of sanitation, increased long-distance travel, ineffective mosquito control, and increased reporting capacity” as the likely primary factors.

Regardless of whether climate is the major driver behind the spread of dengue, social, biological, and environmental factors, such as poor sanitation or water retaining receptacles (e.g., tires), will be where solutions will be found; controlling dengue will most likely succeed with ecologic and public health interventions. CDC notes studies on the US-Mexico border that suggest transmission is reduced by low housing density, air conditioning, and screens. Vaccine research is underway. In the meantime, pesticides could also help.

As with so many pressing issues for which climate change is attributed a larger role than warranted, the linkage draws limited attention and resources away from finding viable solutions. Furthermore, by looking at the world through the climate change lens, the focus becomes what to do about climate change – something that has proved elusive and politically charged.

Instead, the focus should be on the very real, immediate, and tangible problems and threats that may or may not be exacerbated by climate change. Besides, with actual and concrete steps to be taken around mosquito control — steps we know can work – a “stop dengue now” campaign has a lot more potential for traction and success than a “stop climate change” campaign ever has.

Erica Rosenberg is an environmental consultant and writer in Washington, DC.