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                              West Nile Virus and Other Mosquito-Borne Diseases
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West Nile Virus and Other Mosquito-Borne Diseases








  
  
    

  
      



Vector-Borne Diseases in the U.S. in 2023

Climate change is one of several factors that can influence the distribution and prevalence of vector-borne diseases. Among vector-borne diseases in 2023, Lyme disease (tickborne) as well as West Nile virus, dengue, and malaria (mosquito-borne) were of public health concern in the United States. Lyme disease is the most common vector-borne illness in the United States, with an estimated 476,000 people diagnosed and treated each year. Traditional Lyme disease surveillance data are available through 2021 from CDC. In 2023, 2,406 West Nile virus disease cases were identified across 47 jurisdictions, including 1,599 neuroinvasive disease cases. There were also 2,556 dengue cases across 52 jurisdictions, including **locally acquired dengue cases in** **Florida (n=168)**, **California (n=2), Texas (n=1)**, and Puerto Rico (n=933; where dengue is endemic). The vast majority of malaria cases in the United States are travel-related, usually by people who travel to countries where malaria is endemic (regularly occurring). During May–October of 2023, though, the United States had a total of **10 cases of locally acquired****malaria****reported in Florida, Texas, Maryland, and Arkansas** (as of October 19, 2023).

Number of West Nile virus human disease cases in the United States over the past decade.

**Figure**: Number of West Nile virus human disease cases in the United States over the past decade. Data from CDC *(2023 data are preliminary and subject to change).*

Number of locally acquired dengue cases in the contiguous U.S. over the past decade. Locally acquired cases occurred among people with no history of travel to a dengue-endemic region in the two weeks before illness onset.

**Figure**: Number of locally acquired dengue cases in the contiguous U**.**S**.** over the past decade. Locally acquired cases occurred among people with no history of travel to a dengue-endemic region in the two weeks before illness onset. Data from CDC *(2023 data are preliminary and subject to change).*
                        West Nile Virus

West Nile virus (WNV) is the most common mosquito-borne disease in the continental United States, with a median of 2,205 cases reported each year (range: 712–9,862). People typically get infected following the bite of a mosquito carrying the virus. While uncommon, WNV has been spread through blood transfusion and organ transplantation, and from mother to baby during pregnancy, delivery, or breast feeding.

Approximately 80% of people infected with WNV will not have any symptoms. About 20% will experience a fever and other flu-like symptoms, and less than 1% will develop severe West Nile neuroinvasive disease (WNND), a condition that can lead to death or long-term disability. Older adults and those with compromised immune systems are at higher risk for WNND. There is currently no available treatment or vaccine for WNV disease. In severe cases, patients may need to be hospitalized to receive supportive treatment.

West Nile Virus Distribution

Most U.S. counties have reported WNV disease cases since its introduction into the U.S. in 1999. However, the incidence of WNV disease varies greatly. The Great Plains and western states are more likely to have high incidence of WNV (defined as more than 1.10 cases per 100,000 people). Six counties with both high incidence and large populations reported 23% of all WNND cases during 2009– 2018 (Cook County, IL; Dallas County, TX; Harris County, TX; Los Angeles County, CA; Maricopa County, AZ; and Orange County, CA). Although certain areas of the country are more likely to have higher WNV incidence, the number of cases reported in a given county varies greatly each year. This makes accurately predicting the number of WNV disease cases that will occur each year and in each county challenging.

**West Nile virus human neuroinvasive disease average annual incidence per 100,000 population by county of residence, 1999–2022***

Since 1999, WNND cases have been reported in the majority of counties in the continental United States. The counties with the highest incidence for WNND cases are mostly located in the Great Plains and western states.
Source: https://www.cdc.gov/westnile/statsmaps/historic-data.html

Figure: Since 1999, WNND cases have been reported in the majority of counties in the continental United States. The counties with the highest incidence for WNND cases are mostly located in the Great Plains and western states.

Which Months Do People Most Often Develop West Nile Virus?

The majority of WNV cases occur during mosquito season, which starts in the summer months and continues through fall. Cases of WNV are most commonly reported in August and September.

**West Nile virus human disease cases reported by month of illness onset, 1999–2022, all disease cases**

WNV human disease cases reported by month of illness onset, 1999–2022.
Source: https://www.cdc.gov/westnile/statsmaps/historic-data.html

Figure: WNV human disease cases reported by month of illness onset, 1999–2022.

Climate Change and West Nile Virus

Climate change has resulted in milder winters, earlier springs, longer and warmer summers, and changes in regional precipitation. These factors could potentially affect WNV transmission through changes in, for example, bird migration and breeding patterns, mosquito population size and biting rates, and human behaviors, such as spending time outdoors. While we do not fully understand how climate change impacts WNV transmission across the United States, seasonal weather patterns can have an effect. This was observed in 2021 in Maricopa County, AZ, when the county experienced the largest-ever WNV outbreak, resulting in a reported 1,487 WNV cases, 1,014 hospitalizations, and 101 deaths. Although the reasons behind this outbreak are likely multiple, one potential contributing factor was the increased rainfall that occurred during a wetter-than-average monsoon season in 2021. This change led to a longer duration and increased amount of moisture that likely resulted in the maintenance of mosquito larval habitat sites, leading to greater WNV transmission. To better understand the impact of climate on WNV and better predict WNV transmission, the Centers for Disease Control and Prevention (CDC) and the National Atmospheric and Oceanic Administration (NOAA) are partnering to develop models that can forecast WNV for the upcoming season.
                        West Nile Virus Transmission Cycle
    In nature, WNV cycles between mosquitoes and birds. Some bird species develop high levels of the virus in their bloodstream, and mosquitoes can become infected by biting these infected birds, continuing the cycle. Humans are “dead end” hosts, meaning they don’t pass on the virus to other mosquitoes that bite them, because they do not develop high enough levels in the blood stream.

West Nile Virus Transmission Cycle
Source: Centers for Disease Control and Prevention
                        Preventing West Nile Virus Disease
    The best way to prevent WNV disease is to protect yourself from mosquito bites. When outside, use an Environmental Protection Agency-registered insect repellent containing one of the following active ingredients: DEET, picaridin, IR3535, oil of lemon of eucalyptus, para-methane-diol, or 2-undecanone, and follow these tips for applying insect repellent on children from the American Academy of Pediatrics; or wear loose-fitting, long-sleeved shirts and pants. Other important prevention measures include using screens on windows and doors, using air conditioning if available, and removing standing water in items such as buckets, planters, or birdbaths.
                        Resources to Reduce Health Risks Associated with West Nile Virus
    The CDC West Nile Virus site has information on disease transmission, prevention, and common symptoms.
	The CDC Fight the Bite site has information on preventing bites from both ticks and mosquitoes.
	The CDC Mosquito Control site has information on taking steps to control mosquitoes both indoors and outdoors.****
                        Malaria

For the first time since 2003, the U.S. has had locally acquired cases of malaria in Florida, Texas, and Maryland. Malaria is a serious mosquito-borne disease caused by different species of *Plasmodium* parasites that infect *Anopheles* species mosquitoes. People typically become infected with malaria following the bite of a mosquito carrying the parasite.

There are about 2,000 reported cases of malaria in the U.S. each year, mostly in travelers returning from other countries. Continuous spread of malaria was eliminated in the U.S. in the early 1950s through mosquito surveillance and control measures; however, as we’ve seen this summer, locally acquired malaria cases do still occur in the U.S.

*Anopheles* species mosquitoes can be found in much of the continental U.S., making local spread possible if people infected in a malaria-endemic country travel to the U.S. and are bitten by local *Anopheles* mosquitoes, which can then become infected and spread the parasite to people who have not traveled. The risk for local transmission is higher in areas where local climatic conditions allow *Anopheles* mosquitoes to survive during all or most of the year. Temperatures also need to be warm enough for the malaria parasite to develop in the mosquito. This summer, the U.S. has had plenty of the hot and humid conditions in which mosquitoes thrive, as well as increased international travel, potentially contributing to local spread.

While local malaria transmission in the U.S. is rare, it is possible that increased temperatures and altered precipitation patterns and humidity due to climate change may lead to increased mosquito populations, increased range of *Anopheles*mosquitoes, a greater number of mosquito biting days, and faster development of the malaria parasite in mosquitoes, all of which could impact local transmission in the U.S. in the future.

The best way to prevent any mosquito-borne disease, including malaria, is to protect yourself from mosquito bites. When outside, use an Environmental Protection Agency-registered insect repellent containing one of the following active ingredients: DEET, picaridin, IR3535, oil of lemon eucalyptus, para-methanediol, or 2-undecanone, and follow these tips for applying insect repellent on children from the American Academy of Pediatrics; and/or wear loose-fitting, long-sleeved shirts and pants. Additionally, if you are traveling to a malaria-endemic country, check out CDC’s malaria prevention recommendations.

Anopheles species mosquito
Dengue

Dengue viruses are spread to people through bites of infected *Aedes*  species mosquitoes *(Ae. aegypti* or *Ae. Albopictus*). There are an estimated 400 million dengue infections each year, but only about 25% of people infected with dengue will get sick. Symptoms typically include fever with aches and pains, nausea and vomiting, or rash. Less than 5% of dengue infections will progress to severe disease, which can lead to hospitalization and death. Early diagnosis and supportive medical care are essential in cases of severe dengue, which can be life-threatening within a few hours. There are currently no medications available to treat dengue.

Chart of Common symptoms of dengue
Figure: Common symptoms of dengue. Source: https://www.cdc.gov/dengue/symptoms/index.html
                        Dengue Distribution
    Map of areas with dengue risk, including frequent or continuous risk, sporadic or uncertain risk, and no evidence of dengue risk.
Dengue viruses are common in most tropical and sub-tropical regions of the world, which experience year-round transmission with seasonal and intermittent major epidemics. *Aedes* mosquitoes have hitchhiked to most of the warm and wet areas of the world where people live. This includes many parts of the U.S., some of which have endemic dengue, including American Samoa, Puerto Rico, the U.S. Virgin Islands, and the freely associated states. In Puerto Rico, there were almost 30,000 confirmed cases reported between 2010 and 2020.

Figure: Map of areas with dengue risk, including frequent or continuous risk, sporadic or uncertain risk, and no evidence of dengue risk. Source: https://www.cdc.gov/dengue/areaswithrisk/around-the-world.html

Occasional dengue outbreaks can occur in non-endemic areas of the U.S. where *Aedes* mosquitoes live, but these outbreaks are rare. Outbreaks can happen when an infected traveler comes home and gets bitten by an *Aedes* mosquito. The mosquito gets infected and then bites healthy people, which can lead to an outbreak. *Aedes* mosquitoes require warm and humid environments for survival and reproduction, yet eggs can survive for up to 8 months in cold and dry conditions. This allows the species to persist through dry periods and winters in many parts of the continental U.S. and other temperate regions of the world. Recent local dengue transmission in the continental U.S. has occurred in Florida, Texas, and Arizona, but most dengue cases reported in the continental U.S. occur in travelers infected elsewhere. In 2019, there were 1,474 travel-associated cases reported in the U.S., but the true annual number of infected travelers is likely many times higher.

Maps showing the potential range of Aedes aegypti and Aedes albopictus in the U.S. as of 2017.
Figure: Maps showing the potential range of *Aedes aegypti*and *Aedes albopictus* in the U.S. as of 2017. These maps represent the CDC’s best estimate of the potential range of *Aedes* species mosquitoes in the U.S., but do not represent risk for spread of disease. Source: https://www.cdc.gov/mosquitoes/mosquito-control/professionals/range.html
                        Climate Change and Dengue

The impact of climate change on dengue is complex since many factors play a role in dengue transmission*. Aedes*speciesmosquitoes require warm temperatures for virus transmission, with optimal transmission estimated to occur at 77-86°F (25-30°C). Mosquitoes can alter their behavior to mitigate adverse environmental exposures, such as resting in shaded areas when temperatures are too high. The global increase in temperatures seems like a recipe for increased dengue due to better conditions for virus transmission.

Temperature is only one aspect of climatic suitability for *Aedes* mosquitoes. Extreme weather events that are increasing with climate change, such as floods and hurricanes, can also impact mosquito populations and dengue virus transmission. It is common for mosquito populations to decrease during and immediately after a hurricane, and then grow rapidly. Water accumulation is critical for immature mosquito development, and humidity is important for adult survival.

However, advances in infrastructure, like improved housing, water, and sanitation services, have helped to make dengue transmission rare in the continental U.S. These advances may help to prevent the spread of dengue, even as climate change makes environmental conditions more favorable for transmission.
                        Prevention
    Mosquito control methods, such as trapping adult mosquitoes; using insecticides and larvicides; and removing standing water from places where mosquitoes lay eggs, such as buckets, planters, or bird baths; all help to stop the spread of dengue. Additionally, improved sanitation services, such as garbage collection and sewage systems, can decrease mosquito breeding sites. You can also prevent mosquitoes from getting into your home by installing screens on windows and doors and using air conditioning when available.

People living in or traveling to areas with higher risk of dengue should take steps to prevent mosquito bites. When outside, use anEnvironmental Protection Agency-registered insect repellent containing one of the following active ingredients: DEET, picaridin, IR3535, oil of lemon eucalyptus, para-methanediol, or 2-undecanone, and follow these tips for applying insect repellent on children from the American Academy of Pediatrics; and/or wear loose-fitting, long-sleeved shirts and pants. Additionally, children aged 9-16 years with laboratory-confirmed previous dengue virus infection and living in areas where dengue is endemic should get vaccinated for dengue.
                        Dengue and Hurricanes

Dengue, a viral disease spread to people through the bite of infected *Aedes* species mosquitoes, is common throughout the tropical and sub-tropical regions of the world, including some U.S. territories and freely associated states. Some continental states have also reported local dengue spread like Arizona (2022), Florida (2013, 2020, 2022), and Texas (2013, 2020). As with other pathogens carried by insects, the spread of dengue is highly dependent on environmental conditions.

While *Aedes* species mosquitoes generally do not survive the high winds and flooding that hurricanes bring, mosquito eggs can survive. Natural and manmade containers filled with rain or used for water storage provide an excellent environment for eggs to hatch and larvae to grow. It is common for mosquito populations to decrease during and immediately after a hurricane, and then grow rapidly, as was seen approximately two weeks after Hurricane Maria in Puerto Rico in 2017. Increasing numbers ofmosquitoes, combined with the destruction of housing and infrastructure, temporary or permanent human migration, and interruptions to mosquito control measures, can raisethe risk of dengue transmission in the weeks that follow a hurricane.

People can take steps to protect themselves and their families from mosquito bites by using Environmental Protection Agency-registered insect repellent; wearing loose-fitting, long-sleeved shirts and pants; controlling mosquitoes in and around their homes; and having children 9-16 years old who live in dengue-endemic areas and have laboratory confirmation of a previous dengue infection get a dengue vaccine.

                    
  


Content created by Assistant Secretary for Health (ASH) Content last reviewed February 8, 2024 Back to top
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