Mosquito Almanac
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Mosquito-borne disease risk by region

Diseases & Health Updated Jul 2026 7 min read
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The short answer

Your mosquito-borne disease risk depends heavily on where you live. Different regions carry different risks because different mosquito species live in different places, and each species carries a different set of viruses. A mosquito that thrives in a Gulf Coast backyard is not the same species that breeds in a northern hardwood swamp or an irrigated western field, and the viruses those mosquitoes spread are not the same either.

That said, one virus is close to universal. West Nile is now reported across nearly the entire lower 48. Beyond West Nile, the picture becomes regional: eastern equine encephalitis and La Crosse in the East, western-transmitted West Nile across the Plains and West, and a rising level of dengue risk in the warmest states and territories.

West Nile is nearly everywhere

West Nile virus is the most commonly reported mosquito-borne disease in the country and is found in nearly every state. In 2023, it accounted for 2,628 reported cases, about 95 percent of all reported domestic arboviral disease that year. Of those, 1,789 (68 percent) were neuroinvasive, the more severe form that affects the brain and spinal cord, and 194 patients (7 percent) died. Cases were reported from 692 counties across 46 states and the District of Columbia [1].

Although West Nile is widespread, the per-capita burden is not evenly spread. In 2023 the highest state incidence rates were in Colorado (5.38 per 100,000), South Dakota (5.00), and Nebraska (4.60) [1]. Preliminary 2024 surveillance showed the same pattern, with incidence highest across the West South Central and West North Central regions [2]. This clustering in the Plains and interior West is tied to Culex tarsalis, an efficient West Nile carrier that is common in irrigated and rural western landscapes [3].

Regional patterns

Once you look past West Nile, the risks sort out by region and by mosquito habitat.

Northeast and Great Lakes. This is the core range for several mosquito-borne viruses that are rare but serious. Eastern equine encephalitis (EEE) transmission is most common in and around freshwater hardwood swamps in the Atlantic Coast states and the Great Lakes region [4]. La Crosse virus and Jamestown Canyon virus also circulate here. In 2023, Jamestown Canyon accounted for 27 reported cases across seven states, with the highest incidence in Wisconsin; most patients were older adults [1].

Southeast and Gulf Coast. EEE is also concentrated along the Gulf and southern Atlantic coasts, again linked to swampy freshwater habitat [4]. The warm, humid climate supports year-round or near year-round mosquito activity, and it is these states that carry the highest potential for locally transmitted dengue (see below). Florida in particular reports the most local dengue transmission of any mainland state.

Western states. West Nile dominates here, spread largely by Culex tarsalis across the Plains, Mountain West, and irrigated valleys [3]. Western equine encephalitis was historically a western concern but has been reported only very rarely in recent decades.

Southwest. West Nile is present, and the Southwest also carries dengue potential because Aedes aegypti, a key dengue mosquito, is established in parts of the region and in warm urban areas.

Appalachia. The Appalachian region is a persistent hot spot for La Crosse virus, which mostly infects children. In 2023, 35 La Crosse cases were reported from 10 states, and 91 percent of patients were under age 18 [1]. Over a longer period, more than 80 percent of reported La Crosse cases have come from just four states: North Carolina, Ohio, Tennessee, and West Virginia, with the disease shifting from its historic Midwest focus into Appalachia [5]. La Crosse is the leading cause of mosquito-borne encephalitis in US children [5].

Travel-associated diseases

Several well-known mosquito-borne diseases are, for most US residents, mainly a travel risk rather than a local one. Dengue, Zika, and chikungunya are widespread in tropical parts of the world, and the large majority of cases diagnosed in the mainland states are in travelers returning from affected areas.

Dengue illustrates this clearly. In 2024, a record 3,483 travel-associated dengue cases were identified among US travelers, an 84 percent increase over the prior year, with the most cases reported in Florida (1,016), California (648), and New York (327) [6]. Global dengue reached about 14.1 million cases that year [6].

But dengue is not only a travel disease. Local, mosquito-transmitted dengue does happen in warm areas. In 2024, locally acquired mainland cases were reported in Florida (91), California (18), and Texas (1) [6]. In the US territories, dengue is endemic: Puerto Rico reported 6,291 cases in 2024, crossed its epidemic threshold, and declared a public health emergency [7].

Malaria is a similar story. Almost all US malaria is travel-related, but in 2023 the country recorded 10 locally acquired cases in Florida, Texas, Maryland, and Arkansas, the first local mosquito transmission reported to public health authorities in about two decades [8].

Why local vector control and surveillance matter

Because risk is so local, the systems that track and reduce it are local too. Surveillance data used above come from ArboNET, the national arboviral surveillance system run jointly by the CDC and state and local health departments [2]. Mosquitoes are trapped and tested, human and animal cases are reported, and that information tells health officials which viruses are active, where, and when.

That local detail is what makes targeted control possible. Where surveillance shows West Nile activity climbing, mosquito control districts can treat breeding sites and, if needed, spray. Where dengue-competent Aedes aegypti is present, control focuses on the small containers around homes where that species breeds. The dengue response in Puerto Rico in 2024 paired vector control with public awareness campaigns as case numbers rose [7]. Without local surveillance and control, none of that can be aimed where it is actually needed.

How to check your local risk

The most useful, current information about your area comes from local sources, not national averages.

  • Your state health department usually publishes seasonal mosquito and arbovirus reports, often with maps showing where West Nile or other viruses have been detected in mosquitoes, birds, or people that year.
  • Your county health department or mosquito control district tracks trapping results and spraying schedules at the neighborhood level.
  • CDC data and maps give the national and state-by-state picture and link out to current-year surveillance [2].

Check these at the start of and during mosquito season, because activity changes year to year and month to month.

Sources

  1. CDC / MMWR. “West Nile Virus and Other Nationally Notifiable Arboviral Diseases, United States, 2023.” MMWR 2025;74:358-364 (via PubMed Central mirror PMC12161518). Claims: 2,628 WNV cases (95 percent of arboviral disease), 1,789 neuroinvasive (68 percent), 194 deaths (7 percent), 692 counties in 46 states plus DC; highest 2023 incidence Colorado 5.38, South Dakota 5.00, Nebraska 4.60 per 100,000; La Crosse 35 cases from 10 states, 91 percent under 18; Jamestown Canyon 27 cases from 7 states, highest in Wisconsin; St. Louis encephalitis 21 cases from 3 states. https://pmc.ncbi.nlm.nih.gov/articles/PMC12161518/ and https://www.cdc.gov/mmwr/volumes/74/wr/mm7421a1.htm (accessed 2026-07-20)
  2. CDC. West Nile Virus Data and Maps / ArboNET national surveillance; preliminary 2024 data showing WNV activity in 1,113 counties across 49 states plus DC, incidence highest across West South Central and West North Central regions; ArboNET is run jointly by CDC and state and local health departments. https://www.cdc.gov/west-nile-virus/data-maps/index.html and https://stacks.cdc.gov/view/cdc/158519 (accessed 2026-07-20)
  3. Wimberly et al., “Spatio-Temporal Epidemiology of Human West Nile Virus Disease” (PMC). Claim: Culex tarsalis is a particularly efficient West Nile vector, with disease clustering in the Dakotas, Nebraska, and the interior West. https://pmc.ncbi.nlm.nih.gov/articles/PMC3863861/ (accessed 2026-07-20)
  4. CDC. Eastern Equine Encephalitis, surveillance and control guidelines / about pages. Claim: EEE transmission is most common in and around freshwater hardwood swamps in Atlantic and Gulf Coast states and the Great Lakes region; average about 11 US cases per year. https://www.cdc.gov/eastern-equine-encephalitis/php/surveillance-and-control-guidelines/index.html and https://www.cdc.gov/eastern-equine-encephalitis/about/index.html (accessed 2026-07-20)
  5. Byrd et al., “Geographically persistent clusters of La Crosse virus disease in the Appalachian region of the United States from 2003 to 2021,” PLOS Neglected Tropical Diseases. Claims: more than 80 percent of reported La Crosse cases from North Carolina, Ohio, Tennessee, and West Virginia; disease focus shifted from Midwest into Appalachia; La Crosse is the leading cause of pediatric arboviral encephalitis in the US. https://journals.plos.org/plosntds/article?id=10.1371/journal.pntd.0011065 (accessed 2026-07-20)
  6. CDC / MMWR. “Increase in Travel-Associated and Locally Acquired Dengue Cases, United States, 2024.” MMWR 2026;75(18). Claims: 3,483 travel-associated dengue cases (84 percent increase), top states Florida 1,016, California 648, New York 327; locally acquired mainland cases Florida 91, California 18, Texas 1; global dengue about 14.1 million in 2024. https://www.cdc.gov/mmwr/volumes/75/wr/mm7518a1.htm (accessed 2026-07-20)
  7. CDC / MMWR. “Dengue Outbreak and Response, Puerto Rico, 2024.” MMWR 2025;74(5). Claims: 6,291 dengue cases, epidemic threshold crossed, public health emergency declared March 2024, dengue endemic in Puerto Rico, response paired vector control with public awareness. https://www.cdc.gov/mmwr/volumes/74/wr/mm7405a1.htm (accessed 2026-07-20)
  8. CDC Health Alert Network (HAN 00494 and HAN 00496) and Blackburn et al., “Outbreak of Locally Acquired Mosquito-Transmitted (Autochthonous) Malaria, Florida and Texas, May-July 2023” (PubMed 37676839). Claims: 10 locally acquired malaria cases in 2023 in Florida, Texas, Maryland, and Arkansas; first locally acquired mosquito-transmitted malaria reported in about two decades (prior cases 2003, Palm Beach County, FL). https://www.cdc.gov/han/2023/han00496.html and https://pubmed.ncbi.nlm.nih.gov/37676839/ (accessed 2026-07-20)

Related

  • Mosquito-borne diseases by US region (regions cluster) [placeholder link]
  • West Nile virus: where it spreads and how to avoid it [placeholder link]
  • Eastern equine encephalitis explained [placeholder link]
  • La Crosse virus and risk to children [placeholder link]
  • Dengue in the US: travel risk versus local risk [placeholder link]
  • How mosquito surveillance and control work [placeholder link]

This page is an independent, non-commercial reference and is not medical advice. For diagnosis, treatment, or guidance on your specific situation, consult a licensed healthcare provider or your local health department. Last reviewed 20 July 2026.