Mosquito-Borne EEEV Disease Outbreak Hits Parts of U.S. in Record Numbers

Mosquito-Borne EEEV Disease Outbreak Hits Parts of U.S. in Record Numbers

Parts of the U.S. are seeing higher than normal cases of infection and death from mosquito-borne eastern equine encephalitis virus (EEEV), a disease that kills one-third of human patients and leave many survivors with mild to severe brain damage. Human cases have been reported in Connecticut, Massachusetts, Michigan, New Jersey, North Carolina, Rhode Island and Tennessee as of October 15 of this year.

EEEV is a rare but serious disease transmitted to humans through the bite of an infected mosquito. In the United States, approximately 5-10 EEE cases in humans are reported annually. This year, at least 33 people have been infected with the disease. That is more than any year since the CDC started keeping track and tops the previous record of 21 cases. At least 10 human fatalities have been reported in 2019. EEE occurs cyclically in affected regions, with two to three years of intensive activity followed by a prolonged period of subsiding infections.

Human EEEV cases occur relatively infrequently, largely because the primary transmission cycle takes place in and around swampy areas where human populations tend to be limited. All residents of and visitors to areas where EEEV activity has been identified are at risk of infection. People who engage in outdoor work and recreational activities in endemic areas are at increased risk of infection. Persons over age 50 and under age 15 seem to be at greatest risk for developing severe disease when infected with EEEV. Overall, only about 4-5% of human EEEV infections result in EEE.

Massachusetts, Michigan and Rhode Island agencies or communities have used insecticides to control mosquitos in some areas. Communities have modified the timing of some public events to minimize exposure to mosquitos, like playing football games earlier in the day. Several states test samples of mosquitos collected from standing pools of water for the presence of the disease as an early detection mechanism.

EEEV is one of many diseases transmitted by mosquitos to humans and other wild or domestic animals, called arboviruses. This family of diseases includes West Nile Virus (WNV), which can also affect humans. To date in 2019, CDC reported 681 human WNV cases with 35 fatalities as of October 15. Minnesota, Wisconsin, and Michigan are conducting coordinated surveillance activities to determine if WNV is affecting populations of ruffed grouse. WNV in ruffed grouse has become a topic of concern because of a recent study in Pennsylvania reporting that the virus may have contributed to population declines in areas of lower-quality habitat or where habitat was scarce.

In counties where EEE and WNV activity is occurring, people spending time outdoors should take precautions to minimize their risk for mosquito bites by:

  • Staying indoors between dusk and dawn, when mosquitoes are most active
  • Wearing long sleeves and pants to limit exposed skin
  • Wearing an effective mosquito repellent when outdoors

The good news for states is that the seasonal window for transmission of mosquito-borne diseases will end with the advent of cooler weather and frost.

October 15, 2019