Equine Influenza

Background
Equine influenza, caused by the orthomyxovirus equine influenza A type 2 H3N8 subtype, is one of the most common infectious diseases of the respiratory tract of horses.

Influenza is endemic in the equine population of the United States and throughout much of the world, with the notable exceptions of New Zealand, Australia, and Iceland. The OIE Expert Influenza Surveillance panel, reviews currently circulating strains and makes recommendations for strain inclusion in the vaccine products.

Equine influenza is highly contagious and the virus spreads rapidly through groups of horses in aerosolized droplets dispersed by coughing or through fomite transmission. The majority of the clinical signs are respiratory and may also include fever, edema and enlarged lymph nodes.

Vaccines:

There are two types of equine influenza virus vaccine currently marketed:

Inactivated (killed) vaccines for intramuscular administration
Modified-live vaccine for intranasal administration

Vaccination Schedules

Adult Horses

Adult horses previously vaccinated against influenza:

o Revaccinate annually. Horses at increased risk of exposure may be revaccinated every 6 months. Some facilities and competitions may require vaccination within the previous 6 months to enter.
USEF Vaccination Rule:
https://www.usef.org/forms-pubs/ANcxoLX1gNs/equine-vaccination-rule-gr845

Adult horses previously unvaccinated against influenza or of unknown vaccine history

o Inactivated vaccine: Dependent upon on manufacturer’s product recommendation, the vaccine may be a two or three dose series with a 3 to 4-week interval between doses (IM injection).
o Modified live vaccine: Administer a single dose (IN application).
o Revaccinate annually.

Pregnant Horses

Pregnant mares, previously vaccinated against influenza:

o Inactivated vaccine: Annually with one dose administered 4 - 6 weeks pre-partum

Pregnant mares, unvaccinated or having unknown vaccine history:

o Inactivated vaccine: Dependent upon on manufacturer’s product recommendation, the vaccine may be a two or three dose series with a 3 to 4-week interval between doses (IM), with the last dose administered 4-6 weeks pre-partum.

Foals

Foals of mares vaccinated against influenza in the pre-partum period

o Inactivated vaccine: Dependent upon on manufacturer’s product recommendation, the vaccine may be a two or three dose series with a 3 to 4-week interval between doses (IM).
o Modified Live vaccine: Administer a single dose (IN) in foals 11 months of age or older

Foals of mares previously unvaccinated against influenza or having unknown vaccine history in the pre-partum period:

o Inactivated vaccine: Dependent upon on manufacturer’s product recommendation, the vaccine may be a two or three dose series with a 3 to 4- week interval between doses (IM) starting at 4-6 months of age.

Other Considerations:

Outbreak Mitigation: Vaccination to boost immunity in the face of an outbreak may be a valuable strategy if the outbreak is detected early enough. Previously vaccinated horses may be revaccinated, particularly if the previous vaccine was administered more than 3 months prior. In unvaccinated horses, or horses with an unknown vaccination history, the early onset of immunity after administration of the intranasal product, may be recommended for use. Unpublished evidence indicates onset of immunity is 7 days post administration of the intranasal vaccine.
Horses having been naturally infected and recovered: Horses with a history of influenza infection and disease are likely to have immunity to the specific strain for more than 1 year, but booster vaccination is recommended 6 months after disease occurrence due to variations in the influenza strain.