Ask the Infectious Disease Expert

This column, brought to you by Merck Animal Health, features insightfulanswers from leading minds.

What’s the outlook for equine influenza this season, and how can I help my clients understand which horses should get vaccinated?

"Season” is the operative word here. Equine influenza virus (EIV) infects horses throughout the year, but cases ramp up between mid-winter and early spring, according to data from the Equine Respiratory Biosurveillance Program.1

The best way to slow the spread of EIV is to ensure horses are properly vaccinated. To help clients understand what’s proper, start by reinforcing the recommendations outlined in the AAEP risk-based vaccination guidelines.2 Second, explain two key points:

• EIV is mutating through antigenic drift • Not all vaccines are created equal

The Biosurveillance Program data illustrates these points. The program, which our lab conducts in partnership with Merck Animal Health, provides long-term and real-time EIV monitoring. We analyze samples submitted by more than 250 enrolled clinics from across the country to tabulate EIV case numbers and finely dissect the expected and unexpected outcomes of every positive case.

An evolving virus calls for evolving vaccines

In early 2010, we observed a pattern of unexpected outcomes: higher numbers of older, vaccinated horses were testing positive for EIV. This led to the hypothesis that the virus was slowly mutating and escaping vaccine-specific immunological responses. To determine whether this was due to the introduction of foreign EIV strains or to the natural selection of EIV strains (antigenic drift), a researcher in our lab compared the heterogeneity of circulating wildtype EIV strains in the United States with Ohio ’03 (OIE recommended clade 1 influenza strain).

The conclusion: Foreign EIV strains have not been circulating in the United States. Only EIV Florida clade 1 strains have been circulating and have evolved separately from clade 2 strains.3

Your clients may not need all this background, but it is important to make them aware of antigenic drift and its effect on vaccination choices. Tell your clients that EIV evolves like human flu but much slower. EIV vaccine failure occurs when the virus changes at certain immunodominant sites (i.e. sites that are important for the virus to escape the immune response). Therefore, horses should receive an EIV vaccination that has evolved along with the virus to account for the most up-to-date flu strains. Ongoing EIV sequencing through the time of this article reflects that Florida ’13 is representative of current circulating U.S. field strains.4

1 Merck Animal Health and University of California, Davis School of Veterinary Medicine (Nicola Pusterla). Infectious Upper Respiratory Disease Surveillance Program. Ongoing research 2008-present. 2 American Association of Equine Practitioners, Risk-Based Vaccination Guidelines, Equine Influenza. https://aaep.org/guidelines/vaccination-guidelines/risk-based-vaccination-guidelines/equine-influenza, accessed Aug. 31, 2020. 3 Lee K, Pusterla N, Barnum S, Martinez-Lopez B. Is Current Vaccine Failure of Equine Influenza Virus Due to Evolution of Endemic Strains or Introduction of Foreign Strains? AAEP Proceedings. 2019 Vol 65. 4 Merck Animal Health Technical Bulletin, December 2019.

About the Author Dr. Nicola Pusterla, DVM, PhD, DACVIM, DAVDC-Equine is a professor of equine internal medicine and dentistry at the University of California, Davis, School of Veterinary Medicine. His research focus is on equine infectious diseases with an emphasis on molecular epidemiology. Dr. Pusterla also leads the UC Davis Equine Infectious Disease Research Laboratory.

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