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Vector-Borne Disease in the US, by the Numbers

By Andrew Rothaus, Founder ·

The CDC counts more than 1 million vector-borne disease cases reported in the United States between 2001 and 2023, and says annual reports more than doubled over that period. Ticks drive most of the volume, Lyme disease most of that. The case counts here are reported counts, and reported counts run well below true infections.

A laboratory microscope with a lit specimen stage

How many vector-borne disease cases get reported in the US each year?

The CDC's summary line: "Cases of VBDs have increased significantly since 2004, with more than 1 million cases reported between 2001 and 2023 in the United States. Annual reported VBD cases more than doubled during this period." States, territories, and freely associated states "voluntarily report cases of 17 VBDs to CDC."

For 2004 through 2016, CDC authors writing in MMWR tallied reports from US states and territories covering 16 vector-borne diseases: "A total 642,602 cases were reported." Of that, 491,671 were tickborne, 150,842 mosquitoborne, and 89 plague, a fleaborne disease. Their summary: "Tickborne diseases more than doubled in 13 years and were 77% of all vectorborne disease reports."

The CDC describes the two halves differently. "Epidemics characterize the mosquitoborne viruses," while "the increasing reports of tickborne disease, which occurs almost exclusively in the continental United States, has been gradual." Annual reports of tickborne bacterial and protozoan diseases went "from >22,000 in 2004 to >48,000 in 2016." A later CDC analysis: "Overall, U.S. tickborne disease cases have increased 25%, from 40,795 reported in 2011 to 50,856 in 2019."

Which single disease accounts for the most cases?

Lyme, by a wide margin. Across 2004 to 2016, "Lyme disease accounted for 82% of all tickborne disease reports," with 402,502 cases in the CDC table. Lyme has been nationally notifiable since 1991.

Then the counting rules changed. Effective January 1, 2022, the Council of State and Territorial Epidemiologists and the CDC revised the national surveillance case definition so that high-incidence jurisdictions could report cases "based on laboratory evidence alone, without the need to collect additional clinical information." MMWR reported 62,551 Lyme cases for 2022, which the authors put at 1.7 times the annual average of 37,118 cases reported during 2017 to 2019.

That jump is a measurement artifact, and the CDC says so: the increase "likely reflects changes in surveillance methods rather than change in disease risk," and that the changes "preclude detailed comparison with historical data."

Geography stays concentrated: "Approximately 90% of all cases are reported from 15 high-incidence jurisdictions in the Northeast, mid-Atlantic, and upper-Midwest regions." For 2023, "Over 89,000 cases of Lyme disease were reported to CDC by state health departments and the District of Columbia." See our Lyme prevention guide.

What do the other tickborne numbers look like?

Smaller counts. Across 2004 to 2016, the CDC table shows anaplasmosis and ehrlichiosis combined rising from 875 reports in 2004 to 5,750 in 2016, and spotted fever rickettsiosis from 1,713 to 4,269. Babesiosis, notifiable only since 2011, ran 1,128 in 2011 to 1,910 in 2016.

A dedicated babesiosis analysis found "a total of 16,456 cases of babesiosis were reported to CDC by 37 states" during 2011 to 2019. State rate changes were large: Vermont "reported two cases in 2011 (incidence=0.3 per 100,000 population) and 34 cases in 2019 (5.4), representing a 1,602% increase in incidence," and Maine went from "nine cases in 2011 (0.7) and 138 cases in 2019 (10.3), a 1,422% rate increase."

Geography splits by pathogen. Per the CDC, "More than 75 percent of spotted fever rickettsiosis cases (including RMSF) occur in seven states: Arkansas, Missouri, North Carolina, Oklahoma, Virginia, Alabama, and Tennessee," while reported cases of Lyme, anaplasmosis, babesiosis, and Powassan virus disease are "concentrated in the Northeast, mid-Atlantic, and upper Midwest, with pockets of risk along the Pacific coast." Start with tick identification.

Alpha-gal syndrome sits outside routine surveillance. An MMWR study of commercial lab testing counted "a total of 110,229 suspected cases" during 2010 to 2022, and noted that "in the absence of a national surveillance system, the prevalence of this condition is largely unknown."

What are the West Nile virus numbers?

The CDC calls West Nile "the leading cause of mosquito-borne disease in the contiguous United States." Its surveillance data page, with 2025 data preliminary and current as of June 2, 2026, puts cumulative human disease cases at 63,155 and deaths at 3,315 for 1999 through 2025.

The year-to-year swing is the story. The 2024 count of 1,808 sat "below the 10-year median of 2,162 cases," with an interquartile interval of 1,132 to 2,628 around that median. Single states move the national total: Texas reported 456 cases in 2024, "more than double the number reported in Texas in 2023 (163)." The CDC's round number is "about 2,000 people in the United States are diagnosed with West Nile" each year, "but this number is an underestimate."

For 2024, CDC surveillance authors reported that 48 states and DC "reported 1,955 arboviral disease cases, including 189 (10%) deaths," with West Nile accounting for 1,808, or 92 percent. That year, "173 (9.6%) patients with reported WNV disease died, 169 (98%) of whom had neuroinvasive disease (case-fatality rate [CFR] = 13% among patients with neuroinvasive disease)." The 13 percent is a rate among neuroinvasive patients, not among everyone infected. More in our West Nile explainer.

How big is dengue in the US, and where does it come from?

Mostly from a plane. Per the CDC, "Most dengue cases reported in the 49 continental US states occur in travelers infected elsewhere," while dengue "is common in the U.S. territories of American Samoa, Puerto Rico, and the U.S. Virgin Islands, and the freely associated states."

2024 broke the record. Per MMWR, ArboNET received an average of 828 dengue cases a year from the 50 states and DC during 2010 through 2023, in a range of 202 to 2,055. In 2024 "a record 3,798 dengue cases were reported, representing a 359% increase above the" 2010 to 2023 average. Of those, "97.2% were associated with travel outside the reporting jurisdiction during the 2 weeks preceding symptom onset; the remaining 2.8% were locally acquired." The locally acquired cases came from Florida (85), California (18), and Texas (two).

Severity, same report: "Among all patients, 36.1% required hospitalization, 2.8% of cases were severe, and six (0.2%) patients died." The authors flag their counts as "preliminary and might not reflect final counts for 2024." See dengue and travel.

What does surveillance data not capture?

The CDC's MMWR analysis is blunt: "The reported data substantially underestimate disease occurrence. NNDSS relies on a person seeking care, a clinician requesting appropriate tests, and providers or laboratories reporting to public health authorities."

Three specific gaps, each with a number attached:

  • Lyme. The CDC says "approximately 476,000 people may be diagnosed and treated for Lyme disease each year in the United States," adding that this "likely includes patients who are treated based on clinical suspicion but do not actually have Lyme disease." Even after the 2022 definition change, reported totals still do "not approach" that estimate.
  • West Nile. CDC authors cite historical ratios of 30 to 70 nonneuroinvasive disease cases for every reported case of West Nile neuroinvasive disease. Applied to 2024, that implies 40,410 to 94,290 nonneuroinvasive cases nationally, against 461 actually reported, which they put at 0.5 to 1 percent of the estimated total.
  • Dengue. "Reported case counts are an underestimate of the true number of infections, because many persons with dengue have mild symptoms or might not seek health care services."

Two more caveats. For West Nile, the CDC notes surveillance data "are reported by county of residence, not the location (county or state) of exposure," and Lyme cases are likewise reported from the infected person's county of residence, so a case counted in one place may have been acquired elsewhere. And during the COVID-19 pandemic, fewer tickborne cases were reported, which the CDC attributes to "the effects of the COVID-19 pandemic on tickborne disease surveillance rather than an actual decrease in the numbers of tickborne diseases."

What does the CDC recommend, given these numbers?

For ticks, the CDC's prevention page says to "Use Environmental Protection Agency (EPA)-registered insect repellents containing DEET, picaridin, IR3535, Oil of Lemon Eucalyptus (OLE), para-menthane-diol (PMD), or 2-undecanone," and to "Treat clothing and gear with products containing 0.5% permethrin." It also says to look for an EPA registration number on the label. The EPA, for its part, states plainly: "EPA and the U.S. Government do not endorse any product or service."

For arboviral disease, CDC surveillance authors write that prevention and control "rely on use of personal protective measures (e.g., using insect repellent registered by the Environmental Protection Agency and wearing protective clothing)," on vector-control activities "both at household and community levels," and on "WNV blood and organ donor screening." See permethrin-treated clothing and standing water control.

When during the year do the numbers peak?

Tightly, and in summer. Across 1999 to 2025, West Nile cases by month of illness onset run 9,967 in July, 28,697 in August, and 17,979 in September. In 2024, 82 percent of all arboviral disease patients had illness onset during July through September, and dengue cases peaked over the same three months, accounting for 41.6 percent of the annual total.

Lyme onset peaked during calendar week 26 in both the 2017 to 2019 period and 2022. Powassan runs differently: 28 of 60 cases in 2024 had onset during April through June, which fits a tick active earlier and later than mosquitoes. More in when tick season actually starts.

Where Longsleeve fits

Longsleeve is an EPA-registered picaridin lotion (EPA Reg. No. 101457-1). It repels mosquitoes and ticks for up to 14 hours. It is fragrance-free, with nine disclosed ingredients.

Common questions

Why did Lyme cases jump so much in 2022?

The surveillance case definition changed. From January 1, 2022, high-incidence jurisdictions could report cases on laboratory evidence alone. The CDC says the increase "likely reflects changes in surveillance methods rather than change in disease risk," and that the changes "preclude detailed comparison with historical data."

How far off are reported case counts?

It depends on the disease. The CDC estimates "only 1 in 10 West Nile virus cases are reported, and the number of treated Lyme disease cases is possibly 10 times higher than the number reported to CDC." For West Nile in 2024, the CDC estimated 40,410 to 94,290 nonneuroinvasive cases against 461 reported, an even wider gap than the 1 in 10 figure.

Is dengue spreading inside the United States?

Mostly it arrives with travelers. In 2024, 97.2 percent of the record 3,798 reported cases were travel associated and 2.8 percent were locally acquired, from Florida, California, and Texas. The CDC notes that limited local spread has been reported in Florida, Hawaii, Texas, Arizona, and California.

What should I do if I think I have one of these illnesses?

See a healthcare provider. The CDC says providers diagnose vector-borne infections based on signs and symptoms, history of possible exposure to mosquitoes, ticks, or fleas, and laboratory testing. Any decision about testing or treatment belongs with your clinician.

Sources

  1. CDC, About Vector-Borne Diseases. https://www.cdc.gov/vector-borne-diseases/about/index.html
  2. CDC MMWR: Vital Signs: Trends in Reported Vectorborne Disease Cases, United States and Territories, 2004 to 2016. https://www.cdc.gov/mmwr/volumes/67/wr/mm6717e1.htm
  3. CDC, Lyme Disease Surveillance and Data. https://www.cdc.gov/lyme/data-research/facts-stats/index.html
  4. CDC, Lyme Disease Case Map. https://www.cdc.gov/lyme/data-research/facts-stats/lyme-disease-case-map.html
  5. CDC MMWR: Surveillance for Lyme Disease After Implementation of a Revised Case Definition, United States, 2022. https://www.cdc.gov/mmwr/volumes/73/wr/mm7306a1.htm
  6. CDC MMWR: Trends in Reported Babesiosis Cases, United States, 2011 to 2019. https://www.cdc.gov/mmwr/volumes/72/wr/mm7211a1.htm
  7. CDC MMWR: Geographic Distribution of Suspected Alpha-gal Syndrome Cases, United States, January 2017 to December 2022. https://www.cdc.gov/mmwr/volumes/72/wr/mm7230a2.htm
  8. CDC MMWR: West Nile Virus and Other Nationally Notifiable Arboviral Diseases, United States, 2024. https://www.cdc.gov/mmwr/volumes/75/wr/mm7529a1.htm
  9. CDC MMWR: Increase in Travel-Associated and Locally Acquired Dengue Cases, United States, 2024. https://www.cdc.gov/mmwr/volumes/75/wr/mm7518a1.htm
  10. CDC, West Nile Virus Historic Data (1999-2025). https://www.cdc.gov/west-nile-virus/data-maps/historic-data.html
  11. CDC, About West Nile. https://www.cdc.gov/west-nile-virus/about/index.html
  12. CDC, Data and Statistics on Dengue in the United States. https://www.cdc.gov/dengue/data-research/facts-stats/index.html
  13. CDC, Press Kit: Understanding Lyme and Other Tickborne Diseases. https://www.cdc.gov/ticks/communication-resources/press-kit.html
  14. CDC, Preventing Tick Bites. https://www.cdc.gov/ticks/prevention/index.html
  15. EPA, Find the Repellent that is Right for You. https://www.epa.gov/insect-repellents/find-repellent-right-you

Last reviewed 2026-08-16 · every claim on this page is sourced above