Earlier this year, Central and South America were hit with one of the worst outbreaks of dengue fever in decades. According to the Pan American Health Organisation (PAHO), over 2 million individuals in the region contracted the disease in the first seven months of 2019, resulting in over 700 reported deaths. Brazil alone saw a 600% increase in the number of cases in comparison to last year. The total number of cases seen so far has already surpassed the numbers seen in 2015 and 2016, two other severe outbreak years.
Graph showing number of dengue cases until July 2019, taken from PAHO's September 2019 Epidemiological Update for Dengue.
Dengue is a viral disease found in over 100 countries worldwide, with around half of the world's population living in affected areas. Mosquitos act as vectors, and in the past few decades, the disease has spread rapidly. The situation is so severe that the WHO named dengue as one of the greatest threats to human health in 2019.
Scientists cannot directly link any of these outbreaks to climate change. However, there is evidence climate change is disrupting weather patterns and increases the risk of extreme weather events. Research from Dr Rachel Lowe, assistant professor at the Faculty of Epidemiology and Population Health at the London School of Hygiene and Tropical Medicine, shows that extreme events such as droughts and flooding can increase the probability of dengue outbreaks by increasing the number of breeding sites for mosquitos.
Floods can damage drainage systems, resulting in pools of stagnant water, creating ideal sites for mosquitos to gather and breed. Counterintuitively, similar environments can be created during droughts. Poorly maintained water storage systems can also create these environments in which mosquitos can multiply.
Climate change won't only affect the intensity of outbreaks in places that mosquitos already live in; it will facilitate the spread of dengue and other mosquito-borne diseases to new areas. Mosquitos thrive around 29 °C, meaning that as the planet warms, regions that were previously inhospitable to mosquitos may become habitable.
“We are going to see more big outbreaks and we are going to see them in new places,” said Colin Carlson, an epidemiologist at Georgetown University, speaking to Thomson Reuters.
“In the next 10 to 20 years, there is absolutely going to be an expansion of Aedes aegypti-borne disease into new places, facilitated by warming temperatures," Carlson said.
The extent of disease spread depends on the amount of greenhouse gases, such as carbon dioxide and methane, we emit into the atmosphere. To illustrate the effects increasing greenhouse gas concentrations in the future, climate scientists have devised a set of representative concentration pathways (RCPs). These can be used in models to approximate different emission scenarios, and will be discussed more in future blog posts. A summary of the RCPs is provided below.
Summary of the RCPs, from Climate: Everyone's Business.
Chinese researchers have used these RCPs with models to try to predict how future climate scenarios will affect mosquito, and thereby dengue, spread in China.
Images adapted from Fan and Lui 2019. The first image represents the current spread of dengue in China. The middle image shows the projected future spread under RCP2.6 (severely mitigating emissions). The third image shows the projected future spread under RCP8.5 (Business as Usual).
As can be seen from the maps, even in a world in which we limit global heating to 1.5 °C, there could be an increase in 'high risk' areas. Due to China's growing population and increased urbanisation, this could affect hundreds of millions of people.
Evidence suggests that the expansion of dengue's range could be limited if we curb our emissions. Doing so could help prevent millions of people from being exposed to this devastating disease. Although there is uncertainty about the precise effects of climate change on dengue spread, it is clear that this issue demands action from governments to mitigate the risks of future epidemics.