Arboviruses Endemic to the Northeast District

West Nile Virus (WNV)

West Nile Virus (WNV) was introduced to New York City in 1999 and within five years had spread to all 48 continental US states.  It was first isolated in Essex County in 2000, and is now endemic throughout eastern MA, particularly in the Boston metropolitan area. Since its first appearance in North America, WNV has caused significant illness to over 39,000 persons in the United States; Table 1 shows WNV cases/fatalities in Massachusetts since 2000. While about 80% of all West Nile virus infections in humans are not symptomatic, approximately 20% of infections are manifested as some form of fever and varying degrees of serious neurological ailments are displayed by less than 1%. These neurological diseases include acute febrile paralysis, encephalitis, and meningitis resulting in death to about 10% of all neurological cases.

Culex pipiens and Cx. restuans are primarily responsible for the transmission of WNV within the bird population. The larvae of both these species develop in “high-organic content” water that accumulate in catch basins, containers, tires, pools and other water-holding structures that are in greater abundance in urbanized areas. Since some water-holding structures are permanent (catch basins) and the water contained cannot often be drained, the water itself must then be treated with larvicides to reduce/eliminate larvae from using the water to develop into adults.

Therefore, the principal strategy used by the District to combat WNV transmission and risk is by reducing and/or eliminating larval development in catch basin and other container-like habitats to ultimately reduce adult vector presence.

Additional information about EEE can be found at the following Federal and State websites:


Eastern Equine Encephalitis (EEE)

EEEV-human infections manifest symptoms similar to West Nile encephalitis and while the human infection rate is lower, the fatality rates are much higher with EEEV infections, about 33%. Also, the recovery rates from EEE disease are longer and most often are incomplete. EEEV seems to attack the young as readily as the elderly unlike WNV which the elderly are far more susceptible.

Like WNV, EEEV is an avian virus, transmitted bird-to-bird principally by Cs. melanura. While Cs. melanura mosquitoes are primarily responsible for the amplification of virus in bird populations, they typically might not bite humans. It is other mosquito species with wider host preferences (“bridge vectors”), when infected (after biting infected birds) can transmit EEEV to humans. 

Additional information about EEE can be found at the following Federal and State websites: