The Increasing Burden of Dengue Fever in a Changing Climate

Figure 1: An infected mosquito bites and infects a person; another, uninfected mosquito later bites the infected person, picking up the dengue virus from them. This allows the virus to replicate in this new mosquito and when it bites a new person, the cycle continues. 

Dengue fever is transmitted by mosquitoes of two species: Aedes aegypti and Aedes albopictus. According to one study, of 250 countries or territories worldwide, Ae. aegypti can survive in 188 and Ae. albopictus can survive in 197. Endemic cases of dengue fever (not associated with travel) were reported in 111 of these countries/territories.

Figure 2: A female mosquito requires a blood meal for egg production. She then lays eggs onto the surface of stagnant water, where the larvae develop in stages and pupate. The pupae then develop into mature adult mosquitoes, which then mate to continue the life cycle.

Figure 3: Estimated annual dengue cases by country, 1990–2019, for the top 15 countries by caseload. Estimated cases have risen sharply in tropical and subtropical zones the last thirty years. Note that these figures differ slightly from the WHO’s estimate for symptomatic infections.

Severe dengue was rare in India before 1996, but India now bears the majority of the global dengue burden. Although dengue follows a general pattern associated with the monsoon season, the annual incidence of dengue in India has increased dramatically in the last ten years. Numerous factors may contribute to this increase. With a lack of financial and medical resources, diagnosis and treatment are delayed. Urbanization may play a large role. Mosquito control programs are insufficient to control the vector population. Climate change in India has increased both the temperature and frequency of heavy rainfalls, both of which promote mosquito activity and expand their geographic range. India is not, however, the only country suffering from a dengue uptick:

• Dengue cases have surged in other endemic areas, such as in Vietnam, where cases in 2022 are four times the number of cases reported in 2021.
• Surges may be the result of weather events, which are exacerbated by climate change. Recent catastrophic flooding in Pakistan, for example, has resulted in a massive increase in dengue cases.
• Dengue is expanding geographically within Brazil. In the past five years, 481 communities reported local transmission for the first time, with 63 new communities affected in just the first three months of 2022. Scientists attribute the rapid expansion of dengue in Brazil to climate change, urbanization, human mobility, and a larger area of the country becoming hospitable to the mosquito vectors.
• In Nepal, dengue has been reported more frequently at higher altitudes in recent years, since mosquitoes are able to survive at these elevations due to a warmer climate.
• Some previously unaffected subtropical countries, such as Niger, have reported their first ever dengue cases.
• Cases are even increasing in areas outside the tropical and subtropical zones. In France, for example, no locally transmitted cases of dengue had been documented prior to 2010, but 65 such cases have been reported in 2022 as of mid-October. This is due in part to the expanded range of Ae. albopictus, which can now be found in two thirds of the country.

Dengue and Covid-19

The interactions between climate and public health have become even more evident with the onset of the Covid-19 pandemic, which has exacerbated the impact of dengue fever in several ways. Overlapping symptoms and coinfections have complicated diagnosis. Facilities and personnel dedicated to vector control, research, and healthcare have been diverted from dengue to Covid-19, placing extra pressure on already strained health systems.

In the summer of 2020, Singapore suffered a particularly bad dengue season, with record numbers of weekly dengue cases. Reasons for the surge include warmer, wetter weather, circulation of an uncommon dengue virus serotype (to which residents had only partial immunity), and Covid-19 restrictions, which resulted in people spending more time at home, where mosquito populations are high.

Conversely, in Australia, dengue cases have declined significantly since the beginning of the pandemic. The decline is attributed to Covid-19 travel restrictions, which have prevented travelers from dengue-endemic areas from bringing the disease into the country.

Beyond Dengue Fever

Dengue is not the only disease that can be exacerbated by climate change. A recent study proposed that 58% of known infectious diseases can be negatively affected by the hazards of climate change. Diseases transmitted by mosquitoes, such as yellow fever, Zika, and chikungunya could be affected in a similar manner as dengue fever. Diseases carried by other vectors, and those with animal reservoirs, may also be affected by climate change, as can those transmitted by water, air, and food.

The many effects of climate change increase the risk for hundreds of infectious diseases by numerous pathways. Whether by land use changes increasing tick-borne Lyme disease, floods affecting schistosomiasis spread by snails, or drought prompting hantavirus-infected mice to invade homes in search of food, climate change is a serious threat to human health. While mitigation efforts to interrupt such individual transmission pathways are certainly helpful, a concerted effort to halt climate change will do much more to reduce the current and future burden of infectious diseases.