Mosquito Borne Illnesses

Eastern Equine Encephalitis (EEE) is a rare but serious disease caused by a virus.

The virus that causes EEE is spread through the bite of an infected mosquito. In Massachusetts, the virus is most often identified in mosquitoes found in and around freshwater hardwood swamps. The Massachusetts Department of Public Health website has more information about different types of mosquitoes that can spread the virus.

EEE virus particularly infects birds, often with no evidence of illness in the bird. Mosquitoes become infected when they bite infected birds. Although humans and several other mammals, particularly horses and llamas, can become infected, they do not spread disease.

EEE is a very rare disease. Just over 115 cases have occurred since the virus was first identified in Massachusetts in 1938. Most cases have typically been from Bristol, Plymouth, and Norfolk counties. However, human cases can occur throughout the state in an active year.

Outbreaks of EEE usually occur in Massachusetts every 10-20 years and typically last two to three years. The most recent outbreak began in 2019 and included twelve cases, six fatal. The outbreak continued in 2020 with five cases, including one fatality.

The first symptoms of EEE are fever (often 103º to 106ºF), stiff neck, headache, and lack of energy. These symptoms appear three to ten days after a bite from an infected mosquito. The most dangerous and severe complication is inflammation and swelling of the brain, called encephalitis.

There is no treatment for EEE. In Massachusetts, about half of the people identified with EEE died from the infection. People who survive this disease will often be permanently disabled. Few people recover completely. Again, EEE is a very rare disease. 

Since the virus that causes EEE is spread by mosquitoes, some things that you can do to reduce your chances of being bitten are:

• Schedule outdoor events to avoid the hours between dusk and dawn, when mosquitoes are most active.
• When outdoors, wear long pants, a long-sleeved shirt, and socks. This may be difficult to do when the weather is hot, but it will help keep mosquitoes away from your skin.
• Use a repellent with DEET, permethrin, picaridin, IR3535, or oil of lemon eucalyptus according to the instructions given on the product label. DEET products should not be used on infants under two months of age and should be used in concentrations of 30% or less on older children. Oil of lemon eucalyptus should not be used on children under three years of age. Permethrin products are intended for use on clothing, shoes, bed nets, and camping gear and should never be applied to the skin. More information on choosing and using repellents safely is included in the Massachusetts Department of Public Health mosquito repellents fact sheet.
• Keep mosquitoes out of your house by repairing any holes in your screens and ensuring that screens are tightly attached to all your doors and windows.
• Remove areas of standing water:
  • Look outside your house for containers and other things that might collect water and turn them over, regularly empty them, or dispose of them.
  • Drill holes in the bottom of recycling containers left outdoors so water can drain.

West Nile virus (WNV) is a mosquito-carried virus that can cause illness ranging from a mild fever to more severe diseases like encephalitis or meningitis. It was first identified in the United States in 1999.

WNV most commonly spreads to people through the bite of an infected mosquito. WNV may also be spread through blood transfusion or organ transplants. There are rare reports of WNV being passed from pregnant or breastfeeding people who are infected with WNV to their babies. Since these reports are rare, the health effects on an unborn or breastfeeding baby are unclear and still being studied.

People do not become infected by having direct contact with other infected people, birds, or animals.

Most people infected with WNV (approximately 80%) will have no symptoms.

A smaller number of people who become infected (~ 20%) will have symptoms such as fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands. They may also develop a skin rash on the chest, stomach, and back.

Less than one percent of people infected with WNV will develop severe illness, including encephalitis or meningitis. The symptoms of severe illness can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis. Persons older than 50 years of age have a higher risk of developing severe illness.

Because most people exposed to WNV have no symptoms, it is difficult to know how many people have been infected. People who develop severe illness with WNV are most often reported. Between 2014 and 2023, 131 people were reported with WNV infection in Massachusetts. Eight of these people died. Cases have been identified from around the state.

Since WNV is spread by mosquitoes, here are some things you can do to reduce your chances of being bitten:

• Schedule outdoor events to avoid the hours between dusk and dawn, when mosquitoes are most active.
• When outdoors, wear long pants, a long-sleeved shirt, and socks. This may be difficult to do when the weather is hot, but it will help keep mosquitoes away from your skin.
• Use a repellent with DEET, permethrin, picaridin, IR3535, or oil of lemon eucalyptus according to the instructions given on the product label. DEET products should not be used on infants under two months of age and should be used in concentrations of 30% or less on older children. Oil of lemon eucalyptus should not be used on children under three years of age. Permethrin products are intended for use on clothing, shoes, bed nets, and camping gear and should not be applied to the skin. More information on choosing and using repellents safely is included in the Massachusetts Department of Public Health mosquito repellents fact sheet.
• Remove areas of standing water. Here are some suggestions:
  • Look outside your house for containers and other things that might collect water and turn them over, regularly empty them, or dispose of them.
  • Drill holes in the bottom of recycling containers left outdoors so that water can drain out.