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"More Americans are at risk than ever before as mosquitoes and ticks are moving into new areas of the country, increasing cases and geographic ranges of vector-borne diseases." said Christopher Braden, M.D., the Acting Director of the Centers for Disease Control and Prevention (CDC) Center for Emerging and Infectious Zoonotic Diseases.

He and others who spoke at ASM’s July 15, 2022, webinar, “Vector-Borne Disease in a Changing Climate,” emphasized the fact that climate change is having a collective impact on all vector-borne diseases, including chikungunya, Zika, West Nile virus, malaria, yellow fever, dengue and many others. , and 10 novel pathogens have been discovered. and an ongoing geographic expansion promoted by warmer summers is allowing ticks, including those carrying bacteria that cause Lyme disease, and other vectors that carry pathogens to migrate farther north.

As of 2024, dengue, a mosquito-borne disease that typically occurs in tropical and subtropical regions of the world, is chief among these migrating vector-borne diseases. From Jan. 1-June 24, 2024, dengue has caused record-breaking numbers of cases worldwide. Over 10 million cases and 5,000 dengue-related deaths have been reported by 80 countries and territories around the world since the beginning of 2024—most of these . According to a , this is approximately double the number of cases that were observed in all of 2023. Some basic knowledge about the virus and how it is transmitted can help us better contain this ongoing threat.

What is Dengue?

, the most common arboviral disease in the world, is caused by 4 viruses (DENV-1, -2, -3 and -4), which are very closely related genotypes. One of the peculiar immunologic characteristics of this disease is that if someone has been infected with 1 type of DENV, they usually develop life-long immunity to that specific DENV but are only protected from infection by the other 3 DENVs for several months to years after primary infection. Even more peculiar is the observation that (i.e., with a different type of DENV from the first infection) carries a higher risk of severe disease than a first, third or fourth infection. However, it should be noted that any DENV infection can lead to severe disease.

The through the bites of infected female mosquitoes, primarily the Aedes aegypti mosquito. Other species within the Aedes genus can also act as vectors, but their contribution is normally secondary to A. aegypti. The Asian Tiger mosquito (A. albopictus) is another species that can spread dengue. 

Dengue is common in many countries globally, including the Americas, Africa, the Middle East, Asia and the Pacific Islands. In the U.S., dengue is a nationally notifiable disease. Six U.S. territories and freely associated states are classified as areas with frequent or continuous dengue transmission: Puerto Rico, American Samoa, the U.S. Virgin Islands, the Federated States of Micronesia, the Republic of Marshall Islands and the Republic of Palau.

The mosquitoes that spread dengue are common throughout many areas of the U.S., so local transmission is possible but has been historically limited, with sporadic cases or small outbreaks in Florida, Hawaii and Texas.

The Current Global Outbreak

However, these historical trends are being challenged by current outbreaks. Many countries in the PAHO region are experiencing case counts that exceed the highest number of dengue cases ever reported in a single calendar year. Brazil has reported the most cases in 2024 (over 8 million), followed by Argentina, Paraguay, Peru and Colombia. Mainland Europe reports imported cases from endemic areas (Germany, Italy and France) but no autochthonous cases (locally acquired) have been reported so far. U.S. territory, over the virus.

In the 49 continental states, most of the reported dengue cases have occurred in travelers who experienced primary exposure outside of the U.S. Yet, confirmed local DENV transmission has been reported by Arizona and California over the past 2 years, and, as of June 25,  in 2024, causing some experts to warn that the southern U.S. could become a hotspot for an outbreak.

Still, according to the CDC, widespread transmission remains low, and large outbreaks are not expected across the U.S. this summer. Additional travel-related cases and small local transmission chains linked to those cases are anticipated. For a complete list and overview of global dengue cases, see the .

Signs and Symptoms of Dengue Disease

About 25% of DENV infections are symptomatic, and the disease can range from mild to severe. The typical incubation of an infection is 1 week (3–10 days). include, a sudden high fever with severe headaches, pain behind the eyes, muscle and joint pain, nausea, vomiting, enlarged glands and a rash that occurs a few days after the fever starts.

Laboratory findings may include, low white blood cell counts, high hematocrit (liver enlargement >2cm.) and other warning signs, which are specific clinical findings that predict progression to severe disease. These signs include, ongoing vomiting, abdominal pain or tenderness, ascites (clinical fluid build-up), mucosal bleeding and lethargy. A small percentage of people (1 in 20) with dengue fever get a severe case called (DHF).
Symptoms of DHF show up as the fever begins to ease and may include, vomiting that does not go away, rapid breathing, blood in vomit, organ failure and bleeding gums.

Patients typically report night sweats, very sore eyes, devastating headaches, enlarged lymph nodes and severe body aches that give the illness the scary moniker of “break-bone fever." Symptoms last for 2-7 days, during which activity is severely limited.

Treatment and Prevention

Dengue is usually mild, but sometimes progresses to severe disease and even death if undetected and/or untreated. Unfortunately, there are no antiviral medications approved to treat dengue. Usually, treatment is supportive (analgesic medication) and requires careful volume management (fluid replacement via oral rehydration or intravenus administration of fluids). If the infection is untreated, the mortality rate for severe dengue fever can be as high as 20%, but rapid initiation of a standardized fluid replacement strategy can decrease mortality from 13% to <1%. 

The  of dengue patients. Likewise,  expectant management of patients at high risk for severe disease, including infants aged ≤1-year, pregnant people, adults aged ≥65 years and people with certain medical conditions.

In June 2021, the , for children aged 9–16 years with laboratory confirmation of previous DENV infection, and living in areas with frequent or continuous dengue transmission, such as Puerto Rico. While the vaccine is considered safe and effective, the citing a lack of demand. Vaccine administration will continue in Puerto Rico until available doses expire in 2026. No vaccines have been recommended for travelers, adults or persons without a previous DENV infection.

The Future of Dengue Fever

Climate change is creating unusually balmy conditions, which contribute to the expansion of mosquito-borne diseases. The  of 2023 (which consisted of increases in temperatures, humidity and rainfall) created ideal conditions for mosquitoes to hatch en masse and carry the virus to new environments (and hosts) around the world. WHO and other experts warn this could be a. Additional factors, such as fragile health care systems, financial and/or political instability, globalization and socioeconomic imbalances only compound the issue. Collaborative local and international vector control and prevention plans will be required to limit the spread of vector-borne diseases, and early detection and supportive therapies will remain critical for improving patient outcomes.

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According to the World Health Organization, , most of which are preventable, account for more than 17% of all infectious diseases and result in the loss of an estimated 700,000 lives annually around the world. Learn what can be done to improve these outcomes.