Monday, May. 20, 2024

Veterinary Corner: The Real Threat Of Equine Flu






The Veterinary Corner series, brought to you by Merck Animal Health, features insights from leading veterinarians on some of the most pressing health issues affecting horses today.

By Duane E. Chappell, DVM, Merck Animal Health


The number of horses infected with equine influenza virus (EIV) has been trending upward since 2008, according to data from the Equine Respiratory Biosurveillance Program.1 This highly contagious, rapidly spreading virus can make a big impact, with effects ranging from medical challenges to economic loss and time away from training and performance. Young horses in active training, competition and travel are especially at risk for EIV because they come into contact with many horses and their immune systems may be weakened due to stress.1

So why the steady rise in equine flu? Because the virus is changing.


Just like human flu, equine influenza evolves but at a much slower rate. Through the Equine Respiratory Biosurveillance Program, researchers found that horses involved in a 2013 influenza outbreak were infected with a new flu strain named Florida ‘13 after the location and date of first isolation.2

Therein lies the threat of today’s equine flu: The virus has evolved and Florida ‘13 most closely reflects the flu that’s circulating in horses today.

To reach this conclusion, researchers compared various equine influenza vaccine strains, revealing that the Florida ‘13 strain differed significantly from older strains including Ohio ‘03 and Kentucky ‘97. Ongoing research through the Equine Respiratory Biosurveillance Program at the time of this article shows Florida ‘13 is the vaccine strain closest to today’s equine influenza virus.3

Watch this video to see how antigenic drift affects equine influenza strains.


In response to these findings, Merck Animal Health updated the PRESTIGE® influenza vaccines to account for the Florida ‘13 strain. PRESTIGE® vaccines are the only vaccines available that contain the Florida ’13 vaccine strain.

The equine influenza virus infects horses by binding to respiratory cells, and vaccines work by producing antibodies that bind to key sites on the virus, preventing cell infection. Antigenic drift decreases a vaccine’s ability to bind to these key sites, leading to reduction in or a total lack of protection.

To help protect time in the saddle — and, most importantly, horses — keep these points in mind:


  • The EIV virus is changing
  • Not all vaccines are created equally
  • Biosecurity measures help reduce horses’ risk

Work with your veterinarian to determine the right time to protect horses with an updated vaccine that includes Florida ‘13. You can also implement a few straightforward biosecurity measures at home and on the road to help reduce horses’ risk of influenza. Remember that even horses that don’t travel are still susceptible to equine flu, especially if they live at a barn with other horses coming and going.

You and your veterinarian can rely on Merck Animal Health to stay up to date on equine flu and other infectious disease threats with ongoing monitoring through the Equine Respiratory Biosurveillance Program. This is part of the Merck Animal Health Unconditional promise to not only provide a portfolio of exceptional products, but also to invest in resources that strengthen the bond between human and horse. Backed by current information and an updated vaccination, you’ll help ward off the threat of today’s equine flu.

Quick Look at Equine Flu:

  • Most common signs: Fever, nasal discharge, cough1
  • Travel status of infected horses (between Jan. – June 2022): 47% Yes, 38% No, 15% Unknown3
  • Median age of infected horses (between Jan. – June 2022): 5 years3
  • Distance a coughing horse can propel the equine influenza virus (EIV): 50 yards
  • Time it takes for signs of EIV to develop: 24 to 72 hours
  • Time it takes horses to recover from EIV: 3 weeks to 6 months depending on severity of infection



  1. Pusterla, N.; James, K.; Barnum, S.; Bain, F.; Barnett, D.C.; Chappell, D.; Gaughan, E.; Craig, B.; Schneider, C.; Vaala, W. Frequency of Detection and Prevalence Factors Associated with Common Respiratory Pathogens in Equids with Acute Onset of Fever and/or Respiratory Signs (2008–2021). Pathogens 2022, 11, 759.
  2. Data on file, Merck Animal Health.
  3. Merck Animal Health and University of California, Davis (Nicola Pusterla). Infectious Upper Respiratory Disease Surveillance Program. Ongoing research 2008–present

Copyright © 2023 Merck & Co., Inc., Rahway, NJ, USA and its affiliates. All rights reserved.

This content was paid for and provided by Merck Animal Health. The views and opinions expressed in this article do not necessarily reflect those of The Chronicle of the Horse.




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