Equine Influenza

What is Equine Influenza?

Equine influenza is a highly contagious respiratory disease caused by two subtypes of the influenza A virus. Other species, like birds, pigs, dogs, and people, are also infected by other influenza A viruses that are related to, but distinct from, the equine influenza virus (EIV).  

EIV is endemic, meaning that it regularly occurs in the equine population. Horses do not become chronic carriers of the virus like they do for other viruses (e.g., equine herpesvirus). Clinical illness is most often seen in young horses, aged 1-5 years old, and those horses that travel frequently. It occurs mostly during the winter months.  

Symptoms of Equine Influenza

Horses experiencing equine influenza may exhibit the following signs: 

• Fever 

• Deep, dry cough 

• Nasal discharge 

• Depression 

• Inappetence, “going off feed” 

• Enlarged lymph nodes under the jaw 

• Muscle pain 

Causes of Equine Influenza

Equine influenza virus is spread both directly and indirectly. Direct transmission occurs when infected horses release viral particles into the air by coughing or snorting. These particles may travel up to 150 feet in the air.  

Indirect transmission occurs through contaminated equipment like water and feed buckets or grooming supplies that are shared among horses, or when people transmit the virus from horse to horse on their clothing or shoes.  

How Veterinarians Diagnose Equine Influenza

Since the signs of equine influenza are similar to other respiratory diseases in horses, molecular tests must be run to confirm an infection. The most common test is a RT-PCR test that detects parts of the virus, the antigen. This test is performed on a sample from a nasal swab.  

Additionally, an ELISA test can be run from a blood sample, but it must be done on paired samples—one at the onset of disease and one 2 weeks later. This test looks for antibodies, or proteins the horse makes in response to having the virus. Testing must be done as soon as possible. If testing is delayed, the results may come back falsely negative.  

Treatment of Equine Influenza

Unless the horse has a complicated case, the main treatment of equine influenza is rest and supportive care. As a general rule, horses should have a week of rest for every day they had a fever; with a minimum of 3 weeks of rest. This will allow time for the cells lining the respiratory tract to heal.

Anti-inflammatories, like phenylbutazone and flunixin meglumine, are used to reduce fever, pain, and inflammation. Antibiotics are recommended for horses that develop a secondary bacterial infection or have a fever for more than 3 days. Thick, yellow nasal discharge is an indication that a bacterial infection is present. Hospitalization may be required in horses that develop pneumonia secondary to the virus.  

Recovery and Management of Equine Influenza

Horses with mild infections of EIV recover fully in 2 to 3 weeks. Severely affected horses may take up to 6 months to recover. Full recovery is facilitated by restricting exercise, controlling dust particles in the air (wetting down hay), and providing good ventilation to the area where the horse is housed. Complications, though they occur rarely, may include laminitis, vasculitis (inflammation of the blood vessels), and myocarditis (inflammation of the heart muscles).  

Any horse showing respiratory signs of disease (cough, nasal discharge) should be immediately isolated. To minimize the occurrence of EIV and prevent outbreaks, strict barn hygiene should be in place. Thoroughly cleaning all equipment and washing all common areas will help decrease the spread of the virus. Any new horses to the environment should be isolated for 14 days before being introduced to the general population.  

Preventing Equine Influenza 

Vaccines are recommended to decrease the occurrence of EIV. There are two types of vaccines, a modified-live vaccine and an inactivated vaccine. The modified-live vaccine is administered as a single dose that goes up the nose and is booster-ed annually. The modified live vaccine contains a weakened version of the virus. The horse’s immune system will respond to it and produce antibodies.  

The inactivated vaccine contains killed viral particles and is given in the muscle. It requires 2 to 3 doses given 3 to 4 weeks apart, based on the manufacturer’s recommendations. Horses with low risk of exposure should be booster-ed yearly, and horses with high risk of exposure (horses traveling to races or shows) should be booster-ed every 6 months. Vaccinated horses may still get EIV, but it is less likely and if they do, the disease should be less severe.