Background
In the United States, equine alphavirus encephalitides for which vaccines are available include eastern equine encephalomyelitis (EEE), western equine encephalomyelitis (WEE), and Venezuelan equine encephalomyelitis (VEE). EEE has been reported in North and South America. WEE have been recorded more commonly in the Western US. VEE causes outbreaks in horses in Central America, South America, Mexico and occasionally in the southern US. Transmission of EEE/WEE/VEE is by primarily mosquitoes, and infrequently by other insects, ticks, or nasal secretion. Although vaccination has reduced the size and number of outbreaks of EEE, WEE and VEE in horses, the impact of these diseases is still significant because of the high mortality rate.
Immunizations against EEE and WEE are core prophylaxis for all horses residing in or traveling to North America. The choice to immunize against VEE is a risk-based assessment of the horse’s potential exposure. VEE is a reportable foreign animal disease. Areas of Southern Texas, California, Louisiana, Mississippi, Alabama and the West Coast of Florida are likely at the most risk for natural VEE encroachment from Central and South America.
The practitioner should consider the following in making recommendations:
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Vaccination against a foreign animal disease may confound testing in the event of an outbreak. An IgM capture ELISA (as we do with EEE, WEE, and WNV) would be used to confirm the diagnosis in a clinical animal. This can differentiate vaccinated (IgG only) versus virulent virus (IgM).
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There is some evidence that vaccination against EEE/WEE may also result in cross-protection to VEE -based on one study. 1
Vaccines
Vaccines for all three diseases are available as killed products and were shown to be highly efficacious in protecting against clinical disease.
Vaccination Schedules
Immunization for EEE/WEE are core vaccines for all horses residing and traveling within the United States. Immunization for VEE is risk-based. The schedules presented here apply to bivalent and multivalent vaccines.
Adult Horses
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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.
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Adult horses, previously unvaccinated against EEE/WEE or of unknown vaccine history: Administer a primary series of 2 doses with a 3- to 6-week interval between doses as per product label. Revaccinate prior to the onset of the next vector season and annually thereafter.
Pregnant Horses
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Pregnant mares, previously vaccinated against EEE/WEE: Vaccinate 4 to 6 weeks before foaling.
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Pregnant mares, unvaccinated or having unknown vaccine history: Immediately begin a 2-dose primary series with a 3 to 6-week interval between doses as per product label. Booster at 4 to 6 weeks before foaling or prior to the onset of the next vector season—whichever occurs first.
Foals
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Foals of mares vaccinated/unvaccinated against EEE/WEE in the pre-partum period: Administer a primary 2-dose series beginning at 4 to 6 months of age. A 4- to 6-week interval between the first and second doses is recommended. A third dose should be administered at 10 to 12 months of age prior to the onset of the next mosquito season.
Other Considerations
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In high risk situations such as an early onset of seasonal disease, increase incidence in a geographic area, or foals of unvaccinated mare, practitioner in consultation with manufacturer, may consider starting earlier vaccination or using a product more frequently.
1 Walton TE1, Jochim MM, Barber TL, Thompson LH. Cross-protective immunity between equine encephalomyelitis viruses in equids. Am J Vet Res. 1989 Sep;50(9):1442-6.
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