Equine Influenza is a highly contagious viral disease that affects the respiratory tract of horses. Influenza, commonly known as the flu, continues to be one of the most important diseases of horses despite the availability and widespread use of equine influenza vaccines for almost 30 years. Influenza occurs worldwide except for New Zealand, Australia and Iceland. All ages of horses can be infected.
Equine Influenza causes a high fever, clear nasal discharge with swollen lymph nodes under the throat and a dry cough. If there is secondary bacterial infection, the nasal discharge may take on a more yellow to green color. Horses can also exhibit stiffness, red eye, and swelling of the lower limbs. In partially immune, vaccinated animals, one or more of these signs may be absent. Horses typically improve in 3 days. However, it takes up to 3 weeks of rest for horses to recover from influenza. Characteristically, influenza spreads rapidly in a susceptible population. The most susceptible groups are densely housed, young, 2-3 year old horses in a high stress environment, such as race track training. In these groups of horses, sickness may be as high as 60-90%, with 20-30% being the norm. Luckily, mortality rates are very low.
Horses contract the influenza virus through inhalation of aerosolized virus. They can also come into contact with the virus through tack, water bowls, pails and people’s clothing or hands. Mature horses, which have the virus, but are not sick, contribute to the spread of the virus in outbreaks. The disease typically spreads very rapidly in a barn or group of horses. The explosive nature of the cough propels the virus great distances. There is a very short incubation period and a high concentration of the virus in nasal secretions.
Treatment of Equine Influenza consists of strict rest for a minimum of three weeks. It is very important for these horses to be rested in a dust free environment to minimize the respiratory tract irritation. Affected animals should be isolated from the healthy animals in the herd. Anti-inflammatory drugs may be used to help abate the fever and reduce the inflammation. Antibiotics are necessary if the fever persists for more than three days or if there appears to be pus in the nasal discharge. When training resumes, there should be no coughing during the exercise period.
Other diseases to consider as possible causes of the above mentioned clinical signs include Rhinopnuemonitis (Rhino), early Strangles, bacterial pneumonia, and COPD (Heaves).
Due to the highly infectious nature of this disease, vaccination should be carried out at the level of the herd, not just the individual. To be fully effective, a vaccination program should therefore provide herd immunity. At Twin Valley VHS, we recommend vaccinating all herds that have horses traveling to shows, rodeos, cutting and trail riding be vaccinated against Equine Influenza. Vaccination is meant to act as an AID to prevention and there are no claims that vaccination against influenza will fully protect all animals. The goal of vaccinating horses against Equine Influenza is to minimize disease in as many horses as possible but also to reduce the clinical signs in those horses that contract the virus. We recommend vaccinating with Calvenza.
If you have any questions regarding the above information or any questions/concerns in general, please contact Twin Valley VHS at 745-6642.
Dr. Justin Noble DVM
Twin Valley Veterinary Health Services