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Equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4) infect the respiratory tract, the clinical outcome of which can vary in severity from sub-clinical to severe respiratory disease. Clinical infection is characterized by fever, lethargy, anorexia, nasal discharge, cough, and mandibular lymphadenopathy. Infection of the respiratory tract with EHV-1 and EHV-4 typically first occurs in foals in the first weeks or months of life, but recurrent clinical infections are seen in weanlings, yearlings, and young horses entering training, especially when horses from different sources are commingled. Equine herpesvirus type 1 can cause major outbreaks of abortion in naïve mares, the birth of weak nonviable foals, or a sporadic neurologic disease (equine herpesvirus myeloencephalopathy-EHM) secondary to lytic infection of endothelial cells resulting in the development of thombi in the small blood vessels supplying the spinal cord and brain.

Both EHV-1 and EHV-4 spread primarily by the respiratory route, by direct and indirect (fomite) contact with nasal secretions, and, in the case of EHV-1 and infrequently EHV-4, by contact with aborted fetuses, placental and fetal fluids, and placentae. Like herpesviruses of other species, these viruses establish latent infection in the majority of horses, which become asymptomatic carriers of one or both viruses.  Such horses may experience reactivation of either virus, resulting in replication in certain white cell elements in the blood and short term shedding of the virus when stressed. Some pregnant mares in which reactivation of virus occurs, may abort.  Existence of a carrier state seriously compromises efforts to control these diseases and explains why outbreaks of EHV-1 or EHV-4 can occur in closed populations of horses.

Because both viruses are endemic in many equine populations, most mature horses have developed some immunity through repeated natural infection; thus, most mature horses do not develop serious respiratory disease when they become reinfected but may be a source of infection for other susceptible horses. In contrast, horses may not be protected against the abortigenic or neurologic forms of the disease, even after repeated infection, and mature or aged horses are in fact more commonly affected by the neurologic form of the disease than juvenile animals.

Recently, a genetic variant of EHV-1 has been described (defined by a single point mutation in the viral DNA polymerase [DNApol] gene) that is more commonly associated with neurologic disease (EHM). This mutation results in the presence of either aspartic acid (D) or an asparagine (N) residue at position 752.  Molecular diagnostic techniques can identify EHV-1 strains carrying these genetic markers. The finding of a neuropathogenic variant of the virus can have implications for the management of EHV-1 outbreaks, or individual horses actively infected with these strains. It is important to understand that both virus genotypes can and do cause neurological disease. However, infection with D752 strains can result in a higher clinical attack rate and a higher case fatality rate. It is estimated that 80-90% of neurological disease is caused by D752 isolates, and 10-20% by N752 isolates.  It is possible that 5-10% of all horses normally carry the D752 form (this estimate is based on limited studies at this time). In the face of an active outbreak of EHV-1 disease, identification of a D752 isolate may be grounds for increased concern about the risk of development of neurological disease.

Primary indications for use of equine herpesvirus vaccines include prevention of EHV-1-induced abortion, and reduction of severity and duration of signs of respiratory tract disease (rhinopneumonitis) in foals, weanlings, yearlings, young performance and show horses that are at high risk for exposure. Many horses produce post-vaccinal antibodies against EHV, but the presence of those antibodies is not indicative of protective immunity. Repeated vaccination appears to reduce the frequency and severity of disease and limits the occurrence of abortion storms.  As with all forms of equine herpes viral disease, biosecurity management is of primary importance for control of abortion caused by EHV-1.

Please check with your state or provincial animal health office about which diseases are reportable.

Vaccines:

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:

I. 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.