Mosquito-borne Illnesses – What are the Risks?

Eastern Equine Encephalitis (EEE) and West Nile virus (WNV) are illnesses that can be transmitted by mosquitoes. The risk of getting ill from EEE and WNV varies by location, age and immune status. EEE and WNV are carried by different species of mosquitoes that breed in different habitats. Mosquitoes encountered in the Metro Boston area do not include the species known to transmit EEE.

The Basics:

Eastern Equine Encephalitis (EEE)

• Rare but serious (30-50% mortality risk)
• Can cause severe illness and possibly death in all ages; under age 15 most at risk
• EEE risk is remote in Cambridge; there is no history of EEE cases in the Metro Boston area
• People visiting high risk areas in the state should take precautions

West Nile Virus (WNV)

• More common
• Most people infected have no symptoms 
• Can cause serious illness, especially for people over 50
• WNV risk is currently high in Cambridge; human infections have occurred in Middlesex County and elsewhere across Metro Boston and this risk will continue through September and October.

Protecting against mosquito bites is the best defense against both EEE and WNV.

• Use insect repellent
• Cover up with socks, long pants and long sleeves
• Take particular care from dawn to dusk
• Eliminate standing water

Background

Eastern Equine Encephalitis (EEE) is a rare but serious disease caused by a virus that can affect people of all ages. EEE is generally spread to humans through the bite of a mosquito infected with the virus. EEE can cause severe illness and possibly lead to death in any age group; however, people under age 15 are at particular risk.  Inflammation and swelling of the brain, called encephalitis, is the most dangerous and frequent serious complication from EEE. 

EEE does not occur every year, but based on mosquito sampling, a high risk currently exists in parts of the state. In Massachusetts, the virus is most often identified in mosquitoes found in and around freshwater, hardwood swamps. Risk levels are elevated in several communities in Bristol, Essex, Plymouth and Worcester counties. EEE risk in Cambridge remains remote, which means that EEE is not usually found in this area.

West Nile virus (WNV), also spread by mosquito bites, is a virus that can cause illness ranging from a mild fever to more serious disease. The majority of people who are infected with WNV (approximately 80%) will have no symptoms. However, persons older than 50 years of age have a higher risk of developing severe illness.

Currently, the risk of WNV is at the moderate level for much of Eastern and Western Massachusetts. In Cambridge and several surrounding communities, the current WNV risk level is high. 

The mosquitoes known to transmit WNV are active in the evening and night time hours. They are not aggressive biters and are quiet, so protecting yourself with repellent and long sleeved clothes while sitting outside after dark is important. Darker, daytime active mosquitos (Tiger mosquitoes) are not known to transmit any viral illnesses in the Northeast.

Avoiding mosquito bites is the best way to prevent EEE, WNV and other mosquito-borne illnesses. Use bug spray; cover up with socks, long pants and long sleeves; take particular care  from dusk to dawn; and reduce standing water.