West Nile Virus Encephalitis Information

 

Updated September 16, 2020

(most up to date information is presented in bold).

West Nile Virus (WNV) encephalitis is a rare disease caused by a virus.  In a very small percentage of people infected by the virus, the disease can be serious, even fatal.  WNV grows in birds, and it is transmitted from bird to bird and from birds to humans by mosquitoes.  Horses bitten by mosquitoes carrying WNV can also become sick.  The virus that causes WNV encephalitis occurs in Europe, Africa and Asia.  

  • It was first identified in the United States in New York during the summer of 1999 and was identified only after a large number of dead crows and other infected mammals were discovered. It is not known how WNV got to the US.
  • By the summer of 2001, WNV had spread to 27 States and the District of Columbia for a total of 149 human cases and 18 fatalities.  By 2002, WNV had been detected in 44 states across the US, 5 provinces in Canada and in Mexico.  There has been complete transcontinental movement (from the East coast to the West coast) of the West Nile Virus in three years.  2002 also saw a marked increase in the number of human cases.  For 2002, there were 3852 human cases with 232 fatalities (reported from 39 States plus Washington D.C.).
  • In 2003, there were 9862 cases reported from 45 States plus the District of Columbia.  264 deaths were attributed to WNV in 2003.
  • In 2004, there were 2539 WNV cases, with 100 fatalities reported from 40 states plus the District of Columbia. 
  • In 2005, there were 2949 cases reported, with 116 deaths.  These cases were reported from 42 states. 
  • In 2006, there were 4,269 cases reported nationwide, with 177 fatalities. These cases were reported from 43 states plus the District of Columbia. 
  • In 2007, there were 3,630 cases with 124 fatalities nationwide from 43 states. 
  • In 2008 there were 1356 cases nationwide, with 44 fatalities.  These cases were reported from 45 States plus the District of Columbia.
  • In 2009, there were 663 cases nationwide with 30 fatalities.  These cases were reported from 34 states.
  • In 2010, there were 981 cases nationwide with 45 fatalities.  These cases were reported from 40 states plus the District of Columbia.
  • In 2011, there were 712 cases nationwide with 43 fatalities.  These cases were reported from 43 states plus the District of Columbia.
  • In 2012, there were 5387 cases nationwide with 243 fatalities.  These cases were reported from 48 states plus the District of Columbia.
  • In 2013, there were 2469 cases nationwide with 119 fatalities.  These cases were reported from 47 states plus the District of Columbia.
  • In 2014, there were 2205 cases nation-wide, with 97 deaths.  These cases were reported from 45 states plus the District of Columbia. 
  • In 2015, there were 2060 cases nation-wide, with 119 deaths.  These cases were reported from 44 states plus the District of Columbia.
  • In 2016, there were 2038 cases nationwide, with 94 deaths.  These cases were reported from 45 states plus the District of Columbia.   For Massachusetts, there were 16 human cases.  There were no fatalities in Massachusetts in 2016. 
  • In 2017, there were 2002 cases nationwide, with  121 deaths.  The cases were reported from  47 states plus the District of Colombia. For Massachusetts, there were 6 human cases. There were no fatalities in Massachusetts.
  • In 2018, there were 2544 cases nationwide, with 137 deaths.  The cases were reported from 48 states plus the District of Columbia.  For Massachusetts, there were 49 human cases.  There were two human fatalities. 
  • In 2019 there were 958 cases nationwide, with 54 deaths. The cases were reported from 43 states plus the District of Columbia. In Massachusetts, there were five human cases from Bristol, Middlesex (2), and Plymouth (2) counties. There were no human fatalities in Massachusetts, from WNV in 2019.  In Massachusetts, 8295 mosquito pools were sampled with 87 samples positive for WNV. In Quincy, one mosquito pool tested positive for WNV.

2020: There have been 89 positive mosquito pools for WNV, to date. There have been seven mosquito pools from Quincy; Quincy's risk level from WNV has been raised to moderate from low.*  There have been seven human cases of WNV, six from Middlesex County and one from Bristol County.

County Town 

Suffolk

 (25)
 Boston (22), Revere, Chelsea (2)

Barnstable 

 
 

Worcester

 
 

Norfolk

  (21)
 Brookline, Needham (3), Westwood (5), Quincy (7), Weymouth, Wellseley, Braintree, Dover, Norfolk

Hampden 

  (1)
 Holyoke

Middlesex  

(25)                      
 Belmont (2), Medford, Newton (6), Arlington (2), Watertown(2), Everett (2), Malden (6), Wakefield, Cambridge (2), Waltham

Essex

 
 

Bristol 

  (12)
 Attleboro (5), Somerset (3), Norton, North Attleboro, Fall River, Easton

Berkshire 

 
 

Hampshire

 
 

 Plymouth

  (5)
Plympton (2), Halifax, Hanson, Middleborough

Franklin

 
                        
 Dukes  

* Moderate risk means WNV is likely or has already occurred.  To prevent WNV, it is recommended to:

-wear mosquito repellent

-wear long sleeves an long pants whenever possible

-use mosquito netting on baby carriages and playpens

-dump standing water at least two times per week