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 WNV 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:


Jamestown Canyon Virus (JCV)

Jamestown Canyon virus (JCV) was first isolated in 1961 from a mosquito in Colorado and was first recognized to cause human disease in 1980. JCV persists among white-tailed deer and 22 different species of mosquitoes including Aedes and Anopheles. The infection occurs in June through September with a peak in mid-June to mid-July. Although rare, this disease has potentially severe and even fatal consequences for those who contract them. Clinical features include mild febrile illness with acute central nervous system infection including meningitis and encephalitis and frequently respiratory system involvement in patients more than 18 years old. There are no vaccines to prevent JCV.

In 2013, of 10 states reporting cases, 8 states (Georgia, Idaho, Massachusetts, Minnesota, New Hampshire, Oregon, Pennsylvania, and Rhode Island) reported their first JCV cases. In Connecticut, human cases have been rare, but mosquitoes in 8 towns, including Stamford and Norwalk, have tested positive for the virus in 2014. In August 2015, the Iowa Department of Public Health announced that one case of JCV has been confirmed.

In 2017, 2 Maine residents from Kennebec and Franklin Counties were diagnosed with JCV. Both had onset dates in June. Also, 2 New Hampshire residents were diagnosed with JCV in 2017 and 1 resident in 2018. Although the 2017 cases were not travel related the single 2018 infection in NH was most likely acquired out of state.

In 2019, 2 New Hampshire residents were diagnosed with JCV; one in August from Kingston and another in October from Laconia. No out-of-state travel history was reported for either case.  New Hampshire has had eight cases since the state’s first report of the disease in 2013.

Also in 2019, Massachusetts reported 4 human cases of JCV from Essex, Worcester, Middlesex and Barnstable Counties. Massachusetts has also had eight human cases since the state’s first report of the disease in 2013.

Additional information about JCV can be found at the following Federal website:

Saint Louis Encephalitis Virus (SLEV)

Saint Louis encephalitis virus (SLEV) is transmitted to humans by the bite of an infected mosquito. Most cases of SLEV disease have occurred in eastern and central states. Culex pipiens are one of the primary mosquito vectors for this bird disease. Most persons infected with SLEV have no apparent illness. Initial symptoms of those who become ill include fever, headache, nausea, vomiting, and tiredness. Severe neuroinvasive disease (often involving encephalitis, an inflammation of the brain) occurs more commonly in older adults. In rare cases, long-term disability or death can result. There is no specific treatment for SLEV infection; care is based on symptoms.  There are no vaccines to prevent SLEV.

The majority of cases have occurred in eastern and central states, where episodic urban-centered outbreaks have recurred since the 1930s. New Hampshire reported one human case in 2006.

To date, there have been no reported cases of Saint Louis Encephalitis in Massachusetts.

Additional information about SLEV can be found at the following website:

La Crosse Encephalitis Virus (LACV)

La Crosse encephalitis is a viral disease spread to people by the bite of an infected mosquito. Most cases occur in the upper Midwestern, mid-Atlantic, and southeastern states. Many people infected have no apparent symptoms. Some of those who become ill develop severe neuroinvasive disease. Severe disease often involves encephalitis and can include seizures, coma, and paralysis. Severe disease occurs most often in children under the age of 16. There are no vaccines to prevent LACV.

LACV is transmitted by the bite of an infected mosquito, Aedes triseriatus “the tree-hole mosquito”. Anyone bitten by a mosquito in an area where the virus is circulating can get infected with LACV. The risk is highest for people who live, work or recreate in woodland habitats, because of greater exposure to potentially infected mosquitoes. LACV is not transmitted directly from person to person.

To date, there have been no reported cases of La Crosse Encephalitis in Massachusetts. In 2019, Newport County in Rhode Island, which borders southern MA, reported the first human case of LACV for the state.

Additional information about SLEV can be found at the following Federal website: