Zoe Berg

In response to rising dengue fever cases throughout Latin America, Yale professors are sounding the alarm on climate change’s impact on the disease’s spread, warning that these conditions make increased spread in the southern United States possible. 

Dengue flu is a tropical viral disease that spreads to humans through the bite of an infected Aedes mosquito. Though dengue is not contagious from person to person, mosquitoes that previously did not carry the disease can become newly infected after biting an infected person, further propagating the disease’s spread through the population. Dengue can cause severe flu-like symptoms — and some cases can be deadly. 

While the disease is most prevalent in South America from June to September, during and after the monsoon season, there is now an alarming rate of cases in Brazil and other South American countries. According to Albert Ko, the professor of public health at the School of Public Health, there have been over two million cases reported in Brazil in 2024.

“2024 [is] one of the largest epidemics this country has experienced,” Ko wrote in an email to the News. 

Ko wrote that he expects the outbreak in Brazil to worsen in the coming months because the northeast part of the country is only now entering peak dengue season. 

The disease has begun to creep northward, causing concern from the CDC. On Monday, the Puerto Rico Department of Health declared dengue an epidemic following a recent case spike. 

“While dengue virus spread is right now mainly an issue in South-East Asia and in Central and South America and equatorial Africa … it is likely that the vectors that transmit viruses like dengue will spread to new geographic areas,” David Martinez, a professor at the School of Medicine who studies viral pathogens, wrote in an email to the News. “We are starting to see the local, albeit limited, transmission of dengue virus in some southern U.S. states that have the climate to support mosquito population growth and spread.” 

Dengue is unique from other mosquito-borne infections, Martinez noted, in that the immune system recognizes each of its four distinct variations differently, meaning that immunity to one form of the virus typically does not grant immunity to the others. 

Further, unlike other infectious diseases, multiple dengue infections can often bring about worse cases. Martinez said that dengue’s specific characteristics make it particularly difficult to counter. 

“Dengue virus has another trick up its sleeve compared to other mosquito-borne [viruses]: it can use host antibody immunity as a “Trojan horse” to exacerbate disease through a phenomenon called ‘antibody-dependent enhancement of disease,’” Martinez wrote. “Therefore, immunity to [one] dengue virus [variant] can make disease worse.” 

According to Martinez and Nathan Grubaugh, an epidemiology professor at the School of Public Health, climate change has contributed to the unprecedented outbreak of dengue this year in several ways. Increased temperature and rainfall have extended mosquito breeding grounds, and mosquitoes are now present in areas that were previously not warm enough for breeding.

Florida, California and Texas — coastal states that have been affected by climate change — have particularly seen an increase in dengue over the past few years. 

These changing climate conditions also lead to longer lifespans for mosquitoes and, thus, longer dengue seasons. A 2023 study led by the University of Michigan School of Public Health found that the transmission potential of dengue may increase by 10 to 20 percent in the next 30 years due to climate change. 

Beyond climate change, Grubaugh explained that this year’s rapid rise in cases may be due to the re-emergence of serotype 3, a previously uncommon dengue variant to which the population has little immunity.

Brazil was the first country to roll out a public vaccine campaign against dengue. However, Grubaugh said this may not be enough to stop the epidemic.

“While it is exciting to see Brazil administer a dengue vaccine (Qdenga from Takada), their supplies are very limited and something like only 10 percent of the population would have access,” Grubaugh wrote. “On top of this, the vaccine is not very effective at protecting against all serotypes and variants of dengue virus. So the likelihood that dengue vaccination actually significantly halts spread in 2024 is quite low.”

Public education in resource-limited areas could help improve the situation, Martinez said. Individuals can help curb its spread by removing common household objects that become reservoirs near the home, as they can become breeding grounds for mosquitoes. 

He also emphasized that there needs to be more “investment in basic science” to understand specifically how the virus causes disease and to help develop antiviral drugs that reduce case severity.

Robert Dubrow, a professor of epidemiology at the School of Public Health, highlighted that the spread of dengue should serve as a warning for the spread of other viral infections due to climate change. 

“As the planet warms, we can expect Aedes aegypti and Aedes albopictus, the two mosquito species that can carry the dengue, chikungunya, and Zika viruses, to spread to higher altitudes and latitudes,” Dubrow wrote in an email to the News. “It’s vital for the public health system to stay vigilant.”

There are approximately 550 cases of dengue fever a year in the continental United States. 

Jessica Kasamoto covers the Yale School of Public Health for the SciTech desk. She is a graduate student in computational biology and bioinformatics.