Understanding Equine Influenza and Rhinopneumonitis

Equine influenza and rhinopneumonitis are two of the most significant viral threats to horse health worldwide. As a responsible horse owner, understanding these diseases and the importance of vaccination can help prevent outbreaks and ensure the health and performance of your horses. This article delves into the nature of these viruses, the vaccination process, and recommended schedules based on AAEP guidelines.

What is Equine Influenza?

Equine influenza, commonly known as horse flu, is a highly contagious respiratory virus that affects horses globally. Caused by various strains of the influenza A virus, it is characterized by:
  • Symptoms: Acute onset of high fever, cough, nasal discharge, and sometimes, swollen lymph nodes.
  • Transmission: Spread through airborne particles over distances up to 50 meters, via direct contact, and contaminated equipment or clothing.
  • Impact: Particularly debilitating in young, elderly, or immunocompromised horses, leading to severe respiratory distress and extended recovery periods.

What is Rhinopneumonitis (Equine Herpesvirus)?

Rhinopneumonitis refers to the disease caused by the Equine Herpesvirus (EHV), primarily types 1 and 4:
  • EHV-1: Causes respiratory disease, neurological disorders, and abortions in pregnant mares.
  • EHV-4: Mainly leads to respiratory illness but can occasionally cause abortions.
  • Symptoms: Fever, lethargy, nasal discharge, and, in severe cases, neurological symptoms such as incoordination.
  • Transmission: The virus spreads via the respiratory route from infected to susceptible horses and can remain latent, reactivating during periods of stress.

How We Vaccinate

Vaccinations are administered primarily as intramuscular injections. The vaccines for flu and rhino are often combined to simplify the protocol and enhance compliance, however, single virus vaccines exist as well as intra-nasal options.

When to Vaccinate

The AAEP recommends the following vaccination schedules:
  • Foals: Begin the initial vaccination series at 6 months, followed by boosters at 7 and 8 months. For foals born to vaccinated mares, the first vaccine can be administered at 9 months, followed by boosters at 10 and 11 months.
  • Adults: Annual or semi-annual boosters, depending on the horse’s exposure risk and environment. Horses at higher risk (those that travel frequently or are housed in large stables) may require more frequent vaccination.

Specific Schedules for Pregnant Mares

To protect against EHV-1 induced abortion, pregnant mares should be vaccinated at 5, 7, and 9 months of gestation.

Which Horses Should Be Vaccinated?

Vaccination is recommended for all horses, regardless of their use, age, or environment. This is particularly crucial for:
  • Competition horses: Due to their frequent exposure to other horses.
  • Leisure horses: Even minimal contact with other horses can pose a risk.
  • Horses in communal environments: Such as boarding facilities, where the disease can spread quickly.


Equine influenza and rhinopneumonitis pose significant risks to the health and performance of horses. Effective vaccination protocols, as recommended by the AAEP, are crucial in managing these diseases. Regular consultations with a veterinarian will ensure that your vaccination schedules are up to date and tailored to your horse’s specific needs.

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