Human Arbovirus Update for Healthcare Providers in Maine

July 08, 2013

Summer is here and arboviral activity has already been detected in the New England region, with a mosquito collection testing positive for West Nile virus (WNV) in Massachusetts. Arboviral diseases, including Eastern equine encephalitis (EEE) and WNV, are very serious infections that are transmitted by the bite of an infected mosquito. Although rare, these diseases have potentially severe and even fatal consequences for those who contract them.

The purpose of this health advisory is to alert clinicians to the potential for human disease activity in Maine and to consider testing for arboviral disease in patients presenting with unexplained encephalitis, meningitis, or high fever (≥100.4°F or 38°C) during the summer and early fall.


EEE and WNV were first detected in Maine in 2001 in birds. In 2009, Maine experienced unprecedented EEE activity, with 19 animals and 2 mosquito pools testing positive. In the fall of 2008, a man vacationing in Cumberland County died of the disease. It is unclear where he contracted the infection. In 2012, Maine experienced arboviral activity in York and Cumberland counties, with 7 mosquito collections testing positive for WNV, 1 pheasant flock testing positive for EEE, and Maine's first human case of locally acquired WNV neuroinvasive illness.

Clinical Features of Mosquito-Borne Infections

EEE: EEE is considered to be the most severe arboviral infection found in the United States. Symptoms of EEE usually appear 4 to 10 days after the bite of an infected mosquito and range from mild influenza-like illness to high fever, encephalitis, coma, and death. The EEE case fatality rate is approximately 35%-50%. It is estimated that 35% of people who survive EEE will have residual neurological deficits.

WNV: Symptoms of WNV infection usually appear 3 to 15 days following the bite of an infected mosquito. Most people infected with WNV are asymptomatic. Symptoms can range from a mild influenza-like illness to headache, high fever, neck stiffness, altered mental status, convulsions, paralysis, coma, and sometimes death.

Risk Groups

Many people infected with EEE or WNV remain asymptomatic. The following groups of people are at higher risk for clinically significant arboviral infection:

  • residents of and visitors to areas with mosquito activity,

  • people who engage in outdoor work and recreational activities,

  • persons over age 50 and younger than age 15.

Diagnostic Tests for Arboviral Infections

Diagnosis relies on a high index of suspicion and on results of specific laboratory tests. EEE, WNV, or other arboviral infections should be considered in any individual — but especially those over age 50 or younger than age 15 &madsh; who has onset of unexplained encephalitis, meningitis, or high fever in the summer and fall. The local presence of EEE and WNV in animals and mosquito pools should further raise the index of suspicion.

If arboviral infection is suspected on the basis of clinical evidence, serum samples and cerebral spinal fluid (CSF) should be submitted for arboviral testing. Maine's Health and Environmental Testing Laboratory (HETL) and many reference laboratories perform arboviral testing. All samples of CSF submitted to HETL should be accompanied by a serum sample. Ideally, an acute and a convalescent serum sample should be submitted for each patient.

  • Acute serum samples should be collected within 14 days of onset of symptoms.

  • Convalescent serum samples should be collected 10 days to 4 weeks following the acute specimen.

Additional Information

More information on arboviral disease

HETL Laboratory Submission Sheet

Weekly arboviral reports will be posted throughout the season on the Maine CDC Web site.

Disease consultation and reporting are available through the Maine CDC at 1-800-821-5821.