Eastern/Western Equine Encephalomyelitis

The risk of exposure and geographic distribution of EEE and WEE vary from year-to-year with changes in distribution of insect vectors and reservoirs important in the natural ecology of the virus. EEE activity in mosquito and birds, and resultant disease in humans and equids, continues to cause concern along the East Coast and demonstrates northward encroachment. WEE has caused minimal disease in horses in the last two decades; however, the virus continues to be detected in mosquitoes and birds throughout the Western states. In addition, variants that cause clinical disease in equids have been detected in the eastern U.S.

VEE is a reportable foreign animal disease. Epidemics of VEE occur when the virus undergoes genetic change and develops greater virulence in avian and mammalian hosts. These viral variants are able to multiply to high levels in the horse and then the horse becomes a reservoir in these outbreaks. Vaccination against VEE is controversial because:

1) Vaccination against a foreign animal disease may confound testing in the event of an outbreak.

2) Experimental and field data have demonstrated that vaccination with bivalent EEE/WEE provides cross protection against VEE.

3) A conditionally available modified live (MLV) vaccine has been released during previous outbreaks. Should an outbreak occur it is likely that this highly attenuated MLV would be released.

Given this combined data, horses that receive annual EEE/WEE vaccines would be partially to completely protected and vaccination with the highly effective MLV product would induce rapid, complete immunity while allowing for accurate surveillance before VEE specific vaccination. Vaccination of horses with killed VEE vaccine should only be performed in very high risk areas of the U.S. under the guidance of state agriculture officials.

Vaccines:

EEE/WEE vaccines currently available are formalin inactivated adjuvanted whole virus products. Early testing of bivalent (EEE/WEE) vaccines was performed by intracranial challenge with either EEE and WEE; the formalin inactivated preparations demonstrated 100% protection.

Vaccination Schedules EEE/WEE:

Adult horses previously vaccinated against EEE/WEE Annual revaccination must be completed prior to vector season in the spring. In animals of high risk or with limited immunity, more frequent vaccination or appropriately timed vaccination is recommended in order to induce protective immunity during periods of likely exposure. In areas where mosquitoes are active year-round, many veterinarians elect to vaccinate horses at 6-month intervals to ensure uniform protection throughout the year, although this practice is not specifically recommended by manufacturers of vaccines.

Adult horses, previously unvaccinated against EEE/WEE or of unknown vaccinal history Administer a primary series of 2 doses with a 4 to 6 week interval between doses. Revaccinate prior to the onset of the next vector season and annually thereafter.

Pregnant mares, previously vaccinated against EEE/WEE Vaccinate 4 to 6 weeks before foaling.

Pregnant mares, unvaccinated or having unknown vaccinal history Immediately begin a 2-dose primary series with a 4-week interval between doses. Booster at 4 to 6 weeks before foaling or prior to the onset of the next vector season—whichever occurs first.

 Foals of mares vaccinated against EEE/WEE in the pre-partum period Administer a primary three-dose series beginning at 4 to 6 months of age. A 4- to 6-week interval between the first and second doses is recommended. The third dose should be administered at 10 to12 months of age prior to the onset of the next mosquito season.

In the southeastern U.S., due to earlier seasonal disease risk, vaccination may be started at 2 to 3 months of age. When initiating vaccinations in younger foals, a series of 4 primary doses should be administered, with a 4-week interval between the first and second doses and a 4-week interval between the second and third doses. The fourth dose should be administered at 10 to 12 months of age prior to the onset of the next mosquito season.

Foals of unvaccinated mares or having unknown vaccinal history Administer a primary 3-dose series beginning at 3 to 4 months of age. A 4-week interval between the first and second doses is recommended. The third dose should be administered at 10 to 12 months of age before the onset of the next mosquito season.

Horses having been naturally infected and recovered: Recovered horses likely develop lifelong immunity. Consider revaccination only if the immune status of the animal changes the risk for susceptibility to infection. Examples of these conditions would include the long term use of corticosteroids and pituitary adenoma.