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September 19, 2025
9 min read
Driven by climate change, population mobility and global trade, mosquito-borne diseases are extending beyond tropical and subtropical areas.
Dengue, chikungunya and Zika virus — all spread by Aedes mosquitoes — have caused numerous outbreaks in North America over the past decade. West Nile virus, spread by Culex mosquitoes, has become the most common mosquito-borne disease in the United States since it was first introduced to the country around the turn of the century.
The diseases all have a wide spectrum of clinical manifestations, with varying levels of severity. The quicker the response to outbreaks, the better the chance to contain them, making it important to spread awareness, including globally, experts said.
“We cannot be sound asleep at the switch,” Jan C. Semenza, PhD, MPH, professor at Umeå University in Sweden and former head of the Health Determinants Program at the European Centre for Disease Prevention and Control (ECDC), told Healio | Infectious Disease News.
Using a multi-disease ecological niche model, researchers from the U.S., United Kingdom and Australia created global environmental suitability maps for mosquito-borne diseases and estimated that 5.66 billion people live in areas currently suitable for chikungunya, dengue and Zika, including 730 million people in the Americas, and an additional 1.54 billion live in areas suitable for yellow fever transmission.
Rubens Belfort Jr., MD, PhD, professor and president of the Vision Institute at Federal University of São Paulo in Brazil, compared outbreaks of mosquito-borne diseases to tsunamis.
“Most of the time, we cannot predict them or avoid them, but we can minimize damage,” he told Healio | Infectious Disease News. “We need sentinel units able to detect the initial cases and trigger health procedures, but we don’t have these systems in place in most countries. Therefore, mosquitoes will continue to be a big problem.”
Invasive species
Aedes albopictus, known as the Asian tiger mosquito, first arrived in the U.S. in the mid-1980s in used tires shipped from Asia, spreading rapidly from the South, according to the Center for Invasive Species Research at the University of California, Riverside. Aedes aegypti, the yellow fever mosquito, has been around much longer, arriving from West Africa via the European slave trade hundreds of years earlier, according to a review published by Yale researchers in 2018.
According to the authors of a multinational study published in Nature Microbiology in 2019, both species will continue to spread in the coming decades even under current climate conditions, “filling unoccupied suitable habitats and posing a risk to human health in the majority of locations where they survive and reproduce.”
Both mosquitoes are major vectors of chikungunya, dengue and Zika virus, and have caused outbreaks of all three in the Americas over the past decade.
Chikungunya
The first outbreak of chikungunya in the Americas — indeed, the first appearance of the virus among non-travelers in the Western Hemisphere — began on the Caribbean island of St. Martin in 2013 and spread to dozens of other countries and territories, according to a summary published in The Lancet Regional Health: Americas in 2024.
Since then, there have been 3.7 million suspected or confirmed cases, according to the researchers, who noted two factors that could increase the likelihood of transmission in the U.S. in the future: a large population of people who have never been infected with the virus, and the impact of climate change, which could expand the range of Aedes mosquitoes.
Dozens and sometimes hundreds of travel-associated cases of chikungunya are reported in the U.S. each year — including 56 this year, as of Aug. 26 — but there has not been a locally acquired case in a U.S. state since 2015, although U.S. territories continued to report cases through 2019.
Chikungunya can cause debilitating pain that persists for months or even years, and there are no medications to treat it. U.S. travelers used to have two vaccine options for chikungunya, but the FDA recently suspended the license for Valneva’s Ixchiq, a live-attenuated vaccine, over serious safety concerns after initially pausing its use among people aged 60 years or older. Months earlier, the FDA approved a virus-like particle vaccine called Vimkunya (Bavarian Nordic), which remains available for people aged 12 years or older and is recommended for travelers to areas where there is an outbreak of chikungunya.
Dengue
Dengue has been around a lot longer in the Americas. A history published in The American Journal of Tropical Medicine and Hygiene in 2012 suggests multiple possible introductions: suspected dengue epidemics in the Caribbean islands of Guadeloupe and Martinique in 1635 and the Central American country of Panama in 1699, and a dengue-like outbreak in Philadelphia in 1780.
Either way, by the 1800s, dengue was common in the Americas, especially in port cities, according to the review. Today, there are up to 400 million dengue infections each year globally, with around half of the world’s population living in areas that are susceptible to transmission, according to WHO, which noted several aspects of climate change — rising temperatures, rainfall and humidity — among the factors that increase the risk that dengue will spread to even more areas.
Dengue outbreaks in the U.S. are mostly associated with tropical territories, including Puerto Rico and the U.S. Virgin Islands, which have been experiencing outbreaks since last year. Among the 50 U.S. states, a few experience locally acquired cases, including Florida, which has reported 10 this year. California, Florida and Texas all reported local cases last year.
Dengue infection can cause a range of symptoms, most commonly fever with aches and pains, nausea, vomiting and rash, according to the CDC, which notes that around 5% of people develop a more severe form of the disease that can result in shock, internal bleeding and death. It is sometimes referred to as “breakbone fever.”
Dengue vaccine development has been challenging for several reasons, including the makeup of the virus itself, which has four serotypes. Infection with one serotype gives a person lifelong immunity to that serotype, but not the others. And because infection with one dengue serotype places a person at risk for more severe disease if they are exposed to another serotype, an ideal dengue vaccine needs to protect against all four.
The only approved dengue vaccine in the U.S. will not be available much longer. Sanofi is discontinuing Dengvaxia due to a lack of global demand, with the existing lots of vaccine set to expire in August 2026.
Because the vaccine is given in three doses over the course of a year, the time to start the series has all but passed. This is mostly an issue for the U.S. territories with endemic dengue, including American Samoa, Guam, Puerto Rico and the U.S. Virgin Islands. Because the vaccine can act as an initial infection, raising a dengue-naive patient’s risk for severe dengue, it was only approved for children aged 9 through 16 years who had a laboratory-confirmed previous dengue infection.
Zika
Once thought to be a relatively benign virus, Zika emerged last decade as a major public health threat, ravaging the brains of infants born to infected mothers and causing central nervous disease in adults.
The Zika virus outbreak that was centered in Brazil but spread throughout the Americas included hundreds of thousands of cases, mostly in 2016, with more than 270,000 reported in Brazil alone. Cases began to decline over the ensuing years. In fact, U.S. territories have not recorded a case of Zika since 2019, and no Zika virus transmission from mosquitoes has been reported in the continental U.S. since 2018, according to the CDC.
Children born to infected mothers are at risk for congenital Zika syndrome, a suite of birth defects and disabilities that include severe microcephaly. Because of the severity of these conditions, women of reproductive age are seen as a prime target of vaccine efforts. Although there is no licensed vaccine, several are in varying stages of development, according to WHO’s vaccine pipeline tracker.
Audina M. Berrocal, MD, director of pediatric retina at Bascom Palmer Eye Institute in Florida, contributed to milestone studies on congenital Zika syndrome and visited Altino Ventura Foundation in Recife, Brazil, in 2016, where she had the opportunity to examine infants suspected to have acquired Zika virus infection in utero. Recife was the epicenter of a large Zika outbreak, leading to an explosion of cases of microcephaly in newborns.
“Because of this experience in Brazil, we were able to diagnose here in Miami the first locally acquired cases of [congenital Zika syndrome] in babies with microcephaly,” Berrocal told Healio | Infectious Disease News.
Infection in the first trimester of pregnancy carries the highest risk of birth defects.
“Babies might be born with microcephaly, atrophy of the optic nerve, pigmentary changes of the retinal pigment epithelium in the fovea, chorioretinal atrophy, optic nerve atrophy and other defects anywhere in the eye,” she said. “Many of these babies are wheelchair bound, have almost a cerebral palsy presentation, and have frequent and often fatal lung infections.”
It is a unique set of challenges requiring lifelong, specialized caregiving. Altino Ventura Foundation has created a multispecialty service that provides early intervention, rehabilitation and free spectacles.
“Luckily, most of the congenital cases we have seen here in the U.S. had milder findings, indicating that the mothers who were infected here must have been further along in their pregnancy,” Berrocal said. “However, we must stay alert because the whole world is changing.”
Don’t forget about West Nile
After the first outbreak of West Nile virus in New York in 1999, it spread across the U.S. and has become endemic in many areas with diverse climates, including New York state, which has a humid continental climate; desert regions in Colorado and Arizona; and in North and South Dakota, which are freezing in winter, according to Healio | Infectious Disease News Editorial Board Member Lyle Petersen, MD, MPH, director of the CDC’s Division of Vector-Borne Diseases.
Petersen had a personal experience with West Nile virus at home in Fort Collins, Colorado. He was bitten by mosquitoes while outdoors at dusk talking with a neighbor, and after 3 days, developed fever, severe muscle pain, fatigue, headache, photophobia and a stiff neck, followed by a typical skin rash as his fever abated.
“The fatigue was terrible,” he said. “I tried to get back to work but lasted only 15 minutes. I could barely walk up the stairs in my house for months.”
His neighbor was also affected, and they were among the initial cases in the first big West Nile virus outbreak in Colorado.
“There was no background immunity in the bird population at the time, and because we have a lot of irrigated farmland, we have a lot of vector mosquitoes,” Petersen said.
Severe neurological conditions are the most concerning manifestation of West Nile, including meningitis, encephalitis and acute flaccid paralysis. They are relatively uncommon, occurring in about 1% of cases, and are related to age.
“In a 20-year-old, the risk is low, maybe one in 1,000, and then in every decade of life, that risk doubles. By the time you are 65 or 70 years of age, there is a substantial risk of developing severe neurologic disease,” Petersen said.
Mosquito-transmitted viruses fall into three categories, Petersen said: viruses such as yellow fever or dengue, in which the primary manifestations are systemic fever and other associated issues; viruses that cause neurologic diseases, such as West Nile and Zika; and viruses that primarily manifest with fever and arthralgia or arthritis, the classic one being chikungunya.
“They fall into those three buckets, but there may be overlap between all of them. For instance, dengue, which primarily just causes severe fever and shock syndrome, can also less commonly cause neurologic disease,” he said.
Mosquito control plans
In the U.S., the CDC, local governments and mosquito control districts have developed mosquito control plans by using an integrated mosquito management approach based on surveillance, reduction of mosquito oviposition sites and use of insecticides.
To dispel concerns about the use of insecticides, Petersen said that the amount used to control mosquitoes is extremely small.
“Following mosquito control operations for West Nile, we tested people in the community and could not find any evidence that they had been exposed because the amount used is so small,” he said. “Insecticides are applied at night because that’s when vector mosquitoes are active, but bees and other insects that we care about are not. The bees are in their hives, so they’re not getting exposed.”
There are more than 2,000 mosquito control operations in the U.S. that range in quality. When applied correctly, control measures usually work well for Culex mosquitoes, but Aedes mosquitoes are hard to control, Petersen said.
“As much as we try, it’s just really difficult to control that species. So, there are some real challenges ahead,” he said.
- References:
- Brathwaite O, et al. Am J Trop Med Hyg. 2012;doi:10.4269/ajtmh.2012.11-0770.
- CDC. About chikungunya. https://www.cdc.gov/chikungunya/about/index.html. Updated May 15, 2024. Accessed Aug. 28, 2025.
- CDC. Chikungunya vaccines. https://www.cdc.gov/chikungunya/vaccines/index.html. Updated Aug. 22, 2025. Accessed Aug. 28, 2025.
- CDC. Current dengue outbreak. https://www.cdc.gov/dengue/outbreaks/2024/index.html. Updated July 29, 2025. Accessed Aug. 28, 2025.
- CDC. Dengue vaccine. https://www.cdc.gov/dengue/hcp/vaccine/index.html. Updated May 15, 2025. Accessed Aug. 28, 2025.
- CDC. Zika cases in the United States. https://www.cdc.gov/zika/zika-cases-us/index.html/ Updated Aug. 26, 2025. Accessed Aug. 28, 2025.
- De Souza WM, et al. Lancet Reg Health Am. 2024;doi:10.1016/j.lana.2023.100673.
- Powell JR, et al. Bioscience. 2018;doi:10.1093/biosci/biy119.
- UC Riverside. Asian tiger mosquito. https://cisr.ucr.edu/invasive-species/asian-tiger-mosquito. Accessed Aug. 28, 2025.
- WHO. Dengue fact sheet. https://www.who.int/news-room/fact-sheets/detail/dengue-and-severe-dengue. Updated Aug. 21, 2025. Accessed Aug. 21, 2025.
- WHO. Tracking the new vaccine pipeline. https://www.who.int/teams/regulation-prequalification/eul/eul-vaccines/tracking-the-new-vaccine-pipeline. Accessed Aug. 28, 2025.
- For more information:
- Rubens Belfort Jr., MD, PhD, of Clínica Belfort, São Paulo, Brazil, can be reached at prof.belfort@clinicabelfort.com.br.
- Audina M. Berrocal, MD, of Bascom Palmer Eye Institute, Miami, can be reached at aberrocal@med.miami.edu.
- Lyle Petersen, MD, MPH, of the CDC, can be reached at lxp2@cdc.gov.
- Jan C. Semenza, PhD, MPH, of Umeå University, Sweden, can be reached at jan.semenza@lateralphc.com.
Sources/Disclosures
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Source:
Healio Interviews
Disclosures:
Belfort, Berrocal, Petersen and Semenza report no relevant financial disclosures.
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