West Nile Virus and Other Mosquito- and Tick-Borne Viral Diseases in the U.S., 2022

Arboviruses — arthropod-borne viruses — reach people mainly through the bite of an infected mosquito or tick, and rarely through blood transfusion or organ transplant. Most infections cause no symptoms; illness ranges from a mild fever to severe neuroinvasive disease (encephalitis, meningitis or acute flaccid paralysis). West Nile virus is the leading cause of arboviral disease in the continental United States; the others cause scattered cases and occasional outbreaks.

This CDC report covers the six nationally notifiable domestic arboviruses reported to ArboNET, the national surveillance system, for 2022: eastern equine encephalitis, Jamestown Canyon, La Crosse, Powassan, St. Louis encephalitis and West Nile. Chikungunya, dengue, yellow fever and Zika are left out, because cases in U.S. states are mostly travel-related.

2022 at a glance

1,247 cases, 968 (78%) hospitalized, 103 (8%) deaths, from 414 of the nation's 3,143 counties in 45 states and the District of Columbia. That is 59% fewer than the 3,035 cases of 2021 — mostly because West Nile cases fell 61%, from 2,911 in 2021, when Arizona had a large outbreak. Cases fell for every virus except Powassan and St. Louis encephalitis.

West Nile virus

• 1,132 cases from 358 counties in 42 states and DC; 61% of patients were male.
• 827 neuroinvasive cases: 772 (93%) hospitalized, 91 (11%) died. National incidence: 0.25 per 100,000.
• Highest incidence: South Dakota (3.96 per 100,000), Colorado (2.24), Nebraska (1.88). Most cases: California (162), Colorado (131), New York (75) — together 44% of the nation's neuroinvasive cases.
• Age and sex. Incidence rose with age, from 0.01 per 100,000 under 10 to 0.78 at 70 and over, and was 68% higher in males (0.32) than females (0.19).
• Three patients caught it through organ transplants from the same donor; none died.

Powassan virus — a record

47 cases, from 39 counties in nine states — the most ever reported in a year (the previous high was 43, in 2019). Vermont reported its first. Most illness began in April–June. The highest neuroinvasive incidences were in Maine (0.29 per 100,000), Connecticut (0.17) and Vermont (0.15). All seven who died were over 60 (ages 61 to 91).

St. Louis encephalitis virus — most since 2003

33 cases from 12 counties in three states — the most since 2003, when there were 49. Illness most often began in July–September (45%), but 39% began in October–December, all of those late cases in the Southwest (Arizona, California, Texas). Arizona (0.16 per 100,000) and California (0.04) had the highest neuroinvasive incidence. All three who died were over 65 (ages 68 to 85).

La Crosse virus — a disease of children

22 cases from 19 counties in five states, with a median age of 9. La Crosse remains the most common cause of neuroinvasive arboviral disease in children. Ohio reported 12 of the 19 neuroinvasive cases (63%) and had the highest incidence (0.10 per 100,000), ahead of West Virginia (0.06) and Minnesota (0.05). Nobody died.

Jamestown Canyon and eastern equine encephalitis

• Jamestown Canyon: 12 cases from 12 counties in five states; nine of the 12 patients were male and all but one had neuroinvasive disease. Wisconsin reported the most neuroinvasive cases (four), and Rhode Island had the highest incidence (0.18 per 100,000). No deaths.
• Eastern equine encephalitis: one case, a woman over 60 who fell ill in August with neuroinvasive disease and was hospitalized.

How many are missed

ArboNET is passive: a case counts only if the patient seeks care, a doctor orders a test and the result is reported. Milder, non-neuroinvasive illness is especially under-reported. Earlier studies estimate 30 to 70 non-neuroinvasive West Nile cases for every reported neuroinvasive one; the 827 neuroinvasive cases of 2022 therefore imply 24,810 to 57,890 milder cases, of which only 305 — 0.5% to 1.2% — were reported. And because ArboNET does not require clinical or laboratory details, some cases may be misclassified.

What it means for doctors and the public

• Test patients with a compatible illness — fever, meningitis or encephalitis — during mosquito and tick season, or after an organ transplant or blood transfusion, and report positive results to the state or local health department. Infections linked to a transfusion or transplant should be reported quickly, so that other products from the same donor can be found and withdrawn.
• Transplants continue to spread these viruses, and no national policy requires deceased organ donors to be screened for them.
• No treatment or vaccine exists for these domestic arboviruses. Prevention relies on repellents and protective clothing against mosquitoes and ticks, insecticides and removing breeding sites at home and in communities, and screening blood donors.

Numbers swing from year to year and place to place, which is why high-quality surveillance matters for aiming prevention messages and mosquito control.

Sources

• Rebekah A. Sutter, Shelby Lyons, Carolyn V. Gould, J. Erin Staples and Nicole P. Lindsey, "West Nile Virus and Other Nationally Notifiable Arboviral Diseases — United States, 2022," MMWR 73(21), 2024. https://www.cdc.gov/mmwr/volumes/73/wr/mm7321a2.htm
• CDC, preventing mosquito bites: https://www.cdc.gov/mosquitoes/prevention/index.html · preventing tick bites: https://www.cdc.gov/ticks/prevention/index.html