upper respiratory diseases in horses

On April 17 Merck Animal Health announced 10 years’ worth of key findings from its ongoing Equine Respiratory Biosurveillance Program.

Recognized as the largest equine infectious respiratory biosurveillance study ever compiled and comprising one of the largest collections of equine influenza isolates in the U.S., the program has provided new information on the major infectious upper respiratory diseases in horses.

Researchers on the ongoing study, which Merck is conducting in partnership with the University of California, Davis (UC Davis), School of Veterinary Medicine, gather data on equine herpesvirus types 1 and 4 (EHV-1 and EHV-4), equine influenza virus (EIV), and Streptococcus equi (S. equi or strangles)—which have been tracked from the study’s inception—along with equine rhinitis A/B viruses (ERAV/ERBV), which were was added in 2012.

Nicola Pusterla, DVM, PhD, Dipl. ACVIM, leads the UC Davis Equine Infectious Disease Research Laboratory where program samples are submitted and analyzed and was integral to the program’s design and implementation.

“When we started this program, we had no idea how much we would gain over this period,” he said. “The study has increased awareness of respiratory pathogens in the veterinary community, provided invaluable epidemiological information pertaining to common and less-characterized respiratory pathogens, and provided sequencing of EIV isolates to monitor how the virus is changing in the field and to evaluate and improve the efficacy of vaccines.”

Tracking an Influenza Uptick

“One of the more notable findings of the program has been the frequency of equine influenza, and our analysis of constantly changing field isolates resulted in the discovery of a clinically relevant new equine influenza strain,” said D. Craig Barnett, DVM, Merck Animal Health director of equine veterinary professional services and program co-founder.

Before the study began, he said veterinarians often reported they weren’t seeing influenza cases too often.

“But throughout the study we’ve been tracking a lot of influenza even in well-vaccinated horses, regardless of age and breed. We continue to see a lot of EIV circulating—it was the most common disease in November 2018 and again in January 2019,” he added.

Barnett said influenza outbreaks in well-vaccinated horses are generally indicative of significant antigenic drift—small changes over time that result in new virus strains the immune system might not recognize—and, therefore, inadequate protection.

“Phylogenetic analysis and sequencing 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,” he added.

A Decade of Data

Researchers have collected more than 8,200 samples since the biosurveillance program began more than 10 years ago. Through December 2018, EHV-4 was the most commonly diagnosed infectious upper-respiratory disease, comprising 33% of all positive samples; EIV and S. equi followed at 28% and 22%, respectively.¹

Other highlights and results from the program include:

  • Five peer-reviewed published papers, as well as 10 abstracts presented at four national and six international conferences;
  • Identifying antigenic drift and isolating a new influenza strain (Florida ’13) from one of the largest collections of EIV isolates ever gathered in the U.S., demonstrating the virus’ high prevalence within the general horse population;
  • Gaining a greater understanding of the demographic and signalment (age, breed, sex, etc.) parameters associated with common upper-respiratory infections in horses, including a recognition that age does not define a horse’s susceptibility to certain pathogens; for example, EIV is no longer considered primarily a young horse disease, and EHV-4 can cause respiratory disease in mature horses as well as in weanlings and yearlings);
  • Finding new insights on strangles, including a high-frequency of S. equi in nontraveling pleasure horses; a higher median age of the horse affected than other upper respiratory diseases (strangles is the most commonly diagnosed upper respiratory disease in horses 6-10 years of age²); and the propensity for coinfection in horses with EHV-4;
  • Learning that EHV-4 is a predominant virus associated with upper airway infection and a major infectious upper respiratory disease threat particularly, but not exclusively, in young horses; and
  • Gaining new insight into the impact of lesser known herpesviruses (EHV-2 and EHV-5), which are often a source of coinfection with other major respiratory pathogens.

“The Merck Animal Health Biosurveillance Program has been invaluable in improving our diagnostic capability,” said Christine Cocquyt, DVM, Dipl. ACVIM, internal medicine associate with Tennessee Equine Hospital, a longtime program participant in the program. “We have been able to quickly and accurately assess potential outbreaks of disease, which has allowed us to quarantine and implement appropriate biosecurity programs when necessary.”

Barnett added, “The program has changed the way we look at equine respiratory pathogens and helped the industry evolve the way it identifies and manages these costly diseases, not only through timely and accurate diagnostic services but also with improved vaccination solutions and disease management measures, bringing the program full-circle to our original vision.”


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.
²Surveillance programme for important equine infectious respiratory pathogens in the USA. N. Pusterla, P.H. Kass, S. Mapes, C. Johnson, D.C. Barnett, W. Vaala, et al. Vet Rec 2011.