Eight Massachusetts Horses Positive for EEE

On Sept. 9, the Massachusetts Health and Human Services (MHHS) announced eight confirmed cases of Eastern equine encephalomyelitis (EEE).

The affected horses came from five counties:

  1. Essex (town of Methuen, with onset of clinical signs Aug. 22);
  2. Hampshire (Granby, onset of clinical signs Aug. 8);
  3. Middlesex (Holliston, onset of clinical signs Aug. 23);
  4. Norfolk (Medfield, onset of clinical signs Aug. 24); and
  5. Worcester (Uxbridge, Brookfield, and Douglas, onset of clinical signs Aug. 16, 22, and 27).

Michael Cahill, director of the Division of Animal Health (DAH) at the Massachusetts Department of Agricultural Resources (MDAR), said none of the horses were vaccinated against EEE, and all were euthanized.

“Based on the information collected, we believe all of the affected animals were infected on their home farms, and that information in combination with any human illness reports is used by the Department of Public Health in determining areas that may need to be targeted for aerial spraying of mosquitoes,” he said. “Quarantines are not an effective means of controlling the disease since horses are dead-end hosts (there is never enough virus in their blood to allow for the transmission of virus to another mosquito).”

EEE 101

Eastern equine encephalomyelitis is caused by the Eastern equine encephalitis virus, for which wild birds are a natural reservoir. Mosquitoes that feed on EEE-infected birds can transmit the virus to humans, horses, and other birds. Horses do not develop high enough levels of these viruses in their blood to be contagious to other animals or humans. Because of the high mortality rate in horses and humans, EEE is regarded as one of the most serious mosquito-borne diseases in the United States.

Tips for preventing mosquito-borne diseases include:

  • Avoid mosquito bites: Use insect repellent when outdoors, especially from dusk to dawn.
  • Look for EPA-labeled products containing active ingredients such as DEET, picaridin (KBR3023), or oil of lemon eucalyptus (p-menthane 3,8-diol).
  • Apply more repellent, according to label instructions, if mosquitoes start to bite.
  • Mosquito-proof homes: Fix or install window and door screens, and cover or eliminate empty containers with standing water where mosquitoes can lay eggs.
  • Protect your horses: Veterinarians recommend commercially available licensed vaccines against EEE for all horses in the U.S. Horses should be vaccinated at least annually (recommendations vary in high-risk areas). It’s not too late this year to vaccinate your horses.
  • Use approved insect repellents to protect horses.
  • If possible, put horses in stables, stalls, or barns during the prime mosquito exposure hours of dusk and dawn.
  • Eliminate standing water, drain water troughs, and empty buckets at least weekly.
  • Stock water tanks with fish that consume mosquito larvae (contact your local mosquito control for assistance), or use mosquito “dunks” (solid “donuts” of Bacillus thuringiensis israelensis[BTi], which are nontoxic to horses) available at hardware stores.