Massachusetts Department of Public Health Reports First Detection of Equine Encephalitis Virus in Mosquitoes in 2004

BOSTON -- The Massachusetts Department of Public Health (MDPH) announced today that eastern equine encephalitis virus (EEEV) was detected in a pool of non-human biting mosquitoes collected on July 28, 2004 in Easton. This is the first detection of EEEV in Massachusetts this season.

West Nile virus (WNV) has not yet been detected in Massachusetts this year in the field surveillance program, which routinely tests mosquitoes, birds and horses. No human infections of either WNV or EEEV have been detected by testing at the State Laboratory Institute, which regularly screens human cases of encephalitis and meningitis.

Eastern Equine Encephalitis (EEE) is a rare but serious disease caused by a virus. Fewer than 50 cases have occurred in Massachusetts since 1940. The virus is carried by birds that live in freshwater swamps, and it is usually found only in birds and in mosquitoes that do not bite people. However, in years when EEEV infects large numbers of birds, the virus may infect other species of mosquitoes that do bite horses and people.

The finding of EEEV in mosquitoes was expected given last years findings. In 2003, nine mosquito pools and five horses tested positive for the presence of the EEEV late in the season. There have been no human cases of EEE since 2001 in Massachusetts. Historically, most human cases have been in Plymouth and Norfolk counties, with smaller numbers in Middlesex, Bristol, Suffolk, Barnstable, Essex and Hampden counties. WNV, a newly emerged virus in North America, has been much more prevalent in recent years. There were 16 confirmed human cases of WNV disease, acquired in Massachusetts in 2003, including one fatality. The Centers for Disease Control and Prevention reports a 2003 nationwide total of 9872 human WNV cases resulting in 264 deaths.

Symptoms of EEE are high fever (103-106 degrees), stiff neck, headache, and lack of energy. These symptoms appear two to 10 days after infection. Swelling of the brain, called encephalitis, is the most dangerous symptom. The disease worsens quickly and patients may go into a coma within a week. Unlike EEE, most WNV infections do not result in serious illness.

In response to the finding of EEEV in mosquitoes, the MDPH will increase collections of mosquitoes in Bristol, Norfolk and Plymouth counties. This will enable MDPH to estimate the prevalence of the virus and provide further updates to the public.

MDPH cautions residents that human infections of WNV and EEEV are transmitted by bites from infected mosquitoes. Mosquitoes get WNV and EEEV by biting infected birds. Infected mosquitoes may then bite and infect humans. Mosquitoes are active from July through October. The period of highest risk occurs in August and September.

Source: Massachusetts Department of Public Health