Annual vaccinations are required for mares that test seronegative for antibody to EVA and are going to be bred to a carrier stallion. There is only one vaccine lisenced for commercial use in horses. Proper documentation must be submitted for approval to vaccinate horses against EVA. Proper documentation indicates if the horse has been tested and determined to be seronegative for antibodies against EVA.

Breeding to an EVA Positive Stud:

At least one month prior to the breeding the mare should be check for antibody titer levels for EVA. The titer results will indicate what steps need to be followed.

  • Negative Antibody Titer (Titer <1:4, non-pregnant)
    Mare should  be vaccinated with a modified live vaccine against EVA. The mare should then be kept in isolation for 21 days following vaccination. The isolation period allows the mare to develop an adequate level of immunity and protects other horses from viral exposure. Following this isolation period the mare can be bred to the stallion, as long as the stallion is not currently shedding.
    If a mare has been bred to a shedding stallion, the mare should be kept in isolation from any other horse with a negative titer for 21 days. The mare does not need to repeat the isolatin period following subsequent breedings. The mare should receive a yearly booster vaccination 21 days before breeding if she is to be rebred to positive stallion the next year.
  • Negative Antibody Titer  (Titer <1:4, pregnant)
    Pregnant mares should not be vaccinated using a modified live vaccine unless she is at risk of high exposure to the virus. A healthy mare can be vaccinated following parturition other wise she should not be vaccinated until her health improves or she recovers from a complicated foaling. The mare should not be vaccinated before the foal is two weeks of age. Also the foal should be in good health if the mare is to be vaccinated. There is minimal risk that the foal will be exposed to the virus after the mare's vaccination.
  • Positive Antibody Titer (Titer >1:4, all mares)
          Mares that test serologically positive do not need to be vaccinated prior to being bred to a shedding stallion. Mares that  are bred via live cover should be kept seperate from other horses for 24 hours following the breeding. This isolation will prevent mechanical transmission from voided semen. If the mare was transported to the stallion for breeding, the vehicle used for transport must be immediately cleaned and disinfected.

Foal effects and Immunity:mare

Severe pneumonia and possible death are the main results of infection in very young foals. EVA is rarely fatal in horses other than very young foals. If a mare is infected and does not abort, the result will be a congenitally infected foal. Infected foals will not become carrier animals. After natural infection foals will develop a strong natural immunity that lasts several years. Foals less than 6 weeks of age should not be vaccinated with a modified live vaccine. This vaccine should only be used if the foal is faced with a high risk of natural infection.


  • AAEP Guidelines for Vaccination of Horses AAEP

  • AAEP Guidelines for Breeding to an EVA Shedding Stallion