Climate change is worsening dengue fever outbreaks

To the list of disasters resulting from climate change, extreme weather, and deforestation in Latin America, scientists have recently added another peril: dengue fever, a mosquito-borne disease in which victims suffer high fever, bleeding, extreme pain, and possible loss of life. Dengue fever is a serious disease afflicting rich and poor alike, as demonstrated by scores of currently overwhelmed hospitals in some of Brazil’s wealthier cities, such as the capital Brasília and coastal Rio de Janeiro.

Brazilian doctors are trying to develop and disseminate vaccines against dengue fever, but the challenges are enormous. Meanwhile, the disease is spreading so fast it could affect Central America, aggravating the already fraught migration crisis at the US-Mexico border.

The World Health Organization (WHO) recently reported that in 2023 alone, over 5 million cases were reported, with 5,000 dengue-associated deaths distributed over 80 countries, a sharp increase since 2019. Scientists agree that the worsening trend in outbreaks is primarily explained by climate change, elevating surface temperatures, especially in tropical areas. Rising temperatures, in turn, are associated with greater proliferation of Aedes Aegypti, the mosquito that carries the virus.

Warmer temperatures, particularly in Latin America’s Southern Cone, are associated with changes in weather patterns resulting from Amazon deforestation, which is largely human-made, driven by illegal logging, farming, and grazing in a region sometimes known as the earth’s “lungs” because of the role that forests play in absorbing heat-trapping carbon dioxide and other gases. The rate of deforestation in the Amazon region fell significantly in 2023, but the felling of trees remains far from negligible. As discussed here, scientists are increasingly concerned that the rainforest may be close to the point beyond which it dies off and turns into a savannah. An indicator of this possibility is when carbon emissions by the rainforest are higher than its ability to capture the greenhouse gases from the atmosphere, a phenomenon witnessed in 2023.

The bulk of recent dengue fever cases – some 80 percent – were registered in the Americas. The confluence of dengue fever incidence, extremely warm temperatures, and the effects of the weather phenomenon El Niño was no coincidence: Research has already established a connection between warmer temperatures and more severe dengue fever outbreaks. Warmer temperatures, in turn, are associated with tropical rainforest deforestation.

Accordingly, the economic and health costs of dengue fever fall disproportionately on the Global South, already plagued by underfinanced health systems, limited capacity to raise public spending, inequality, and impoverished populations. Raising awareness about the connections between deforestation and dengue fever outbreaks is one of the critical pillars of what needs to be a multi-pronged approach to fighting the disease and its consequences.

Dengue fever and its incidence in Latin America over time

Dengue fever is caused by the most common among the arboviruses – viruses that are transmitted by a mosquito vector (Aedes aegypti) – and its symptoms include fever, severe body aches, and vomiting that can lead to dehydration, especially in the elderly and the very young. The hemorrhagic variant of the disease is often fatal if not treated in a hospital setting.

There are four serotypes of the virus, and an infection by one of them does not confer protection against the others. In fact, infection by one of the serotypes can develop into hemorrhagic dengue when another serotype subsequently infects someone. There is no cure, although two new vaccines will be deployed through Brazil’s public health system as the country faces its worst outbreak on record (see figure below).

The two vaccines are similar: One was developed by the Brazilian Instituto Butantan, a public research institute, while the Japanese pharmaceutical company Takeda formulated the other. The Takeda vaccine named Qdenga is already being distributed, though in limited supply. Because both vaccines involve live attenuated virus, they are not recommended for the most vulnerable: young children, the elderly, and immunocompromised individuals.

The figure above shows how sharply the incidence of dengue fever has increased in South America’s Southern Cone. Although the graph captures only a portion of Latin America and the Caribbean, similar trends can be seen in other parts of the region. Brazil accounts for most of the cases shown above.

Climate change, deforestation, and dengue fever

If the Amazon is indeed close to its so-called tipping point, temperatures will rise not only in Latin America’s Southern Cone but in much of the region, possibly also affecting the United States. The likelihood of even more severe outbreaks of dengue fever will increase, and the mosquito will also migrate to regions that are currently less affected by the disease.

Since Aedes aegypti is not only a carrier of the dengue virus but also of the Zika and West Nile viruses, and, notably in urban areas, the virus that causes yellow fever, the public health burden is difficult to overstate. The implications could be grave. As more cases of these diseases spread in the poorest countries of Central America, patterns of migration could be affected, exacerbating what we are already witnessing at the US-Mexico border. It is also possible that diseases not previously endemic in the southern United States will become more prevalent.

Addressing these issues requires recognizing how intertwined climate change and vector-borne diseases truly are. As already mentioned, vaccines for dengue fever are few and far between and in any case, can only be given to relatively young and healthy people. Vaccines for yellow fever are highly effective, but there are no immunizations for the Zika or West Nile viruses. A coordinated effort in the Americas to fight deforestation is urgently needed, alongside a well-articulated plan to address climate change–driven epidemics. Unfortunately, we are currently neglecting all of these problems at our peril.