Key Respiratory Findings Released From Merck Biosurveillance Research

An ongoing Merck study revealed important respiratory disease findings from 10 years of research, including notable changes in influenza.

Strangles was the second-most commonly diagnosed infectious upper respiratory disease of horses in this Merck study. Amy Dragoo

Merck Animal Health announced 10 years 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 United States, the program has provided new information on the major infectious respiratory disease threats that has shaped industry management and preventative strategies. The comprehensive, ongoing national surveillance study managed by Merck Animal Health in partnership with University of California, Davis (UC Davis), monitors equine herpesvirus types 1 and 4 (EHV-1, EHV-4), equine influenza virus (EIV) and Streptococcus equi (S. equi or strangles), which have been tracked from the inception of the program, and equine rhinitis A/B viruses (ERAV/ERBV), which was added in 2012.

Nicola Pusterla, DVM, DACVIM, who was integral to the Respiratory Biosurveillance Program’s design and implementation and leads the UC Davis Equine Infectious Disease Research Laboratory where program samples are submitted and analyzed, said the program is a testament to the value of pursuing diagnostic testing and disseminating that information.

“When we started this program, we had no idea how much we would gain over this period,” said Pusterla. “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. What Merck Animal Health has done is and will remain an unmatched service to the entire U.S. equine community.”

Tracking an Uptick in Influenza

“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. “Prior to implementation of this study, we commonly heard from veterinarians that they weren’t seeing much influenza in horses. 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.”

In 2018, Merck Animal Health launched the updated PRESTIGE® vaccine line containing a new influenza strain, Florida ’13, which was identified and isolated through the Equine Respiratory Biosurveillance Program.

“Influenza outbreaks in well-vaccinated horses are generally indicative of significant antigenic drift and inadequate protection,” added Barnett. “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.”

Findings from a Decade of Data

More than 8,200 samples have been collected 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 percent of all positive samples, followed closely by EIV at 28 percent and S. equi at 22%. 1 

Program highlights include:

  • Five peer-reviewed published papers, as well as 10 abstracts presented at four national and six international conferences
  • Identification of important antigenic drift and isolation of a new influenza strain (Florida ’13). The study houses one of the largest collections of equine influenza isolates ever gathered in the U.S., demonstrating the high prevalence of EIV within the general horse population
  • Greater understanding of the demographic and signalment parameters associated with common upper respiratory disease infections in horses, including recognition that age does not define 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
  • New insights on strangles, including a high-frequency of S. equi in non-traveling pleasure horses; higher median age than in horses with other upper respiratory diseases (strangles is the most commonly diagnosed upper respiratory disease in horses 6-10 years of age 2); and the propensity for co-infection with EHV-4
  • 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
  • New appreciation of the impact of lesser known herpesviruses—EHV-2 and EHV-5, which are often a source of co-infection with other major respiratory pathogens

“The Merck Animal Health Biosurveillance Program has been invaluable in improving our diagnostic capability,” said Christine Cocquyt, DVM, DACVIM, and internal medicine associate with Tennessee Equine Hospital and a longtime 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.”

Findings from the Merck Animal Health Equine Respiratory Biosurveillance Program are being reported on a bi-weekly basis through the Equine Disease Communication Center (EDCC) at In addition, Merck Animal Health publishes a bi-annual newsletter highlighting cumulative and six-month disease trends.

“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,” said Barnett.

For more information on the Merck Animal Health Equine Respiratory Biosurveillance Program, talk to your Merck Animal Health sales representative or visit

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, a division of Merck & Co., Inc., Kenilworth, N.J., USA, 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 ranges of veterinary pharmaceuticals, vaccines and health management solutions and services as well as an extensive suite of digitally connected 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 or connect with us on LinkedIn, Facebook, and Twitter at @MerckAH.


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. Surveillance 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.

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