Cases of dengue fever are surging at a record pace across South America and Puerto Rico. The Pan American Health Organization recently confirmed that 3.5 million cases of dengue and more than 1,000 deaths have been reported across South and Central America in the first three months of this year. This is compared to a total of 4.5 million reported cases in the region in all of 2023. The outbreak has reached as far north as Puerto Rico, which recently declared a dengue epidemic due to a sudden spike in cases and hospitalizations.

Dr. Albert Ko, MD, an infectious disease epidemiologist in the Yale School of Public Health’s Department of Epidemiology of Microbial Diseases, specializes in community-based interventions to epidemics such as dengue, Zika, meningitis, and leptospirosis. He has conducted extensive research on infectious diseases in Brazil, where the vast majority of dengue cases have been reported. In addition to serving as Yale’s Raj and Indra Nooyi Professor of Public Health, Ko is a collaborating researcher at the Oswaldo Cruz Foundation of the Brazilian Ministry of Health. Ko shares his insights about the current dengue public health threat below.

Dengue fever is a viral disease that is transmitted to humans by bites from infected mosquitos, usually Aedes aegypti, which also has been known to carry the Zika and chikungunya viruses. The mosquitos breed in standing water and areas of poor sanitation. Dengue is most often found in hot and humid climates. In addition to Brazil and Puerto Rico, countries reporting outbreaks this year include Peru, Argentina, Colombia, and Paraguay. Many dengue infections are asymptomatic or cause mild illness such as high fever, headaches, muscle and joint paint, and nausea. The virus is unique in that it has four distinct variations and having one form of dengue fever does not offer protection against the other variations and can, in some cases, make the infection worse. In severe cases, dengue fever can cause internal bleeding and death.

What is driving the current outbreak of dengue in the Americas?

AK: Many parts of South America are experiencing a large epidemic of dengue because of unusually warm and humid weather, which is in part attributable to an El Nino phenomenon, and which promotes the expansion of the Aedes aegypti mosquito. The mosquito is the vector for dengue as well as other mosquito-borne viruses. In Brazil alone, over 2 million cases and 600 deaths due to dengue have been reported. This number will likely rise as the epidemic spreads.

The ongoing outbreak in Puerto Rico is unusual since dengue typically occurs in the wetter summer to fall months. The spread of dengue during this period may be due to people who are infected with dengue from regions experiencing the outbreak in the Americas but are asymptomatic and are traveling to locations such as Puerto Rico. This is introducing the virus in these settings which have mosquitoes that transmit the virus. It’s not exactly clear why the dengue outbreak is occurring in this typical “low” season but it’s surely a warning sign.

Is there a chance the surge in dengue will spread to the continental U.S. and if so, how soon might it happen?

We have had local transmission of dengue in Florida, Texas, and California so there is a risk that we will see cases in this region. It is much cooler and dryer in these states than in Puerto Rico so we may not see an increase in cases until we reach the summer.

Is there a vaccine to protect against dengue and what other steps can people take to reduce the risk of infection?

In the U.S., we have one licensed vaccine, Dengvaxia, but its use is restricted to people who already experienced a dengue infection. Outside of Puerto Rico, the vast majority of Americans have never been exposed to dengue, so the use of this vaccine is negligible. There are new vaccines such as Qdenga which have been licensed in other parts of the world but have not been licensed and is not available in the U.S.

For those who travel to regions where dengue is occurring, people can take precautions by wearing protective clothing (such as long sleeves and pants) and using insect repellant. Bed nets won’t be of much use since the Aedes aegypti mosquito bites during the day, especially in early morning and late afternoon/evening.