The Climate Connection to Dengue Fever
Dengue virus makes you really sick and may even kill you. If you are curious about what it’s like to have this illness, just ask anyone who has lived in the tropics. My brother, who had Dengue twice while living in Vietnam, told me: “I was so sick and it’s a weird kind of sick. I was freezing cold, lying in bed covered in sweat for two days, despite the fact that it was 95 degrees outside.”
The news these days suggests we’d better get more familiar with Dengue, also known as “break-bone fever.” In Florida, an outbreak has been detected, the first since 1934. Dengue outbreaks occurred with some regularity in the 1800s and early 20th century in the Southeast, but with improved mosquito control programs and changed socio-economic conditions (e.g. more spread out cities, increased access to air conditioning and window screens) incidents diminished by the 1950s. Since August 2009, however, there have been 28 verified cases of locally acquired Dengue, and blood tests suggest that about five percent of the Key West population has been infected. Currently there is no vaccine or drugs for treatment, although efforts are underway to create a vaccine.
Dengue’s resurgence in the US does not come as a surprise to health experts, as Greenwire/The New York Times reported on June 30th. The risk of Dengue fever is growing because of three upward trends: urbanization, increased travel, and climate change.
Dengue is a vector-borne infectious disease that spreads best in warm climates. The virus is transmitted between people by mosquitoes. Like malaria, the mosquito gets infected when it bites an infected human. The virus must then incubate for eight to ten days inside the mosquito, at which point the mosquito becomes infectious and can transfer the disease to anyone it bites.
Only two species of mosquito are suitable vectors for Dengue virus. The primary one is Aedes aegypti, also known as the yellow-fever mosquito. Ae. aegypti is recognized by white stripes on its legs. It is particularly successful at spreading Dengue because it feeds almost exclusively on humans, is active during the day, and loves urban areas, where it breeds in any container holding water, like planters or waste tires. The secondary vector is the Asian tiger mosquito, Ae. albopictus, which is a recent invader to the US.
There are a few reasons why climate change may increase Dengue risk. First, the incubation period of the virus shortens in warmer temperatures, which means a mosquito doesn’t have to survive as long to have a chance of becoming infectious. Second, the range of the mosquito is increasing due to global warming. In general, climate is a key factor controlling where a species can live. When climate changes, individuals move to stay in a suitable habitat.
The UN Intergovernmental Panel on Climate Change’s Fourth Assessment Report summarized research showing that various arthropods that carry vector-borne diseases, such as ticks, mosquitoes, and sandflies, have all been moving into more northern latitudes in response to recent warming.
Public health researchers Simon Hales and colleagues published a study in the medical journal The Lancet in 2002, in which they examined links between Dengue virus risk and climate change. They found that 89 percent of the distribution of Dengue fever cases was predicted by vapor pressure, which is a measure of humidity. Using this correlation, they projected how disease risk would change by 2100 if carbon dioxide (CO2) concentrations continue to increase at a rate of one percent per year.
They project the world population at risk for Dengue virus will approximately double by 2085, with risk spreading further into Northern Africa, Europe, and North America.
We can visualize this increased risk and its spread across much of the US in a simulation prepared by a group of researchers at The Research and Development Group at the Environmental Systems Research Institute (ESRI) in collaboration with Mary Hayden and Olga Wilhelmi at the National Center for Atmospheric Research (NCAR).
This animation shows the projected range expansion of Aedes aegypti in North America under changing climate conditions. The areas in red show population centers where outbreaks are most likely. Yellow shows areas at risk for transmission. See below for more details on the model.
In the case of Dengue, the climate has been habitable for the Ae. aegypti in the southern parts of the US for a long time. As alluded to earlier, the main reasons outbreaks have been rare is because of good mosquito control programs and other behavioral changes, like widespread access to window screens and air conditioning. But any slip in these measures, and mosquitoes may reach high enough numbers to cause a disease outbreak.
Ae. aegypti only spreads Dengue if there is Dengue in the population to spread. If there is no Dengue in the population, it's just another annoying mosquito bite. This might suggest that increasing mosquito populations wouldn't be a problem if you don't have any preexisting infection in the population. But the more of these mosquitoes that are around, and the more people there are in close contact and traveling internationally in today’s globalized society, then the more likely the mosquitoes are to bite someone who does have the Dengue virus circulating in their system, and then spread the disease to innocent bystanders.
This illustrates why climate matters to disease risk. Warmer temperatures increase the chances that the vector and the virus will thrive. If you live somewhere where a vector for a disease (i.e. mosquitoes, ticks) can’t survive or incubate the disease because it is too cold, then you have very little risk. But if the climate warms, the risk increases, regardless of whether you have access to good vector control programs or not, and the efficacy of these programs themselves are in jeopardy as the burden of disease increases and funds for control programs decrease. Across the board, funds for public health have decreased in the US in recent years.
So, upon further consideration, it seems Dengue risk is increasing because of at least four trends: urbanization, increased travel, climate change and decreased funding for public health measures.
Obviously there are some quick fix band aides we can put on this problem to help. There is lots of enthusiasm in Florida now to improve mosquito control problems that perhaps have grown too lax. And there is the medical quest for a vaccine.
But these societal trends raise longer-term, systematic questions, such as: How are we going to re-design our ways of living in the 21st century so that trajectories of change are not compounding each other to make people more vulnerable to risks but rather less vulnerable?
Details on Model
In the model, Ae. aegypti are assumed to survive only at temperatures above 10°C in winter, and to not live at high elevations (above 1500 meters). Assumptions are based on research reported in Aedes aegypti (L.), the yellow fever mosquito: its life history, bionomics and structure, pp 550 and Identification and Geographical Distribution of the Mosquitoes of North America, pp. 226 –228. Human population density interacts with the mosquito distribution to affect the liklihood of outbreak and transmission. Future human population growth is based on the the LandScan 2000 global population database (which was extrapolated into the future using US Census and United Nations population projections for 2050). Winter temperature projections are based on the climate change simulations from the Community Climate System Model (CCSM), using the SRES A2 emission scenario, whose hypothesized greenhouse gas emission rates over the rest of this century are similar to the rates of global emissions recorded in most recent years.