Midwest Equine

Feature Article

Updated Equine Influenza Vaccines from Merck Animal Health - The Next Generation of Flu Protection

By Bryant Craig, D.V.M., Merck Animal Health

Article Archives LinkUpdated Equine Influenza Vaccines

Updated Equine Influenza Vaccines

Equine influenza (EI) is not only an acute, extremely contagious viral disease in horses, it’s one of the most common causes of equine viral respiratory tract disease. Historically, flu outbreaks have caused significant economic loss and made a devastating impact on the horse industry. Today, equine influenza continues to be a substantial international threat.

Vaccinating is one of the most important ways to protect horses from equine influenza virus (EIV). However, influenza viruses are constantly changing at the molecular level as they pass from one host to another. This process of minor evolutionary mutations and alterations, known as antigenic drift, leads to new virus strains that a horse’s immune system may not recognize. Due to the changes that can occur with EIV, it is paramount to use vaccines with relevant strains to aid in providing the best protection against viruses currently circulating.

All current circulating strains of influenza in the horse are subtype A2 (H3N8). These strains originated from the American lineage and what’s known as the Florida sublineage. The Florida sublineage is divided into two separate groups called clade 1 and clade 2. (See accompanying diagram: Today’s Equine Influenza.) Equine influenza isolates (strains) are named according to the location and year of isolation.

Evidence confirms that the clade 1 strain primarily circulates in the United States while clade 2 predominantly circulates in Europe. (To date, no outbreaks of clade 2 have occurred in the United States.) The World Organisation for Animal Health (OIE) Expert Surveillance Panel on Equine Influenza currently recommends that vaccines contain both clade 1 and clade 2 viruses of the Florida sublineage.


In February 2013, the annual Ocala Horse Shows in the Sun (HITS) event nearly came to a halt due to an outbreak of equine herpesvirus (EHV-1) and equine influenza virus (EIV) that impacted a large number of horses. While the initial quarantine was established due to a positive EHV-1 case, it quickly became apparent through PCR testing of nasal swabs that many more horses were impacted by EIV.

The highly pathogenic strain of influenza that impacted these horses was ultimately isolated and identified as Florida ‘13 through the Merck Animal Health Respiratory Biosurveillance Program. Florida ‘13 represents a current, clinically significant strain responsible for a large influenza outbreak in well-vaccinated horses. Influenza 6 - Midwest Veterinary Supply│Spring-Summer Midwest Grazer 2018│www.midwestvet.net/midwestequine outbreaks in well-vaccinated horses are generally indicative of significant antigenic drift and inadequate protection. Phylogenetic analysis of the Florida ‘13 strain confirmed that significant antigenic drift had indeed occurred and that this isolate was significantly different from viruses contained in current vaccines.

The Florida ‘13 isolate is the foundation of the next generation of flu-containing killed virus vaccines from Merck Animal Health.

The new flu-containing vaccines are headlined by the Prestige® brand and now include both clade 1 and clade 2 strains.

• Florida ‘13 Clade 1: Based on a highly pathogenic isolate from the 2013 Ocala, Fla., influenza outbreak that impacted a large number of well-vaccinated horses – exclusively identified and isolated through the Merck Animal Health Biosurveillance Program

• Richmond ‘07 Clade 2: Meets World Organisation for Animal Health (OIE) and American Association of Equine Practitioners (AAEP) guidelines for clade 2 influenza protection

In addition, Kentucky ‘02 will remain part of our flu-containing vaccine line


Beginning with the updated flu-containing killed virus vaccines, Merck Animal Health is introducing the new Prestige® brand family of vaccines. The entire vaccine line (apart from Flu Avert® I.N.) will become part of the Prestige line. This naming system will rollout throughout 2018, along with new packaging.

The Prestige line of vaccines has a long-standing reputation for safety and efficacy, shown to be 98 percent reaction-free in field safety trials.1 This is because every killed virus vaccine includes our proprietary Antigen Purification System™ (APS) and exclusive Havlogen® adjuvant. This renaming signifies to veterinarians and horse owners that every Prestige vaccine they purchase includes these technologies.

- Havlogen stimulates the immune system to produce high, long-lasting levels of protection through the slow release and gradual absorption of antigen. Due to the composition of Havlogen, the vaccine maintains suspension without separation and settling in the vial — resulting in consistency and potency in every dose.

- APS removes unwanted protein and cellular debris from the vaccine antigen. Using this method of filtration purification allows concentration of antigen, while minimizing the presence of extraneous protein and cellular debris that can contribute to vaccine associated adverse events.

More to come as this update unfolds over the course of 2018. For more information on the new line of Prestige® flu-containing vaccines from Merck Animal Health, contact your sales representative or call 800-521-5767.

About the author
Bryant Craig, D.V.M., of Piedmont, Okla., received his doctorate of veterinary medicine from Oklahoma State University. He joined the Merck Animal Health Equine Veterinary Professional Services team in 2013. Prior to joining Merck Animal Health, Dr. Craig owned and operated a private equine veterinary practice in Oklahoma before joining the Lazy E Ranch in Guthrie, Okla. as resident veterinarian in 2005. At the Lazy E, he oversaw the ranch’s reproductive and yearling sales prep services and cared for some of the Quarter Horse racing industry’s premier stallions.

About Merck Animal Health
For more than a century, Merck, a leading global biopharmaceutical company, has been inventing for life, bringing forward medicines and vaccines for many of the world’s most challenging diseases. Merck Animal Health, known as MSD Animal Health outside the United States and Canada, is the global animal health business unit of Merck. Through its commitment to the Science of Healthier Animals™, Merck Animal Health offers veterinarians, farmers, pet owners and governments one of the widest range of veterinary pharmaceuticals, vaccines and health management solutions and services. Merck Animal Health is dedicated to preserving and improving the health, well-being and performance of animals. 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 or connect with us on LinkedIn, Facebook and Twitter at @ MerckAH.