Influenza in Horses

Equine influenza is one of the most frequently encountered infectious diseases of horses.
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Editor’s Note: This is from Understanding Equine Preventive Medicine by author and veterinarian Bradford G. Bentz, VMD. 

Equine influenza is one of the most frequently encountered infectious diseases of horses. It affects the upper respiratory tract, producing illness associated with fever and coughing. There are numerous strains, and the occurrence of outbreaks may be associated with waning immunity of the equine population and emergence of a new strain. The risk of infection increases with horses that live in high-density and high-stress situations such as racetracks, training facilities, boarding stables, breeding farms, and show grounds. Equine influenza is introduced into a group of horses by a symptomatic or asymptomatic horse that is shedding the virus. The virus’ highly contagious nature is facilitated through the spread of the pathogen by coughing or exposure to such objects as contaminated feed buckets or feeding equipment, grooming implements, or tack. The equine immune response is capable, however, of rapidly eliminating the virus. Infection can, therefore, be controlled or prevented from entering a horse population by strict quarantine of new horses for 14 days and by appropriate vaccination. Horses that are infected with an influenza virus shed the organism in the nasal secretions for up to 10 days.

Influenza is highly transmissible and is, therefore, important to consider in the vaccination program of any horse that regularly encounters new horses, is undergoing stressful situations in shipping and/or competitions, or is likely to encounter high-risk situations for disease transmission.

Today there are various manufacturers of equine influenza vaccines. Furthermore, both intramuscular (killed vaccines) and intranasal (modified live) vaccines are now available. The use of killed vaccines in the horse is associated with increases in circulating antibody in the vaccinated animal, but repeated vaccination with certain licensed vaccines of this type has failed to provide protection during outbreaks or reduce the severity of the disease.  This may be related to the fact that these killed-virus vaccines cannot produce a local immune response at the level of infection (the respiratory system mucosa). Furthermore, the circulating antibody produced by the killed vaccines does not appear to be at high or persistent levels. However, these levels appear to be higher than those produced by the modified live vaccine (intranasal), and the killed vaccines are recommended for pregnant mares to facilitate the production of circulating antibody that is subsequently concentrated in the colostrum

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Written by:

Brad Bentz, VMD, Dipl. ACVIM, ABVP, ACVECC, owns Bluegrass Equine Performance and Internal Medicine in Lexington, Ky., where he specializes in advanced internal medicine and critical care focused on helping equine patients recuperate at home. He’s authored numerous books, articles, and papers about horse health and currently serves as commission veterinarian for the Kentucky State Racing Commission.

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