CDC evaluates eastern equine encephalitis virus outbreak in New Hampshire

May 2006

Although eastern equine encephalitis virus (EEEV) is infrequent among humans, New Hampshire health officials reported seven human cases in 2005 alone, the first New Hampshire outbreak in nearly 41 years of national surveillance, according to a public health investigation presented at the 2006 International Conference on Emerging Infectious Diseases, held in Atlanta.

“Clinicians should be aware of recent EEEV transmission in New Hampshire, test patients from New Hampshire with aseptic meningitis or encephalitis for EEEV infection and promptly report cases,” wrote Eileen C. Farnon, MD, of the division of vector-borne infectious diseases at the CDC, and colleagues in their abstract.

Humans, horses and other mammals are dead-end hosts; birds are amplifying hosts. Many infected people are asymptomatic; however, in some, symptoms develop that range from mild flulike illness, encephalitis or meningitis, coma and death.

About 35% to 75% of human cases are fatal and about 50% of those who survive will suffer from mild-to-severe permanent neurological damage, according to previously published studies. Due to the high mortality rate, the CDC regards EEEV as one of the most serious mosquito-borne diseases in the nation.

In-depth look at cases

Farnon and colleagues evaluated cases of EEEV among New Hampshire residents.

A suspect case-patient was defined as someone who developed meningitis or encephalitis between July 1 and Sept. 30, 2005, according to the abstract. Case-patients had to have a reactive enzyme-linked immunosorbent assay–identified anti-EEEV IgM in acute or convalescent serum to be considered a probable case. Researchers considered a case as confirmed if it had neutralizing antibodies to the virus based on use of a plaque reduction neutralization test or anti-EEEV IgM in cerebrospinal fluid, according to the abstract.

They conducted personal and environmental reviews, which consisted of medical record reviews, case-patient interviews and evaluation of the residence and potential sites of exposure. They also took serum samples from 26 case-patients’ contacts. The New Hampshire Department of Health and Human Services tested animals suspected to have EEEV and tested mosquito pools.

Findings confirmed seven cases, two of which were fatal, in three southeast counties. Prodromal symptoms occurred in 86% of the cases about four to 15 days before hospital admission; cerebrospinal fluid samples were taken from six cases, of which all had pleocytosis.

Disease onset occurred from Aug. 6 to Sept. 12, 2005, and all seven patients resided in wooded areas located within a half mile of a swamp. Patient age varied from 4 to 80 years, with a median of 35 years, and 71% were males, according to the report.

EEEV IgM was not detected in the serum of patients’ contacts; however, 15 mosquito pools, 54 birds, 89 horses, four alpacas and one llama harbored EEEV.

“The reasons for EEEV emerging in New Hampshire remain speculative,” Farnon and colleagues concluded in their abstract.

Prevalence

Between 1964 and 2004, 220 confirmed cases of EEEV were reported in the United States, according to the CDC. States with the largest number of cases included Florida, Georgia, Massachusetts and New Jersey. However, cases in New Hampshire remained low until last year. The virus is most prevalent in and around freshwater and hardwood swamps in the Atlantic and Gulf Coast states and the Great Lakes region.

According to the CDC, those most at risk for EEEV disease are:

• Residents and visitors to endemic areas;
• People who engage in outdoor work and recreational activities in endemic areas; and
• Those younger than 15 years and older than 50.

Optimal medical care consists of hospitalization and supportive care such as expert nursing care, respiratory support, prevention of secondary bacterial infections and physical therapy.

“No human EEEV vaccine is currently licensed and it is unlikely that one will be available in the foreseeable future,” according to the CDC.

EEEV acquisition may be avoided by using personal and household protective measures. Improved diagnostic and viral detection tests, development and evaluation of therapeutic drugs, improved public awareness and improved predictive capabilities are a few research priorities that are greatly needed, according to the CDC.

“Public health interventions in areas affected by EEEV should focus on veterinary and mosquito surveillance, targeted vector control and education about personal protection from mosquito bites,” wrote Farnon.

For more information:

• Farnon EC, Stull JW, Talbot EA, et al. Outbreak of human eastern equine encephalitis virus infection in a new setting – New Hampshire, 2005. Abstract 332. Presented at: 2006 International Conference on Emerging Infectious Diseases; March 19-22, 2006; Atlanta.