Cases of dengue have risen dramatically in recent years, reaching record levels in the first eight months of 2024 and more than doubling the number of cases in all of 2023. The Americas have been particularly affected by the surge in infections, including Brazil, where nearly three-quarters of all cases worldwide have been detected; Puerto Rico, which declared an epidemic in March; and California, which has reported “unprecedented” cases that are unrelated to travel.

The surge shows no signs of slowing down. On October 3, the World Health Organization released a global strategy to combat the growing problem of dengue and other arboviruses (viruses carried by arthropods like mosquitoes and ticks). The release came alongside an October 7 alert from the Pan American Health Organization warning of the growing dengue threat in the region.

Here is everything you need to know about the rapid rise in dengue and how it could impact you.

What is dengue?

Dengue is a disease spread by bites from Aedes mosquitoes, which also carry yellow fever and Zika viruses. It is the most common mosquito-borne disease, infecting up to 400 million people worldwide each year. The dengue virus causes dengue fever, characterized by a high fever, rash, and muscle and joint pain. In severe cases, dengue fever can cause internal bleeding, organ failure, and death. 

Dengue is endemic, or regularly occurs, in tropical and subtropical climates, where Aedes mosquitoes thrive. The disease is most common in South and Central America, the Caribbean, Southeast Asia, and the South Pacific.

Dengue cases have been on the rise since 2000

Global dengue cases increased tenfold between 2000 and 2019. The first two years of the COVID-19 pandemic saw a slight decline in cases, only for 2023 and 2024 to yield the worst years for dengue on record. Although much of the increase was in endemic countries, a rising number of cases has also been recorded in non-endemic regions.

“We’re seeing both increases in cases in areas that have seen dengue before and also dengue appearing in new areas,” said Gabriela Paz-Bailey, chief of the Dengue Branch at the Division of Vector-Borne Diseases at the Centers for Disease Control and Prevention and a participant at a recent Harvard T.H. Chan School of Public Health panel on the growing global threat of dengue.

How climate change is impacting mosquito-borne illnesses like dengue

Experts have long warned that climate change can impact infectious disease transmission. A 2021 CDC report found that rising air and water temperatures have increased the risk of infectious disease outbreaks in the U.S. Cases of West Nile, malaria, and eastern equine encephalitis are recent examples of this phenomenon. 

Dengue-carrying mosquitoes prefer warmer climates, require access to standing water, like puddles and reservoirs, to breed, and are unable to survive cold winters. As climate change has driven up global temperatures, milder winters and longer warm seasons have allowed the mosquitoes—and the diseases they carry—to spread to previously unaffected regions. 

An August 2024 study found that climate shifts in temperature and humidity since 1979 have made some geographic regions more suitable for dengue transmission. In the Global South, population growth has increased dramatically within these “high suitability” areas, while in the Global North, dengue’s range has expanded to non-endemic regions in North America, East Asia, and the Mediterranean.

In a preprint study, Harvard and Stanford researchers estimated that climate change has already increased global dengue incidence by 18 percent. The study predicts that by the middle of the century, climate change will have increased dengue incidence by up to 57 percent. Notably, the study found that the effect of rising temperatures on dengue spread is most potent in places with cooler temperatures.

Non-climate factors are also at play in dengue spread

“Climate is not the only factor behind the global rise in dengue,” noted Marcia Castro, chair of the Department of Global Health and Population at the Harvard T.H. Chan School of Public Health, at the panel. 

Castro described Brazil’s current historic dengue outbreak as a “perfect storm” caused by pandemic-related disruptions to disease prevention efforts, mosquito-favorable weather conditions, government mismanagement, and rapid growth of high-density urban areas without proper infrastructure in areas bordering the Amazon. Patterns of urban growth without reliable access to clean water and waste disposal exist throughout the Global South and create ideal environments for mosquito populations to thrive. 

Paz-Bailey of the CDC emphasized that incomplete human immunity to dengue also complicates efforts to contain the disease. She explained that there are four distinct but closely related dengue viruses: Infection with one dengue virus provides lifelong immunity to that virus but only temporary (two to six months) immunity to the others. That means that a person with an infection can develop immunity to one dengue virus during an ongoing outbreak, only to get sick with a different one during the next outbreak. 

A novel and highly effective strategy first implemented in 2011 is the release of mosquitoes infected with the bacteria Wolbachia. The bacteria is harmless to humans and other animals, but in the mosquito species that carry dengue, Zika, and chikungunya, it prevents their eggs from hatching and reduces the population. 

Wolbachia-infected mosquitoes have been released in 14 countries, including parts of Indonesia, where it reduced the Aedes mosquito population, decreasing dengue cases by 77 percent and hospitalization by 86 percent. 

Castro, however, was careful to note that any intervention to fight dengue must address issues related to infrastructure and systemic inequities. Even in a scenario where “every single mosquito has Wolbachia and transmission goes down,” she said, “the root cause of the problem is still there. And this affects other diseases as well.”