Important Information about Eastern Equine Encephalitis (EEE) – October 2019

Eastern Equine Encephalitis (EEE) has been identified in human-biting mosquitoes in our area and this year, Connecticut saw the first fatalities from EEE in six years. Here are some common questions and answers about EEE to help you protect yourself and your family.

Why was this such an unusual year for EEE in Connecticut?

EEE outbreaks strike at irregular intervals and are typically preceded by sustained and increased populations of the main mosquito vector (Culiseta melanura) that amplifies the virus in resident bird populations. We have seen high numbers of C. melanura in our trap collection over the last two years. High rainfall and accumulated water in freshwater swamp habitats have provided ideal conditions for mosquito breeding in the tested sites.

  • The increase in EEE virus activity this year is part of a region wide increase affecting Massachusetts and Rhode Island. The last time we had a high level of EEE activity in Connecticut was in 2013, which is when we had our first ever documented human case of EEE.
  • In the United States, most cases of EEE have been reported from Florida, Massachusetts, New York, and North Carolina. EEE transmission is most common in and around freshwater hardwood swamps in the Atlantic and Gulf Coast states and the Great Lakes region. Michigan is also experiencing increased EEE activity.


What is the outlook for EEE infection risk over the next few weeks?

  • In Connecticut, the period of peak risk for transmission of EEE virus to humans was between August 17 and September 8.
  • Based on the results of mosquito testing results from the Connecticut Agricultural Experiment Station (CAES), mosquito populations in Connecticut have been decreasing since mid-September, as have the numbers of the mosquitoes that have tested positive for EEE. Mosquitos testing positive for the virus in recent weeks are bird biting, not human biting.
  • Based on the forecast of colder weather beginning this weekend, the risk of a Connecticut resident becoming ill as a result of being bitten by a mosquito infected with EEE virus is low but not zero; the risk will not be zero until the first hard frost.


In which areas of the state is the risk for EEE infection the highest?

  • There is a low but not zero risk of EEE transmission in the southeastern part of the state from the lower Connecticut River valley to the Rhode Island border region. The entire eastern portion of the state is considered at elevated risk.


What should residents know about taking personal precautions?

  • Residents who live in towns or near towns where EEE virus has been found in mosquitoes and/or where there has been a confirmed case of EEE involving a human, horse or bird (e.g. pheasants and partridges) are advised to protect themselves and their children by: (1) taking personal precautions to prevent mosquito bites and (2) consider minimizing outdoor activity from dusk to dawn, when mosquitoes are most active. If outdoor activity is unavoidable, all personal precautions to prevent mosquito bites should be taken.


Would aerial spraying be effective at reducing risk at this point in the year?

  • At this time of year, aerial spraying to prevent EEE transmission would likely be ineffective because of the decline in mosquito populations.


Would ground spraying be effective at reducing risk at this point in the year?

  • Many mosquito breeding areas of high concern are not accessible by truck-mounted ground sprayers.
  • At this time of year, ground spraying to prevent EEE transmission would likely be ineffective because of the decline in mosquito populations.


How does the state decide when it would be appropriate to conduct aerial spraying, and how come that did not happen this year?

  • The State of Connecticut Mosquito Management Program is a collaborative effort involving the Department of Energy & Environmental Protection, the Connecticut Agricultural Experiment Station, the Department of Public Health, the Department of Agriculture, and the University of Connecticut Department of Pathobiology and Veterinary Science. These agencies are responsible for monitoring the potential public health threat of mosquito-borne diseases.
  • DEEP is responsible for the systematic identification and monitoring of mosquito breeding sites, the provision of technical assistance to municipalities and private property owners regarding mosquito control, and the collection and communication of information and data.
  • The Connecticut Agricultural Experiment Station (CAES) is responsible for the trapping, identification, and arbovirus testing of mosquitoes.
  • The Department of Public Health (DPH) is responsible for reviewing all mosquito test data and consulting with the DEP and the CAES regarding the epidemiologic significance of such results.
  • Based upon evaluation of the potential human health risks, the DPH advises as to appropriate personal, municipal, and state actions to reduce such risks. An EEE/West NileVirus (WNV) Working Group, consisting of staff from the DEEP, DPH, CAES, and DAG modifies, as necessary, the State EEE/WNV surveillance and response plans and reports to and advises the Commissioner of DEEP regarding their implementation.
  • In Massachusetts, Rhode Island, and Connecticut, aerial spraying to prevent EEE is considered when there have been confirmed human EEE cases and the populations of EEE-infected human-biting mosquitoes are high. This year, both of these conditions were met in Massachusetts and Rhode Island in August. Massachusetts had four confirmed human EEE cases by August 30, 2019. On September 16, 2019, the Connecticut Department of Public was notified by the Centers for Disease Control and Prevention that the cause of illness for the patient from East Lyme was EEE virus. At that time, mosquito populations in Connecticut were already declining.
  • In Connecticut, local municipalities make their own decisions about the use of local resources to control mosquito populations. At the local level, most mosquito control activities are done primarily for the purpose of nuisance mosquito control and not for the purpose of preventing mosquito-borne disease.
  • In Connecticut, the EEE/WNV Working Group evaluates the utility of using state resources to control mosquito populations for public health purposes. For example, this could include using an Ultra Low Volume (ULV) fogging application to knock down mosquito vectors within the region of the trap site and/or the application of larvicide to known mosquito breeding areas. The isolation of the virus from bird feeding mosquitoes alone does not pose a significant health threat to the public that would warrant pesticide spraying. Any recommendation to use pesticide is determined in consideration of weather conditions, the number of virus isolations, mosquito species with EEE virus, mosquito population estimates, and breeding cycles.
  • Pesticide applications are completed by State of Connecticut staff and/or by certified commercial pest control operators.
  • Affected communities, including municipal officials and the general public, are notified in advance of any pesticide application.
  • The EEE/WNV Working Group, if necessary, determines the most appropriate method of adulticide pesticides application to target adult mosquito populations: aircraft or truck mounted spraying. Truck mounted application is limited by access to the intended area. Aerial application can affect large areas and numbers of adult mosquitoes but is less discriminating, exposes more people and wildlife, and is more expensive and less easily repeated. Any decision to apply pesticides from trucks or the air would be made only after evaluation of the multiple factors which contribute to risk of transmission of EEE to people and after discussion with officials from the potentially affected community.


How has the state responded to this year’s EEE cases?

  • The Department of Public Health (DPH) has notified local health directors when positive mosquitoes are first identified or when a human or veterinary case is confirmed in a town. Local health directors have been asked to notify the municipal officials of these findings. Conference calls have been hosted by Mosquito Management Plan partner agencies (DPH, DEEP), Connecticut Agricultural Experiment Station (CAES), and Department of Agriculture) weekly since August 13, following the first findings of EEE in mosquitoes in Southeastern CT. Local health directors and municipal officials for towns with positive mosquito findings, human cases, or veterinary cases have been invited to each of these calls which provide updates on mosquito, human, and veterinary surveillance and opportunities to discuss messaging to constituents.
  • The CAES posts results of mosquito surveillance weekly ( Also posted to the website is a map indicating all towns with EEE activity. Maps are updated weekly or as new positive mosquitoes, human cases, and veterinary cases are identified. Local health directors, municipal officials, and member of the public can access these surveillance results and maps to understand current EEE activity in or near their municipality.
  • Partner agencies have issued press releases to update and inform the public and municipal leaders. The first press release, reporting the first identification of EEE in mosquitoes in 2019, was issued by CAES on August 5, 2019. Since late August, press releases have been issued by CAES, DPH, or the Department of Agriculture at least weekly to report new mosquito findings, equine cases, and human cases. Each press release has included recommendations for personal protective measures, information about the infection and illness, and links to additional state and national resources.
  • DEEP used truck-mounted sprayers to spray (“adulticide”) the road network and campgrounds in Pachaug State Forest on Aug. 26, 2019 and again on Sept. 18, 2019. We have provided technical assistance to municipalities, schools and parks regarding spraying of limited areas (ball fields, etc.) or for special events. Some towns/recreation commissions/schools have contracted with private applicators to spray areas for events.


What kind of technical assistance of other support can DEEP or other agencies offer to municipalities over the next week or two?

  • Based on surveillance data (diminished mosquito and virus activity) and weather patterns, we are at the tail end of this mosquito/EEE season. We do not recommend ground spraying under most conditions. However, towns who are interested in discussing this option further, especially in relation to major public gatherings, may reach out to the director of DEEP’s mosquito control program, Roger Wolfe. He can be reached at 860-418-5987 or [email protected]. DEEP’s program can provide technical assistance to towns and would like to hear from municipalities about their needs over the next week or two.