Fact sheet updated: June 2021
What is Eastern equine encephalitis (EEE)?
Eastern equine encephalitis is a rare but serious disease caused by a virus. EEE is generally spread to people by the bite of a mosquito infected with the virus. EEE can cause severe illness and possibly death in people of all ages. However, people under age 15 are especially at risk. EEE does not occur every year, but outbreaks lasting 2-3 years are known to occur. 2019 was likely the first year of one of these outbreak cycles.
What does it mean to find EEE virus in mosquito samples?
The identification of EEE virus in mosquitoes indicates the virus is present during the current mosquito season. Both bird-biting and mammal-biting species of mosquitoes can carry EEE. When the virus is found in mammal-biting mosquitoes, the risk for human disease increases, because these mosquitoes are much more likely to bite people. The risk from EEE is also likely to increase during a summer when there are abundant populations of both bird-biting and mammal-biting mosquitoes
What can be done to reduce risk of EEE?
Understanding the risk and being vigilant about practicing personal protection are the best ways to prevent EEE. Personal protection means consistently using bug spray, wearing clothes that cover your skin while outdoors, and ensuring window and door screens are in good shape. It’s also important to reduce the amount of mosquitoes around your home and yard by removing places where mosquitoes breed. This includes dumping out water that can collect in garbage cans, flower pots, bird baths, discarded car tires or other containers.
In communities that belong to a Mosquito Control District (MCD), the local Board of Health works with the MCD to make decisions about control activities that may include reducing populations of mosquitoes while they are still in their immature or larval state, or reducing populations of adult, flying mosquitoes using truck-based ground spraying.
When is aerial spraying of insecticides considered?
In communities that belong to a Mosquito Control District, truck-mounted ground spraying occurs when virus is found in mosquitoes. In situations where there is a high risk of human disease over a large geographic area – and the risk is not effectively being reduced by use of personal protection and truck-based spraying -- the response may include an aerial pesticide sprayed in the evening and overnight hours to reduce the number of infected, adult mosquitoes in areas of high risk. This allows for the rapid treatment of large areas of high concern that are not accessible by truck-mounted ground sprayers.
Please note: although aerial spraying may sometimes be necessary to reduce human risk, it will not eliminate risk. It is critical that residents protect themselves from mosquito bites by applying insect repellent when outdoors, draining standing water where mosquitoes breed, repairing screens, and – when risk is greatest-- avoiding outdoor activity during peak mosquito hours. You can also take steps to protect animals and pets.
How is aerial spraying conducted?
Aerial spraying is conducted by airplane or helicopter between dusk and dawn (approximately 7pm-4am depending on the time of year) in areas of concern. Mosquito control professionals apply an approved pesticide such as an ultra-low volume (ULV) spray. ULV sprayers dispense very fine aerosol droplets that stay in the air and kill adult mosquitoes on contact. Most droplets don’t reach the ground and there is no residual effect of the product.
What pesticide product is used in aerial spraying?
The pesticide used is Anvil 10+10, a product extensively tested and used to control mosquitoes in both ground-level and aerial spraying in the U.S. Anvil 10+10 contains two ingredients: Sumithrin and piperonyl butoxide. Sumithrin is an ingredient similar to the natural components of the chrysanthemum flower and is also found in other pesticide products used indoors, in pet shampoos, and tick control treatments. It is rapidly inactivated and decomposes with exposure to light and air, with a half-life of less than one day in the air and on plants. In soil, it breaks down rapidly and has proven to be extremely effective in killing mosquitoes worldwide for over 20 years. Piperonyl butoxide (PBO) serves to increase the ability of Sumithrin to kill mosquitoes.
The product is registered by the US Environmental Protection Agency (EPA) and in Massachusetts for this use. It has been used in aerial applications for mosquito control in 2006, 2010, 2012, 2019, and 2020, and is also used by some of the Mosquito Control Districts for ground applications.
Are these pesticides used elsewhere to control mosquitoes?
Yes. Other states including Alabama, Arkansas, Delaware, Florida, Illinois, Louisiana, New Jersey, New York, North Carolina, and Texas regularly apply these same products.
Can these targeted ground and aerial sprays with adulticides harm insects or wildlife?
The (EPA) has evaluated these pesticides for their safety and has determined that they do not pose an unreasonable risk to birds or mammals, if used according to the product label directions. The spraying equipment spreads the pesticide as a fine mist with droplet sizes only large enough to kill an adult flying mosquito that comes into contact with a droplet, and small enough to reduce the likelihood of harm to larger insects. Spraying is also done during the early evening and overnight hours to maximize the impact on the mosquito species of concern and reduce the impact to other insects.
Anvil 10+10 and other similar pesticides are toxic to land-dwelling and water-dwelling invertebrates (e.g., dragonflies, beetles) and fish. There is less risk to fish in larger ponds than in smaller ones and the risks in large natural water bodies are minimal. Due to the fact that small ornamental ponds are confined areas with little aeration and water distribution, people with these ponds in their yard may want to cover them during a night when aerial spraying is taking place in their area.
What about my animals, are they safe outdoors?
Please note that it is NOT necessary to bring animals indoors during the spray although reasonable precautions are recommended. Water supplies in open containers used for watering of animals should be discarded and fresh water provided following the aerial application. You may keep pets indoors during spraying to minimize their risk of exposure. Pets that remain outdoors could be exposed to small amounts of pyrethroids, but would not be expected to experience adverse health effects from the spraying. Again, there are many pesticide products (e.g., flea collars, pet shampoo, dips) containing sumithrin that are used directly on pets to control ticks and insects.
Is there a risk to drinking water sources?
No. Aerial spraying is not expected to have any impacts on surface water or drinking water. Surface drinking water sources are mapped to ensure that aerial spraying will not occur over these bodies of water. Also, the product is rapidly inactivated and decomposes in sunlight and air, does not dissolve easily in water, and is broken down by microorganisms in streams and water bodies that receive sunlight. Monitoring of drinking water supplies during aerial spray events in Massachusetts has confirmed that product is rarely found in samples after a spray and has only been found at levels that do not pose a concern for human health.
Are there any health impacts associated with exposure to Anvil 10+10?
Aerial Ultra Low Volume (ULV) application of Anvil during nighttime generally results in negligible exposure to humans. As a result, the potential for adverse health effects to the general public is likely to be very low... Responsiveness in some sensitive individuals is hard to predict. Upon direct contact with Sumithrin, PBO and petroleum solvents, some sensitive people may develop temporary eye, skin, nose or throat irritation, or breathing problems. People with known sensitivities to chemicals, or with existing respiratory conditions.
Are there precautions I should take if aerial spraying will occur in my area?
No special precautions are recommended. Aerial spraying is conducted at night and the active ingredients of the pesticide product used for aerial application for mosquito control generally break down quickly and leave no residue.
Although aerial spraying is considered necessary to reduce human risk, it will not eliminate risk. It is critical that residents protect themselves from mosquito bites by staying indoors during peak mosquito hours, applying insect repellent when outdoors, draining standing water where mosquitoes breed, repairing screens, and protecting animals and pets.
Even if no precautions are required, are there extra steps people can choose to take if they are still concerned?
Although not necessary, if people are concerned, there are steps that can be followed in areas where aerial spraying is scheduled to take place. These include:
- Close windows and turn off fans in spray areas. In very hot weather, you can open the windows or turn fans back on soon after the aerial spraying is completed.
- Air conditioners do not have to be turned off, because they circulate indoor air.
- Keep pets indoors during spraying. Although pets that remain outdoors could be exposed to small amounts of Anvil 10+10, they are not expected to experience adverse health effects from the spraying. There are many pesticide products (e.g., flea collars, pet shampoo, dips) containing similar ingredients that are used directly on pets to control ticks and insects. The suggestion to keep pets indoors is to ensure that they do not get scared, since the planes fly approximately 300 feet above the ground.
- If clothes or outdoor items are exposed during spraying, wash them with soap and water.
Are there any restrictions on consuming fruits and vegetables from home gardens or local farms?
No. The EPA has established a tolerance (acceptable level) for the product that allows wide-area mosquito application on food crops, fodder crops, pasture and grazing areas. Livestock may graze in treated areas following the application. As always, people should rinse any homegrown or purchased fruits and vegetables with water before preparation or consumption.
If I am a beekeeper, should I take special precautions to protect the bees before or after aerial spraying?
Aerial spraying takes place at night partly to reduce the chance of negative impacts on honey bee colonies. However, if bees are congregating outside hive boxes, consider applying a cover to the hive entrance or over the entire hive box using a loose wet cloth (burlap, cotton sheet, etc.) to prevent bees from exiting, thus preventing them from having direct contact with the spray during the application. Remove covers and additional boxes placed on hives as soon as possible the morning following an aerial spraying event.
If miticides have been applied and there is concern about reduced ventilation during covering, consider adding an empty box on top to increase ventilation within the hive during the application. Remove covers and additional boxes placed on hives as soon as possible the morning following application.
The product being applied has a very short half-life (one day) and breaks down rapidly in sunlight. The Department of Agricultural Resources has conducted monitoring of honey bee hives during past aerial spraying events and has seen no negative effects on honey bees.
For further questions regarding bees, please see the Frequently Asked Questions (FAQ) For Honey Beekeepers.
How will I know if the application will take place over my house?
When aerial spraying is planned, the Department of Agricultural Resources posts a map at massnrc.org/spray-map indicating what areas will be sprayed that evening. This map is updated each subsequent morning to show the area that was sprayed the previous evening.
What if I think that I am experiencing an adverse reaction to pesticide spraying?
If you believe you may be experiencing any health effects from pesticides, call your health care provider or the Massachusetts/Rhode Island Poison Control Center at 800-222-1222. If symptoms are severe, call 911 for assistance.
Who do I contact to learn more about aerial spraying in my area?
Your local health department will be aware of any plans for aerial spraying. Updates will be provided via local media outlets, social media, the DPH website, and other channels.
For questions about aerial spraying, contact MDAR Crop and Pest Services at: MosquitoProgram@mass.gov (508) 281-6786 or visit the MDAR Comment Form page.
For the most updated information on EEE risk, contact the DPH Division of Epidemiology (617) 983-6800 or visit the DPH website at mass.gov/MosquitoesAndTicks for updated mosquito results, maps and incidence of positive mosquito samples.
For general information on mosquito control, see information from the State Reclamation and Mosquito Control Board.