Climate Change, Travel Fuel Dengue’s Spread

Cases of dengue fever are surging, bolstered by a rapidly changing climate and frequent international travel. At least half the organizations in Direct Relief’s Latin America network have expressed concerns about the disease’s growing reach and case numbers. 
 
Dengue is also spreading further into the continental United States, where providers not yet used to seeing the disease may be more likely to dismiss it as a run-of-the-mill fever. 
 
In response, Direct Relief developed a dengue treatment kit in 2024 and has been providing it to partners in affected regions, including to healthcare organizations in Puerto Rico.
 
Last week, the CDC issued an advisory attempting to make healthcare providers and the public more aware of the growing prevalence of dengue fever in the United States. More than 2,500 cases have been reported this year. Above 1,500 of those were in Puerto Rico, which saw 900 hospitalizations and two deaths. The situation in Puerto Rico was declared a public health emergency. 

In 2023, the CDC reported a total of 3,036 cases. 
 
In Latin America, the reported case counts have increased by more than 230% compared to the same time last year. More than 10.4 million cases have been reported so far this year, according to PAHO. 

This year’s increases across the Americas reflect a trend that has emerged over the past four decades. From 1.5 million total cases throughout the 1980s decade, the number surged to 17.5 million cases between the years 2010 to 2019, the World Health Organization reported. 

A Changing Landscape

Dengue is a viral illness transmitted through mosquito bites that can cause fever, body aches, fatigue, nausea, headaches, rash, minor bleeding, and vomiting. Severe cases can include shock, organ failure, uncontrolled bleeding, and death. 
 
Vulnerable populations, especially the young, elderly, and those with underlying health conditions, are more likely to experience severe or fatal cases. 
 
Experts say several factors are responsible for the recent surge in reach and case numbers: In particular, climate change, the difficulty of combating dengue-carrying mosquitos, and increased international travel each play a major role. 
 
“A key driver has been the expansion of the mosquito vector,” said Dr. David Freedman, professor emeritus of infectious diseases at the University of Alabama at Birmingham. 
 
Dengue fever is transmitted by two types of mosquitos: Aedes aegypti and Aedes albopictus. The mosquitoes have been difficult to combat and are now at record levels, according to Freedman. 
 
While malaria-carrying mosquitos tend to breed indoors and can be combated with residual insecticides, dengue-carrying mosquitos prefer the outdoors, making them harder to target. 
 
In Singapore, for example, Aedes mosquito populations have not been successfully controlled, despite seemingly ideal conditions: The setting is largely urbanized, repeated spraying for mosquitoes has been carried out, and there has been a high level of compliance with regulations such as removing standing water near houses. 
 
“It’s a little bit discouraging…People look at Singapore’s result and ask, ‘Well, does anybody have a chance at vector control?’” Freedman said. 

By contrast, the malaria-carrying Anopheles group of mosquitos has been targeted by widespread and largely successful eradication campaigns over the past decades. However, a downward trend in malaria cases that began in 2000 has been complicated by recent increases in case numbers.

The African continent recorded 94% of cases last year. In addition, Pakistan saw the largest annual increase, with more than 2.1 million additional malaria cases.  
 
The world’s changing climate is also helping to grow mosquitoes’ numbers and range. 
 
“It’s a perfect storm of increasing temperatures and changing rainfall patterns due to climate change, which creates ideal breeding conditions for mosquitoes,” said Michael von Fricken, an associate professor at the University of Florida’s Department of Environmental and Global Health. 
 
Von Fricken also identified Covid-19 as having changed immune system profiles across the population. 
 
“Think about how much time we spent during Covid sequestered and isolated. You likely have changes in immune profiles, due to human behaviors, that impact the underlying proportion of the population that is susceptible to exposure,” he said. 

A notable decrease in reported dengue cases during the height of the pandemic was most likely due to a combination of reduced travel and decreases in mosquito-borne disease surveillance, von Fricken explained. The overwhelming majority of U.S. cases are associated with travel, not locally acquired. Puerto Rico, which is experiencing an ongoing epidemic, is an exception. 

A Complicated Disease

Dengue fever has four subtypes. If a person contracts one subtype, they will generally be immune to that specific subtype. However, exposure to other subtypes can lead to more severe symptoms, including hemorrhagic fever. 
 
“We might see an uptick in a couple of years as different serotypes move through cycles,” he said. 
 
While dengue cases have increased, not all vector-borne diseases have followed a similar trend. 
 
Zika cases have declined in recent years and chikungunya cases have varied, albeit not approaching their peak numbers from 2014 to 2016. Von Fricken said this could be due to the diseases “burning through their wood,” or essentially having infected a significant proportion of the population already.  Freedman pointed out that chikungunya only has one serotype, so after people catch it, they become immune. Still, local pockets of concern exist, including chikungunya outbreaks in parts of Brazil and Paraguay. 
 
Because dengue has multiple serotypes, immunity is a much more complex picture.  
 
However, a cure for dengue fever does not exist and current vaccines are not universally approved for all populations. One vaccine is recommended only for those who have previously contracted the disease. Another commercially available option was withdrawn from FDA approval process last year but has been approved in several European, South American, and Asian nations. Several vaccines are in development. 
 
In the absence of a cure or readily available vaccine, Freedman and von Fricken said increasing awareness and education is important to curtail outbreaks. 
 
“Dengue is not endemic in the U.S., but there are areas where it is, and we’re importing it. We need to make sure clinicians are keeping an eye out for this, and not just dismissing it as a fever,” von Fricken explained. “We also need to tell people to stay inside when they have it, so they don’t spread it.” 
 
The disease is not transmissible between people, but mosquitoes can contract it from an infected person and then spread it to others. 
 
“It spreads very easily when mosquitos are present,” Freedman said. “And the geographic region seems to be spreading and populations are mobile.” 
 
In response to the growing threat, Direct Relief has developed a comprehensive dengue kit to support its partners in Puerto Rico and Latin America.

As there is no cure for dengue fever and the vaccination picture is complicated, the kit focuses on supportive care. Each kit is designed to treat mild cases of dengue in 100 people, and includes mosquito repellents, oral rehydration salts, acetaminophen to control fever and aches, and thermometers.  
 
Health providers were consulted on how to optimize the kits to best serve their patients. Dozens of the kits have been shipped across the Americas to date, including 12 to organizations in Puerto Rico.