Spring Signals a Rise in Equine Influenza

Expert tips for preventing spring respiratory disease outbreaks

RAHWAY, N.J., February 11, 2025 – As winter transitions to spring, horse owners and barn managers should be on alert for equine influenza (EIV), a highly contagious respiratory disease that has historically surged during this time of year (see figure 1). Recent biosurveillance data underscores this seasonal pattern, reinforcing the need for proactive disease prevention and biosecurity measures.

Figure 1

          The latest annual analysis (January–December 2024) from the Equine Biosurveillance Program, an ongoing national surveillance study managed by Merck Animal Health in partnership with the University of California, Davis, Equine Infectious Disease Research Laboratory, reveals that equine influenza remains one of the most frequently diagnosed respiratory diseases, alongside equine herpesvirus type 4 (EHV-4) and Streptococcus equi subspecies equi (strangles).1 This trend aligns with the program’s cumulative findings spanning 17 years, which show that EIV consistently ranks among the “big three” respiratory pathogens affecting horses (see figure 2).

Figure 2

           “Every year, we see an uptick in equine influenza cases as horses begin traveling for competitions and training in the spring,” explains Duane E. Chappell, DVM, Merck Animal Health. “The virus spreads rapidly through direct horse-to-horse contact, shared equipment, and even human handlers who inadvertently carry the virus between horses.”

Who is most at risk?

          Analysis of recent equine flu cases confirms that horses traveling for events and competitions are at increased risk. According to the latest Biosurveillance Program update, 58% of horses diagnosed with EIV had a recent history of travel.1 Show horses were the most commonly affected group, accounting for 42% of reported cases.

           “Young horses, those under stress from travel, and those in high-density environments like showgrounds are particularly vulnerable to equine influenza,” says Dr. Chappell. “Vaccination remains the best line of defense against severe illness.”

Symptoms of equine flu can appear suddenly and may include:

  • Fever (often exceeding 102°F)
  • Harsh, dry cough
  • Nasal discharge
  • Lethargy
  • Loss of appetite

          If you suspect equine flu, immediate isolation of the horse is crucial. “It’s essential to separate symptomatic horses from the rest of the herd and implement strict biosecurity protocols,” advises Dr. Chappell. “Many upper respiratory diseases can present with similar clinical signs, so owners should contact their veterinarian for diagnostic testing and supportive care recommendations.”

How to prevent spring outbreaks

          Given the seasonal rise in cases, veterinarians urge horse owners to take proactive steps to protect their animals. Key prevention measures include:

  • Vaccination: Ensure horses are up to date on their influenza vaccinations, particularly those that travel frequently. Consider extending vaccination to resident horses who may encounter horses that come and go, including senior horses who may experience declining immune response.
  • Biosecurity: Limit direct contact with unfamiliar horses and avoid sharing tack, water buckets or grooming tools.
  • Monitoring and quarantine: New arrivals or horses returning from events should be monitored for at least two weeks before full herd integration. Daily temperature checks during travel can help identify early signs of infectious disease.
  • Hygiene practices: Handlers should wash hands (or use hand sanitizer) and disinfect equipment regularly.

           “Influenza is a serious yet preventable disease,” emphasizes Dr. Chappell. “By staying vigilant with vaccination and biosecurity, owners can significantly reduce the risk of outbreaks and keep their horses healthy this season.”

          Horse owners are urged to talk to their veterinarian about individualized vaccination recommendations. Get more equine flu facts, and learn more about the Equine Respiratory Biosurveillance Program and regional disease threats with the new equine disease outbreak map.

About Merck Animal Health

          At Merck, known as MSD outside of the United States and Canada, we are unified around our purpose: We use the power of leading-edge science to save and improve lives around the world. For more than a century, we’ve been at the forefront of research, bringing forward medicines, vaccines and innovative health solutions for the world’s most challenging diseases. Merck Animal Health, a division of Merck & Co., Inc., Rahway, N.J., USA, is the global animal health business of Merck. Through its commitment to The Science of Healthier Animals®, Merck Animal Health offers veterinarians, farmers, producers, pet owners and governments one of the widest ranges of veterinary pharmaceuticals, vaccines and health management solutions and services as well as an extensive suite of connected technology that includes identification, traceability and monitoring products. Merck Animal Health is dedicated to preserving and improving the health, well-being and performance of animals and the people who care for them. It invests extensively in dynamic and comprehensive R&D resources and a modern, global supply chain. Merck Animal Health is present in more than 50 countries, while its products are available in some 150 markets. For more information, visit www.merck-animal-health.com and connect with us on LinkedIn, Facebook, X (formerly Twitter) and Instagram.

1Merck Animal Health and University of California, Davis (Nicola Pusterla). Infectious Upper Respiratory Disease Surveillance Program. Ongoing research 2008–present.