Last year, researchers estimated that 60% of the global population will be at risk for dengue by 2080, citing climate change as a potential factor in the disease’s spread. We asked Duane J. Gubler, ScD, FAAAS, FIDSA, FASTMH, chair of the Global Dengue & Aedes-Transmitted Diseases Consortium and emeritus professor of emerging infectious diseases at Duke-NUS Medical School in Singapore, if climate change impacts the spread and transmission of dengue.
The past 50 years have seen a dramatic global increase in the frequency, magnitude and geographic spread of epidemic dengue, chikungunya, Zika, yellow fever, West Nile and other mosquito-transmitted diseases. Climate change is frequently blamed as a major cause of this dramatic emergence of epidemic disease. Anthropogenic climate change’s effect on epidemic vector-borne diseases is a controversial issue, the acceptance of which has benefitted from the adage “If you repeat something often enough, it becomes fact, whether it is true or not.”
Dengue is the most important arboviral disease of humans, with an estimated 400 million infections occurring annually in 124 countries. Aedes aegypti, a highly domesticated urban mosquito, is the most important vector. This mosquito and the closely related but less important species Aedes albopictus have expanded their geographic distribution in the past 30 years.
Historically, the natural geographic boundaries of Aedes-transmitted diseases have been limited by temperature because the activity of mosquitoes, which are cold-blooded animals, is climate sensitive. Therefore, transmission of Aedes-transmitted viral diseases usually occurs only in tropical and subtropical areas. Exceptions occurred in the past when A. aegypti infested northern cities during summer months, allowing transient epidemic transmission in temperate areas. A. albopictus, which can overwinter in temperate regions, has transmitted small outbreaks in areas of Europe and the United States.
Climate change is frequently listed by WHO and national and international health agencies as one of the major causes of this global emergence and spread of dengue and other Aedes-transmitted viral diseases. This seems intuitive, but it has been a controversial issue for over 30 years because there are no hard data to support that conclusion — only a correlation of mosquito infestation and limited transmission of dengue and chikungunya in some temperate areas. More often cited to support this view are modeling studies that project expanded distribution of the mosquito vectors into temperate regions, and then assume that the presence of the mosquito will result in epidemic disease. These modeling studies are flawed because they focus only on one variable — temperature — but ignore many other important variables that also have strong influences on the transmission dynamics of these diseases. Also not mentioned in these studies is that epidemics of these diseases have occurred in temperate regions for over 300 years but were effectively controlled in the 1950s to 1960s by a combination of improved living standards, good public health and environmental management, even though the mosquito vectors were not eliminated.
On the other hand, there are several observations that suggest other demographic, environmental and societal factors — as opposed to climate change — have been the major drivers of the dramatic emergence and spread of Aedes-borne viral diseases in the past 50 years. These include the following:
– Dengue first began to emerge in the post-World War II era with economic expansion, population growth and unprecedented urban growth in Asia in the 1950s and 1960s. A. aegypti was already present, but unplanned urban growth, inadequate housing, water and waste management provided the ideal ecology for the mosquito to proliferate.
– The 1970s and 1980s brought modern transportation and a new and rapid mechanism to transport people, commodities, mosquitoes and viruses among the cities of Asia, and from Asia to the rest of the world. Both mosquito vectors and viruses began to populate new areas, resulting in epidemic transmission.
– Complacency and apathy during the post-World War II years resulted in the deterioration of the public health infrastructure to control vector-borne diseases. The emergence and spread of dengue were ignored, surveillance was poor and control, when it was attempted, failed.
– Aedes-transmitted viral diseases are an urban problem of the tropics and subtropics. Outbreaks in temperate regions, when they do occur, are small and transient.
– Transmission occurs year-round in the tropics, but peak transmission is seasonal, occurring during the rainy season in most endemic countries.
– The rainy season is usually associated with moderated temperatures and higher humidity from the preceding hot dry season, allowing increased densities and survival of adult mosquitoes and thus increased probability of transmission.