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  • Writer's pictureP.K. Peterson

Dengue Fever, a Barometer of Climate Change

“For the first half of 2024, dengue cases in the Americas region have already surpassed yearly maximums, and cases have already doubled those of 2023.”

CIDRAP News, June 20, 2024


“Repellent sold out everywhere, there were literal clouds of mosquitoes.”

Laura Ledesma, Buenos Aires resident

 


Since my last post on dengue less than a year ago, cases of dengue have soared at a dizzying pace. (See August 23, 2023 Germ Gems, “Dengue Virus Descends on Florida: Why and So What?”). Late this June, the Centers for Disease Control and Prevention (CDC) alerted health providers and the public about the increased risk of dengue virus infection in the U.S. (On June 27, 2024, the CDC reported at least 2,241 confirmed cases of dengue in the U.S. and Puerto Rico.) Therefore, it seemed like a good time to revisit dengue fever with the goals of refreshing our memories about this mosquito-borne disease and exposing a culprit lurking behind the scene—climate change. 

What is dengue fever (a recap). Dengue fever is an acute viral infection caused by one of four serotypes of the RNA dengue virus, designated DENV-1, DENV-2, DENV-3, and DENV-4. All four serotypes are currently circulating.


Dengue virus is one of the pestiest of the arboviruses, meaning it is arthropod-borne. The virus is transmitted by one of two Aedes mosquitoes—A. aegypti, the mosquito that is now causing problems throughout urban areas of Miami-Dade County in Florida, or A. albopictus, a more cold-tolerant mosquito that’s been found as far north as Minnesota.


Dengue fever is characterized by a sudden onset of high fever, severe headache, muscle and joint pains, rash, nausea, and vomiting.  Because victims of dengue often have contortions due to the intense joint and muscle pain, it is also called “break-bone fever.”


The disease can range from asymptomatic to severe forms, including dengue hemorrhagic fever which is associated with internal bleeding, increased vascular permeability, and multi-organ failure. Mortality from dengue can be as high as 13%, and children are at greatest risk of severe disease.     


There are no antiviral drugs available to treat dengue. Instead, management of the disease is focused primarily on supportive care such as fluid replacement, pain and fever relief, and bed rest.


Global burden of dengue. About half of the world’s population is at risk of dengue; there are 100-400 million cases reported each year worldwide. A large majority of dengue cases occur in tropical or subtropical countries—areas of the world where Aedes mosquitoes thrive. In the U.S., most cases are acquired by travelers returning from these tropical or subtropical areas. The CDC recently reported an increase—745 cases to date—in the number of dengue cases among U.S. travelers.   


In recent decades, the incidence of dengue has grown dramatically around the world. Since the beginning of 2023, ongoing transmission, combined with an unexpected spike in cases resulted in a historic high of over 6.5 million cases and more than 7300 dengue-related deaths.

Prevention of dengue. The World Health Organization and other public health organizations recommend three strategies for the prevention of dengue. They are: vaccination programs; eradication of the A. aegypti mosquito; and the use mosquito repellents.


As of June 2024, there were two main dengue vaccines: Dengvaxia (approved for use in the U.S.) and Qdenga (approved for use by the European Commission). Dengvaxia is a live-recombinant tetravalent vaccine, i.e., includes all four serotypes, but it requires pre-vaccination screening for previous dengue infection—a practical limitation to its use. In the U.S., the Advisory Committee on Immunization Practices recommends Dengvaxia “for children aged 9-16 years with confirmed prior dengue infection living in endemic areas of the U.S. and its territories (Puerto Rico, American Samoa, the U.S. Virgin Islands, the Federated States of Micronesia, the Republic of Marshall Islands, and the Republic of Palau) but not for travelers to these regions.”


Qdenga is a live-attenuated tetravalent vaccine. It is more versatile than Dengvaxia as it doesn’t require prior screening for dengue infection. Additionally, the Brazilian Butantan Institute has developed a new dengue vaccine that is showing promise.


A second, and quite innovative, approach to prevent dengue is to biologically control it with the use of the bacterium Wolbachia. Strains of Wolbachia are introduced into A. aegypti mosquitoes to prevent transmission of the dengue virus either directly by blocking the virus or by decreasing the life span of the A. aegypti mosquito. This approach to mosquito eradication, fostered by the non-profit organization World Mosquito Program, is underway in several areas of the world. Recently, it was associated with a significant reduction of dengue cases in Colombia.


But, “[t]he best way to prevent dengue is to protect yourself from mosquito bites.” (See CDC website).  Therefore it is not surprising that the third strategy for dengue prevention also targets the mosquito vector, i.e., the use of mosquito repellents.  


In the U.S., the CDC recommends using Environmental Protection Agency-registered insect repellents which, when used as directed, are proven safe and effective. Unfortunately, mosquito repellents are not available currently in certain areas of the Americas. In a January 25, 2024 Reuters Daily Briefing article “South America dengue spike prompts vaccination drive as bug spray runs out,” journalists Ricardo Brito and Lucinda Elliott reported that the case loads of dengue in Brazilia “have leapt by as much as 646% in the first 20 days of January versus last year.” They also reported that many stores had run out of mosquito repellents.

Why is dengue virus winning? Greenhouse gas emissions are causing temperatures to rise globally allowing mosquito populations to grow in areas that were previously uninhabitable to them. Dengue is now seen in regions that were previously too cold for the A. aegypti mosquito to survive.


When asked why the number of dengue cases is accelerating in Brazil and beyond, Thais dos Santos, who specializes in arboviral diseases at the Pan American Health Organization, pointed out that the El Nino weather pattern in the Pacific contributes to global warming. He also said,  “Climate change has expanded the range for mosquitos to breed, both in the Americas and globally.”


The record breaking surge of dengue fever cases in the Americas provides extra support for a full-fledged campaign to put the brakes on climate change.

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Main Page images courtesy of Shuxian Hu, MD. Dr. Hu is a scientist in the Neuroimmunology Research Laboratory at the University of Minnesota.

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