Equine influenza is a common and highly contagious respiratory illness that affects horses, donkeys, and mules. Vaccination stands as a primary preventive measure against this disease, helping to reduce the incidence and severity of outbreaks within equine populations.
Understanding Equine Influenza Disease
Equine influenza is caused by specific influenza A viruses, primarily the H3N8 subtype, which has circulated globally in horses since the 1960s. This virus spreads rapidly through aerosol droplets released when an infected horse coughs or sneezes. Direct contact with infected animals or indirect contact via contaminated equipment and clothing can also facilitate transmission.
Common symptoms of equine influenza include a high fever, lethargy, and a dry, harsh cough. Nasal discharge may also occur. The disease poses a threat to equine health, particularly in performance horses or those that frequently travel, due to economic losses and missed training or competition time.
How Equine Influenza Vaccines Work
Equine influenza vaccines are designed to stimulate the horse’s immune system to recognize and fight off the virus. Different types of vaccines achieve this through distinct mechanisms, each aiming to induce a protective immune response, primarily targeting the virus’s surface proteins like hemagglutinin (HA) and neuraminidase (NA).
Inactivated vaccines contain whole equine influenza virus particles chemically treated to render them unable to replicate. These “killed” viruses prompt an antibody response. Adjuvants, chemical components that enhance the immune response, are added to inactivated vaccines, which are administered via intramuscular injection.
Modified-live vaccines (MLVs) contain a live, attenuated form of the virus that can replicate to a limited extent without causing the full-blown disease. Administered intranasally, these vaccines stimulate both local immunity in the nasal passages and a broader systemic immune response, mimicking a natural infection. This replication in the nasal mucosa can induce a strong immune response.
Recombinant vaccines use genetic engineering to produce specific viral proteins, such as hemagglutinin, as the vaccine antigen. These vaccines offer precise targeting of key viral components. These three types form the basis of current equine influenza vaccination.
Equine Influenza Vaccination Schedules
Vaccination protocols for equine influenza vary depending on the horse’s age, risk of exposure, and reproductive status. Consulting with a veterinarian is recommended to tailor a schedule to individual horse needs.
Foals born to vaccinated mares receive their first influenza vaccine dose around 6 months of age, followed by a booster 4 to 6 weeks later. If the mare was not vaccinated, foals may begin their vaccination series earlier, between 1 and 4 months of age, with additional boosters.
Adult horses with low exposure risk, such as those kept in closed herds with minimal travel, require annual boosters. However, for performance horses, show horses, or those housed in facilities with frequent movement of horses, more frequent vaccination is advised, such as every 6 months. This increased frequency helps maintain higher levels of protection in horses at greater risk of exposure.
Pregnant mares should receive an inactivated influenza vaccine dose 4 to 6 weeks prior to their foaling date. This timing ensures that the mare produces antibodies that can be passed to the foal through colostrum, providing passive immunity during the foal’s first few months of life. Mares with unknown vaccination histories might require a two or three-dose series before foaling.
Considerations for Vaccine Efficacy
Several factors influence the real-world effectiveness of equine influenza vaccines. Vaccine efficacy is assessed by monitoring clinical signs, viral shedding, and antibody titers.
Antigenic drift is a natural mutation process where the influenza virus changes its surface proteins. These alterations can lead to new viral strains that may not be fully covered by existing vaccines, necessitating periodic updates to vaccine strains to maintain effectiveness. The World Organisation for Animal Health (WOAH) convenes an Expert Surveillance Panel to recommend which strains should be included in current vaccines.
Individual horse factors play a role in vaccine response. Variations in immune response occur due to a horse’s age, overall health status, and stress levels. While vaccines aim to induce a protective response, some horses might mount a less robust immunity than others.
Proper vaccine administration, including correct storage and handling, is important for efficacy. Incorrect techniques compromise the vaccine’s integrity and reduce its effectiveness. Widespread vaccination within a population contributes to herd immunity, which protects unvaccinated or less responsive individuals by reducing virus circulation.
Vaccination can lead to mild, transient side effects, such as soreness or swelling at the injection site, or a slight fever and lethargy. These reactions resolve within a few days. More serious reactions, though rare, can include hives or anaphylaxis. Monitor horses for any adverse reactions after vaccination and contact a veterinarian for concerns.