Equine Flu Infects Quarter Horses Most Often

Written by Duane E. Chappell, DVM, Merck Animal Health
Sponsored by Merck Animal Health

A new study from the Equine Respiratory Biosurveillance Program shows that the Quarter Horse is the breed most commonly infected with equine influenza virus (EIV).¹ While equine flu affects all ages and breeds of horses, Quarter Horses between 1 year old and 9 years old with a recent travel history during winter and spring seasons are most commonly infected.¹

The overall number of horses infected with EIV has been trending upward since 2008, according to data from the Equine Respiratory Biosurveillance Program.² 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.

So why are more horses being affected by 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.

Florida ‘13 and Vaccine Efficacy
To reach this conclusion, researchers compared various equine influenza 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® line of 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 to prevent cell infection. Antigenic drift decreases a vaccine’s ability to bind to these key sites, leading to reduction or a total lack of protection.

To help protect time in the saddle — and, more importantly, Quarter 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 affected breed: Quarter Horse¹
Most affected age: 1-9 years¹
Use of most affected horses: Pleasure 40.1%, Competition 35.6%2
Most common signs: Fever, nasal discharge, cough¹
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. Chappell, D.; Barnett, D.C.; James, K.; Craig, B.; Bain, F.; Gaughan, E.; Schneider, C.; Vaala, W.; Barnum, S.; Pusterla, N. Voluntary Surveillance Program for Equine Influenza Virus in the United States During 2008-2021. AAEP Proceedings. November 18-22,2022;56-57; San Antonio, TX.
2. 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. https://doi.org/10.3390/pathogens11070759.
3. Merck Animal Health and University of California, Davis (Nicola Pusterla). Infectious Upper Respiratory Disease Surveillance Program. Ongoing research 2008–present.