Eastern equine encephalitis risk

EEEV in a mosquito saliva gland

Eastern equine encephalitis (EEE) is caused by the eastern equine encephalitis virus (EEEV), which is transmitted by infected mosquitoes. As implied by its name, the virus primarily affects horses, but human cases sometimes occur. The Centers for Disease Control and Prevention (CDC) in the United States considers EEE one of the most severe mosquito-borne illnesses because its mortality rate is approximately one-third of all individuals who develop EEE. The disease has a sudden onset with swelling and inflammation of the brain (encephalitis), causing headache, fever, chills, and vomiting. However, only 4 to 5 percent of human infections with EEEV are thought to actually develop into EEE.

The transmission cycle of the virus tends to occur in and around swampy areas, including the freshwater hardwood swamps of the Unites States - the Atlantic and Gulf Coast states of the United States have endemic areas of virus transmission, and the endemic areas also now include around the Great Lakes. Wetland birds are infected with EEEV, which then infects the mosquitoes that bite them. When the virus is present in high enough quantity in the bird population, mosquitoes that bite humans may also become infected, allowing them to spread the virus to humans. Horse epidemics of the disease tend to occur in Summer and Fall. The CDC has received reports of only an average of 6 human cases of EEE per year in the United States. Since 1964, the most number of cases in a given year was earlier this past decade with nearly 25, and the average reported per year appears to be increasing.

Florida, Georgia, Massachusetts, and New Jersey have reported the most cases of EEE (neuroinvasive EEEV infection) since 1964, and were some of the earliest areas of the country affected by the disease. However, between 1964 and 2010 cases of EEE have also been reported in individuals living in Wisconsin, Michigan, Indiana, Pennsylvania, New York, New Hampshire, Rhode Island, Delaware, Maryland, Virginia, North Carolina, South Carolina, Alabama, Mississippi, Louisiana, and Texas (see a distribution map). The breakdown of number of cases per year per state available from the CDC indicates that the areas of viral transmission have been spreading since 1964.

Massachusetts is considered particularly vulnerable to the chance of an EEE outbreak. The density of residents, coastal area, history of EEE cases, and confirmed presence of EEEV in mosquitoes in a number of counties in the state create a situation that requires preventative measures to avoid human transmission. Outbreaks occur in the state every 10 to 20 years. The last outbreak in Massachusetts was 2004 to 2006.

States that have not previously had experience with EEE may be even more at risk because they do not have the surveillance system in place to monitor the viral load in mosquito populations. The economic conditions of some areas may also hamper proper preventative measures. As horse outbreaks occur (as in Michigan in 2010), the risk of human infection increases in any area of the country.