Risk-Based Equine Vaccines: Equine Herpesvirus

While most young horses have been exposed to types of herpesviruses, there is a need to protect horses of all ages against respiratory, abortogenic and possibly neurologic herpesvirus.

Vaccinate pregnant mares during the fifth, seventh and ninth months of gestation using an inactivated EHV-1 vaccine licensed for prevention of abortion. iStock/Purple Queue

One of the diseases that the American Association of Equine Practitioners (AAEP) considers an “at-risk” vaccine is for equine herpesvirus type 1 (EHV-1). The respiratory form is known as rhinopneumonitis and the neurologic form is known as equine herpesvirus myeloencephalopathy (EHM).

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Usually horses are infected with a couple of types of herpesvirus (EHV-1 and EHV-4) at an early age, with estimates that 80-90% of the horse population has encountered herpes viral exposure before the age of two. Herpes viral presence persists for the life of a horse following an initial infection. 

EHV-1 undergoes a period of latency, during which time an affected horse shows no clinical signs of infection yet still may actively shed virus. EHV-1 is highly infectious and transmissible through fomites, aerosols, an aborted fetus or placental parts, or through direct horse-to-horse contact.

Latent virus hides out in lymphocytes and/or sensory nerve cell bodies within the trigeminal ganglia (swelling at the end of a nerve) in the head. Reactivation of virus—particularly during periods of stress—enables it to spread to susceptible horses through the respiratory tract. Not all “infected” horses show signs of illness; some are simply silent viral shedders.

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In its respiratory form, EHV-1 leads to fever, lethargy, inappetence and respiratory disease signs such as nasal discharge and cough. In addition, a herpesvirus infection interrupts movement of horses, training and competition schedules not just for the affected horse, but for others in the barn or on the premises when a quarantine is imposed. 

Silent shedders—horses carrying the virus but not displaying any clinical sign— transmit virus to immunologically naïve individuals. 

Herpesvirus also is known for its propensity to cause abortion in the last trimester. And, of great concern, is that mutant strains of EHV-1 are responsible for outbreaks of severe neurologic disease (EHM). 

EHM has a high mortality rate while also impacting large groups of exposed horses with quarantines that interrupt movement, training and competition schedules.

While only up to 10% of EHV-1 infected horses develop equine herpesvirus myeloencephalopathy, when it does occur, it appears 1-2 weeks after infection with or without respiratory signs. 

Current EHV-1 vaccines do not have the ability to protect against development of EHM. However, EHV-1 rhinopneumonitis respiratory vaccines do help limit the degree of respiratory illness, particularly when implemented through the principles of herd immunity. 

Herd immunity is defined as: “The resistance to the spread of a contagious disease within a population that results if a sufficiently high proportion of individuals are immune to the disease, especially through vaccination.” 

Some rhinopneumonitis vaccines specifically target the abortogenic form of herpesvirus and should be given to mares at specific intervals during pregnancy.

A horse previously exposed to or immunized against EHV-1 experiences less clinical severity of respiratory disease and for a shorter duration. AAEP advises that “repeated vaccination against EHV-1 appears to reduce the frequency and severity of disease and limits the occurrence of abortion storms.” 

Usually the respiratory form runs its course within a couple of weeks, although occasional individuals develop mild forms of equine asthma.

The AAEP offers a Q&A on equine herpesvirus.

Vaccination Information

The following is from

Inactivated vaccines: A variety of inactivated vaccines are available, including those licensed only for protection against respiratory disease, and two that are licensed for protection against both respiratory disease and abortion,. Performance of the inactivated respiratory vaccines is variable, with some vaccines outperforming others. Performance of the inactivated abortion/respiratory vaccines is superior, resulting in higher antibody responses and some evidence of a cellular response to vaccination.

Modified live vaccine: A single manufacturer provides a licensed modified live EHV-1 vaccine. It is indicated for the vaccination of healthy horses 3 months of age or older as an aid in preventing respiratory disease caused by equine herpesvirus type 1 (EHV-1).

EHM: None of the available vaccines have a label claim to prevent the neurologic form of EHV-1 infection. It has been suggested that vaccines may assist in limiting the spread of outbreaks of EHM by limiting nasal shedding of EHV-1 and dissemination of infection. For this reason some experts hold the opinion that there may be an advantage to vaccinating in the face of an outbreak. If this approach is pursued, only afebrile and asymptomatic horses should be vaccinated and protection against clinical EHM should not be an expectation. The vaccines with the greatest ability to limit nasal shedding and viremia of the neuro virulent strain include the vaccines licensed for control of abortion (Pneumabort-K® & Prodigy®), the MLV vaccine (Rhinomune® & Calvenza®).

Vaccination Schedules

The following information is provided by

Adult, non-breeding, horses previously vaccinated against EHV: Frequent vaccination of non-pregnant mature horses with EHV vaccines is generally not indicated as clinical respiratory disease is infrequent in horses over 4 years of age. In younger/juvenile horses, immunity following vaccination appears to be short-lived. It is recommended that the following horses be revaccinated at 6-month intervals:

  • Horses less than 5 years of age.
  • Horses on breeding farms or in contact with pregnant mares.
  • Horses housed at facilities with frequent equine movement on and off the premises, thus resulting in an increased risk of exposure.
  • Performance or show horses in high-risk situations, such as racetracks. More frequent vaccination than at 6 months intervals may be required in certain cases as a prerequisite for entry to the facility. See here for USEF Vaccination Rule.

Adult, non-breeding horses unvaccinated or having unknown vaccinal history: Administer a primary series of 3 doses of inactivated EHV-1/EHV-4 vaccine or modified-live EHV-1 vaccine. A 4- to 6-week interval between doses is recommended.

Pregnant mares: Vaccinate during the fifth, seventh and ninth months of gestation using an inactivated EHV-1 vaccine licensed for prevention of abortion. Many veterinarians also recommend a dose during the third month of gestation and some recommend a dose at the time of breeding.

Vaccination of mares with an inactivated EHV-1/EHV-4 vaccine 4 to 6 weeks before foaling is commonly practiced to enhance concentrations of colostral immunoglobulins for transfer to the foal. Maternal antibody passively transferred to foals from vaccinated mares may decrease the incidence of respiratory disease in foals, but infection is common in these foals and may result in clinical disease and establishment of the carrier state.

Barren mares at breeding facilities: Vaccinate before the start of the breeding season and thereafter based on risk of exposure.

Stallions and teasers: Vaccinate before the start of the breeding season and thereafter based on risk of exposure.

Foals: Administer a primary series of 3 doses of inactivated EHV-1/EHV-4 vaccine or modified-live EHV-1 vaccine beginning at 4 to 6 months of age and with a 4- to 6-week interval between the first and second doses. Administer the third dose at 10 to 12 months of age.

Immunity following vaccination appears to be short-lived and it is recommended that foals and young horses be revaccinated at 6-month intervals.

The benefit of intensive vaccination programs directed against EHV-1 and EHV-4 in foals and young horses is not clearly defined because, despite frequent vaccination, infection and clinical disease continue to occur.

Outbreak Mitigation: In the face of an outbreak, horses at high risk of infection, and consequent transmission of infection, may be revaccinated. Administration of a booster vaccination is likely to be of some value if there is a history of vaccination. The simplest approach is to vaccinate all horses in the exposure area—independent of their vaccination history. If horses are known to be unvaccinated, the single dose may still produce some protection. It is essential to understand that strict quarantine, isolation and monitoring protocols are more effective at controlling outbreaks than any vaccination protocol.

Controversy persists among experts regarding possible association between frequent vaccination against EHV and the risk of developing EHM. The absence of any controlled challenge studies designed to examine this question makes it unwise to offer any definitive conclusion

Horses having been naturally infected and recovered: Horses with a history of EHV infection and disease, including neurological disease, are likely to have immunity consequent to the infection that can be expected to last for 3 to 6 months (longer in older horses). Booster vaccination can be resumed 6 months after the disease occurrence.

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