Ongoing Merck equine study reveals 10 years of data

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Merck Animal Health's Equine Respiratory Biosurveillance Program has reached the 10-year mark. The findings, reported biweekly, focus on new information regarding major infectious respiratory disease threats.

The program has collected more than 8,200 samples in the last 10 years, making it the largest equine infectious respiratory biosurveillance study and one of the largest collections of equine influenza isolates in the United States.

The study monitors equine herpesvirus types 1 and 4, equine influenza virus, Streptococcus equi or strangles, and equine rhinitis A/B viruses. The monitoring is managed by Merck in partnership with the University of California-Davis.

"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 efficacy of vaccines," said Dr. Nicola Pusterla, professor of medicine and epidemiology at the University of California-Davis and project lead for the Equine Infectious Disease Research Laboratory, where samples are submitted and analyzed, in an April 17 press release.

The program's findings have been published in five papers and 10 abstracts. Some of its key findings over the last 10 years include the following:

  • 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 nontraveling 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), and the propensity for coinfection with EHV-4.
  • The finding 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.
  • New appreciation of the impact of lesser-known herpesviruses—EHV-2 and EHV-5—which are often a source of coinfection with other major respiratory pathogens.

"One of the most notable findings of the program has been the frequency of equine influenza," said Dr. D. Craig Barnett, program co-founder and Merck Animal Health director of equine veterinary professional services, in the press release. "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."

The findings from the program can be found at at

Bar chart: Disease Incidence 2008-2018 (As a percentage of total positive samples) - Source: Merck Animal Health

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USDA releases equine industry study results (March 01, 2017)