A sourced data hub for how often mosquito-borne diseases are actually reported in the United States, drawn from CDC ArboNET and CDC surveillance reports. Every figure below is tied to a numbered source.
The short answer
Mosquito-borne disease is real but uncommon as a reported event in the United States, and the reported counts are a floor, not a true total. West Nile virus is the workhorse of the domestic numbers: in 2023 it accounted for 2,628 of the 2,770 domestic arboviral disease cases reported to CDC, or about 95 percent (1). Everything else that is spread by local mosquitoes (eastern equine encephalitis, La Crosse, St. Louis encephalitis, Jamestown Canyon) shows up in the dozens of cases per year, not the thousands (1).
The diseases most people worry about from travel, dengue, Zika, chikungunya, and malaria, are largely brought back by travelers rather than caught from a mosquito at home. Dengue set a record in 2024 with 3,798 reported cases, and roughly 97 percent of those were tied to travel (2). Malaria runs near 2,000 reported cases a year, almost all imported (3).
The important caveat: surveillance undercounts. Most people infected with these viruses never feel sick, or feel only mildly sick, and never get tested. So the counts on this page describe reported, mostly medically attended illness. They do not describe the full number of infections.
West Nile virus by the numbers
West Nile virus is the most common mosquito-borne disease spread by mosquitoes within the United States. The figures below are for 2023, the most recent full year in CDC’s annual arboviral surveillance summary.
| Measure (2023) | Reported figure | Source |
|---|---|---|
| Total West Nile virus disease cases | 2,628 | (1) |
| Share of all domestic arboviral cases | about 95% | (1) |
| Neuroinvasive disease cases | 1,789 (68% of WNV cases) | (1) |
| Deaths | 194 (7% of reported cases) | (1) |
The neuroinvasive share is high here because of how surveillance works, not because the virus usually attacks the brain. Most West Nile infections cause no symptoms and are never reported. People with the severe neuroinvasive form (meningitis, encephalitis, or paralysis) almost always end up in the medical system, so they are heavily represented in the reported totals. Year to year the count swings widely; reported WNV cases rose 132 percent from 1,132 in 2022 to 2,628 in 2023 (1).
Other domestic arboviruses
These viruses are also spread by mosquitoes within the United States, but each is far less common than West Nile. Counts vary substantially from year to year. The figures below are the reported case counts for 2023 (1).
| Disease (2023) | Reported cases | Source |
|---|---|---|
| La Crosse virus | 35 | (1) |
| Jamestown Canyon virus | 27 | (1) |
| St. Louis encephalitis virus | 21 | (1) |
| Eastern equine encephalitis (EEE) | 7 | (1) |
Eastern equine encephalitis is the rarest of these but the most severe. CDC reports that on average about 11 cases of eastern equine encephalitis are reported in the United States each year (4). La Crosse virus is a leading cause of arboviral disease in children and clusters in the Appalachian and Midwestern states. Because these diseases are so uncommon, a single active season in one region can double a year’s national count, which is why any single year should be read as a snapshot, not a trend.
Travel-associated diseases
Dengue, Zika, chikungunya, and malaria are mostly diagnosed in travelers returning to the United States rather than acquired from local mosquitoes. Locally acquired cases do happen, but they are the exception in the reported data.
| Disease | Recent reported figure | Travel vs. local | Source |
|---|---|---|---|
| Dengue (2024) | 3,798 cases (record year) | 97.2% travel-associated, 2.8% locally acquired | (2) |
| Malaria (2022) | 1,999 confirmed cases, 10 deaths | Almost entirely imported (mostly travel to Africa) | (3) |
| Chikungunya (2018 to 2023) | 612 travel-associated cases reported over the period, roughly 100 per year on average | Overwhelmingly travel-associated; no locally acquired cases in US states in recent years | (5) |
| Zika (2023) | 7 cases reported in US travelers | Travel-associated; no local mosquito transmission in the continental US since 2018 | (6) |
The 2024 dengue total of 3,798 was a 359 percent increase over the 2010 to 2023 annual average of 828 cases (2). Of those 2024 cases, about 36 percent of patients were hospitalized and six died (2). Malaria in the United States is almost entirely imported: of the 2022 cases, the large majority were among people who had traveled, most often to West Africa (3). A notable exception came in 2023, when the country recorded its first locally acquired, mosquito-transmitted malaria cases in about 20 years, a small cluster of Plasmodium vivax cases in Florida and Texas plus a separate Plasmodium falciparum case in Maryland (7).
How to read these numbers
Reported cases are not the same as infections. Two things separate the two:
- Most infections are mild or silent. For West Nile virus, the large majority of people who are infected never develop symptoms, and only a small fraction develop severe disease. The people who get tested and reported skew heavily toward the sickest, which is why reported counts look small relative to how many mosquito bites carry these viruses.
- Reporting is passive and voluntary. A case only enters the national totals if a clinician orders the right test, the test is positive, and a state health department reports it to CDC. Mild dengue or chikungunya in a returning traveler who never sees a doctor simply never appears.
Because of this, treat every number on this page as a reported minimum for medically attended, tested illness. Rising or falling counts can reflect real changes in transmission, but they can also reflect changes in testing, travel patterns, or reporting, so a single year should be read with caution.
Where the data comes from
Most domestic figures here come from ArboNET, the national arboviral surveillance system maintained by CDC and state and local health departments. State health departments voluntarily report cases of the nationally notifiable domestic arboviral diseases (including West Nile, EEE, La Crosse, St. Louis encephalitis, Jamestown Canyon, and Powassan) to CDC through ArboNET using standard case definitions that combine clinical and laboratory criteria (1). Dengue, Zika, and chikungunya cases are also reported through ArboNET for the 50 states and the District of Columbia (2). Malaria is tracked separately through CDC’s National Malaria Surveillance System, with annual reports published in MMWR and on CDC’s website (3).
CDC publishes an annual summary of the domestic arboviral diseases in its Morbidity and Mortality Weekly Report (MMWR), and posts current-year and historic case counts on its disease-specific data-and-maps pages. Those primary sources are cited below.
Sources
- CDC MMWR. “West Nile Virus and Other Nationally Notifiable Arboviral Diseases, United States, 2023.” https://www.cdc.gov/mmwr/volumes/74/wr/mm7421a1.htm. Accessed 2026-07-20. Supports: 2,628 WNV cases in 2023 (about 95% of 2,770 total domestic arboviral cases); 1,789 (68%) neuroinvasive; 194 (7%) deaths; 132% rise from 1,132 cases in 2022; 2023 counts of La Crosse 35, Jamestown Canyon 27, St. Louis encephalitis 21, EEE 7; ArboNET description.
- CDC MMWR. “Increase in Travel-Associated and Locally Acquired Dengue Cases, United States, 2024.” https://www.cdc.gov/mmwr/volumes/75/wr/mm7518a1.htm. Accessed 2026-07-20. Supports: 3,798 dengue cases in 2024; 97.2% travel-associated and 2.8% locally acquired; 2010 to 2023 annual average of 828; 359% increase; about 36% hospitalized; six deaths; ArboNET reporting for 50 states and DC.
- CDC. “Malaria Surveillance, United States, 2022.” https://www.cdc.gov/malaria/php/surveillance-report/2022.html. Accessed 2026-07-20. Supports: 1,999 confirmed malaria cases and 10 deaths in 2022; predominantly imported, travel to Africa most common; National Malaria Surveillance System.
- CDC. “Data and Maps for Eastern Equine Encephalitis.” https://www.cdc.gov/eastern-equine-encephalitis/data-maps/index.html. Accessed 2026-07-20. Supports: on average about 11 EEE cases reported annually in the US.
- CDC. “Areas at Risk for Chikungunya (Data and Maps).” https://www.cdc.gov/chikungunya/data-maps/index.html. Accessed 2026-07-20. Supports: chikungunya in the US is overwhelmingly travel-associated; general surveillance status. (Exact recent annual case counts flagged for verification.)
- CDC. “Zika Cases in the United States.” https://www.cdc.gov/zika/zika-cases-us/index.html. Accessed 2026-07-20. Supports: no local mosquito-borne Zika transmission in the continental US since 2018; recent cases are travel-associated. (Exact recent annual case counts flagged for verification.)
- CDC MMWR. “Notes from the Field: Locally Acquired Mosquito-Transmitted (Autochthonous) Plasmodium falciparum Malaria, National Capital Region, Maryland, August 2023,” with CDC Health Alert Network HAN 00494. https://www.cdc.gov/mmwr/volumes/72/wr/mm7241a3.htm and https://www.cdc.gov/han/2023/han00494.html. Accessed 2026-07-20. Supports: first locally acquired malaria cases in the US in about 20 years (2023), a P. vivax cluster in Florida and Texas plus a separate P. falciparum case in Maryland.
Related
- West Nile virus: symptoms and risk [placeholder]
- Eastern equine encephalitis explained [placeholder]
- Dengue and the return of travel-related illness [placeholder]
- Understanding CDC ArboNET surveillance [placeholder]
- How mosquito-borne viruses spread [placeholder]
Disclaimer: This page is an independent, non-commercial reference for general information only. It is not medical advice. Case counts are reported surveillance figures that undercount true infections; consult CDC and your state or local health department for the latest data, and a clinician for any health concern.
Last reviewed 20 July 2026.