The American Association of Equine Practitioners (AAEP) divides equine vaccines into two categories: core vaccines recommended for all horses regardless of use or location, and risk-based vaccines recommended based on geographic exposure, horse use, and lifestyle. Understanding the distinction, what each vaccine protects against, and the current AAEP schedule helps owners and veterinarians build an appropriate protocol for each individual horse.
| Core Vaccine | Mortality Unvaccinated | Minimum Schedule |
|---|---|---|
| EEE (Eastern Encephalomyelitis) | ~90% in clinically affected horses | Annual; bi-annual in high-exposure regions |
| Rabies | 100% once clinical signs develop | Annual — also a human health concern |
| Tetanus | ~75% even with treatment | Annual toxoid; antitoxin at injury if unvaccinated |
| West Nile Virus | 30–40% in clinical cases | Annual; bi-annual in high-exposure areas |
Core Vaccines: Recommended for All Horses
Eastern/Western Equine Encephalomyelitis (EEE/WEE): mosquito-transmitted viral encephalitides with high mortality rates (EEE: approximately 90% mortality in unvaccinated horses; WEE: approximately 50%). EEE is endemic in the eastern and Gulf Coast U.S. and poses genuine mortality risk annually. Both EEE and WEE are also transmissible to humans via mosquito vectors (not horse-to-human directly). Annual vaccination is the AAEP core recommendation; bi-annual in high-mosquito-exposure areas or where the geographic season is long.
West Nile Virus (WNV): another mosquito-transmitted arbovirus causing neurological disease with 30-40% mortality in clinically affected horses. First appeared in the United States in 1999, now ubiquitous across North America. Annual vaccination effective; some protocols vaccinate twice yearly in high-exposure regions or in horses with expected repeated mosquito exposure (show horses, trail horses in endemic areas).
Rabies: 100% fatal once clinical signs develop in all mammals including horses. Horses occasionally contact rabid wildlife and can transmit rabies virus to humans. Annual vaccination is the AAEP core recommendation. Rabies is a reportable and notifiable disease -- an unvaccinated horse that bites a person may require lengthy quarantine protocols with significant liability implications for the owner.
Tetanus: Clostridium tetani spores are ubiquitous in soil and horse manure. Any wound, however minor, can introduce tetanus organisms. Tetanus in horses has approximately 75% mortality even with treatment. Annual vaccination with tetanus toxoid provides reliable protection. At time of injury, an unvaccinated horse should receive tetanus antitoxin immediately; tetanus antitoxin in vaccinated horses is controversial (over-administration of antitoxin in vaccinated horses carries rare risk of serum hepatitis) and not routinely recommended for current vaccinations in good standing.
Risk-Based Vaccines
Equine Influenza: a highly contagious respiratory virus causing fever, nasal discharge, and cough. Risk-based vaccination is recommended for horses with exposure to other horses -- show horses, racehorses, horses in boarding facilities. AAEP recommends every 6 months for high-exposure horses. Horses in isolated or low-exposure situations may vaccinate annually. Two main vaccine types: inactivated and modified-live intranasal (FluAvert IN). The intranasal modified-live vaccine provides mucosal immunity in addition to systemic and offers broader cross-protection against drifted strains.
Equine Herpesvirus (EHV-1/EHV-4): EHV-1 causes respiratory disease, abortion storms, and neurological disease (equine herpesvirus myeloencephalopathy, EHM). EHV-4 primarily causes respiratory disease. Vaccination reduces clinical respiratory disease but does not fully prevent EHM -- an important limitation to communicate to owners. Risk-based vaccination: breeding farms (to reduce abortion risk), horses with frequent exposure to other horses, facilities that have had EHV outbreaks. Pregnant mares should be vaccinated at months 5, 7, and 9 of gestation with the killed EHV-1/EHV-4 vaccine specifically labeled for abortion prevention.
Strangles (Streptococcus equi equi): vaccination is risk-based, recommended for horses with high exposure risk. Two vaccine types: intramuscular killed (Pinnacle IN) and intranasal modified-live (Pinnacle IN). Neither provides complete immunity but reduces disease severity. Not recommended in facilities with active strangles outbreaks (risk of guttural pouch empyema in chronically infected horses that receive vaccine during incubation). M-protein titers should be checked before vaccinating horses with history of strangles to assess risk of immune-mediated myopathy (purpura hemorrhagica).
Sources: AAEP Vaccination Guidelines 2022 update; USDA APHIS equine disease surveillance data; Merck Veterinary Manual equine infectious disease chapters; Wilson and Watson, Equine Infectious Diseases, 2nd edition (Saunders Elsevier); Journal of Equine Veterinary Science strangles vaccination efficacy studies.