During the summer months, mosquitoes are regularly monitored at sites around the state for mosquito-borne diseases. Each week, mosquitoes are collected at various locations by the Vermont Agency of Agriculture. These collections of mosquitoes are tested for arborviruses - mosquito-borne viruses - that may affect Vermonters, such as West Nile Virus and Eastern Equine Encephalitis. The Health Department summarizes mosquito testing results and reports of human and animal illnesses every week during the summer and early fall, and compiles data each year in an annual surveillance report.

Check out the Mosquito Surveillance Map

2023 Surveillance for Viruses Spread by Mosquitoes

Graph depicting 2023 Vermont Mosquito Testing Results for West Nile Virus and Eastern equine encephalitis against historic WNV/EEE trend lines
What does this graph tell me?
  • Lines represent historic (2002–2022) average percentage of mosquito pools positive for WNV (solid line) and EEE (dotted line).
  • Bars show the percentage of mosquito pools positive in the current year.
  • Historically, the percentage of mosquito pools positive for an arbovirus increases in late August and peaks in mid-September.

Mosquito Surveillance Results

Collection week# of Mosquito Pools* Tested# of EEE Positive Pools# of WNV Positive Pools
June 25-July 115900
July 2-820800
July 9-1512700
July 16-2216200
July 23-2917103 (Alburgh, St. Albans, Vergennes)
July 30-August 518200
August 6-121684 (Alburgh, Swanton)2 (Colchester, Vergennes)
August 13-1916500
August 20-262101 (Swanton)0
August 27-September 23525 (Alburgh, Swanton)2 (St. Albans, Vergennes)
September 3-93371 (Highgate)1 (Vergennes)
September 10-163222 (Swanton)1 (New Haven)
September 17-2328900
September 24-303821 (Swanton)2 (Vergennes, Whiting)
October 1-738100
October 8-1428600
October 15-2113000

*A mosquito pool is a group of 1-50 mosquitoes of the same species, collected at the same trap location, on the same date.

Animal Surveillance

  • One animal case of EEE was reported in Franklin County (Swanton).
  • One animal case of WNV was reported in Franklin County (Fairfax).

Human Surveillance

  • No human cases of WNV were reported in 2023.
  • No human cases of EEE were reported in 2023.

Aedes albopictus Surveillance

  • The Aedes albopictus (Asian tiger) mosquito can carry and transmit Zika, dengue, and other arboviruses. It has an estimated geographic range that includes southern Vermont.
  • Aedes albopictus mosquito eggs were found at one site in Windham County in 2022 among 18 targeted trapping sites in Vermont.
  • After four consecutive years of detections at a single trap site in Windham County, spanning several weeks of detection each year, Aedes albopictus is considered established (locally-reproducing) at this location.

Annual Surveillance Reports


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