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Keeping you at the forefront of
infectious upper respiratory disease

Knowledge is Power

During an acute outbreak of infectious upper respiratory disease, an accurate and timely diagnosis is critical to ensure proper treatment of affected horses. It is also extremely important in providing guidance regarding vaccination, management, and biosecurity.

The Merck Animal Health Equine Biosurveillance Program offers veterinarians tools to sample horses with clinical signs of upper respiratory disease, so they can provide the most accurate and timely diagnosis possible to their clients.

Equine veterinarian examining horse

About the Merck Animal Health Equine Biosurveillance Program

Since 2008. Merck Animal Health in collaboration with the University of California, Davis School of Veterinary Medicine (UC Davis) has been conducting a voluntary longitudinal study of the prevalence and epidemiology of relevant viral and bacterial respiratory pathogens in horses.

More than 10,000 samples from U.S. equids of numerous ages, genders, and breeds presenting with fever and signs of acute upper respiratory disease and/or acute neurological disease have been collected since the study began. Samples are tested via quantitative PCR at UC Davis for six infectious upper respiratory disease pathogens:

  • Equine herpesvirus types 1 and 4 (EHV-1, EHV-4)
  • Equine influenza virus (EIV)
  • Streptococcus equi subspecies equi (Strangles)
  • Equine rhinitis A/B viruses (ERAV/ERBV)

The results are returned within 24 hours of receipt, allowing participating veterinarians to:

  • Provide a timely and accurate diagnosis for clients
  • Prescribe optimal treatment for horses with infectious upper respiratory disease
  • Make recommendations about vaccination, management, and biosecurity during an infectious respiratory disease outbreak, including the potential need for quarantine
  • Evaluate the efficacy of current vaccination protocols

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Regular Reports to Keep You Informed

The Merck Animal Health Equine Respiratory Update is a biannual newsletter that keeps the equine industry up-to-date on the latest trends and historical information from the Equine Biosurveillance Program. The report also features technical advice, case management advice, and support from the Merck Equine Veterinary Professional Services team and UC Davis.

Access current and past issues of the Merck Animal Health Equine Respiratory Update below.

In addition, the Equine Disease Communication Center (EDCC) issues a biweekly report of findings from the Merck Animal Health Equine Respiratory Biosurveillance Program. Check out the EDCC News page at

Equine Respiratory Update Current and Past Issues

Merck Animal Health Equine Respiratory Update issue 9, spring 2023

Newly Published Data on Epidemiology of Streptococcus equi subsp. equi

Spring 2023: In the last issue (Fall 2022) of this newsletter, a rising trend in S. equi cases was reported. That trajectory has continued through the most recent 6-month reporting period.

Merck Animal Health Equine Respiratory Update issue 8, winter 2022

Research Published from 13 Years of the Equine Respiratory Biosurveillance Program

Winter 2022: Equine influenza and equine herpesvirus type 4 are the two most prevalent viruses

Merck Animal Health Equine Respiratory Update issue 8, spring 2022

A Case Study (or two) on Strangles

Spring 2022: Two practitioners share their approach to managing an Streptococcus equi diagnosis


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Key Program Findings and Management Tips

To date, the study has revealed numerous insights with regard to disease trends and epidemiology, and has also delivered a new highly relevant Clade 1 influenza vaccine strain, Florida ’13.1

EHV-4 and EIV are the most prevalent infectious upper respiratory pathogens.

EHV-1 tends to grab the headlines because of the neurologic form of the disease, but EHV-4 and EIV are responsible for the largest portion of positive tests.

Florida ‘13

Identified through the Equine Biosurveillance Program, Florida ’13 is the most current Clade 1 strain available in U.S. EIV inactivated vaccines.
Learn more here.

Biosecurity plus vaccination is best.

Vaccination alone cannot provide complete disease protection. Particularly for highly contagious diseases like equine herpesvirus, influenza and strangles, sound biosecurity practices will provide a broader net of disease protection.

Age does not preclude infectious disease.

The study has debunked the long-held belief that many infectious upper respiratory diseases such as influenza are only prevalent in young horses. The latest changes in the equine influenza virus (antigenic drift) seem to have made adult horses more susceptible to the disease.

Travel is a significant risk factor.

Both the traveling horse and those exposed to traveling horses are at an increased risk for respiratory disease.

A definite seasonality to disease outbreaks is present.

  • Generally, respiratory disease outbreaks peak in spring and fall, and they valley in the heat of summer.
  • EHV-4 appears to be most prevalent in the fall months, in contrast to the other primary respiratory pathogens that are more prevalent in the winter and spring months.
  • Equine flu season is from December to April.

1. Merck Animal Health and University of California, Davis (Nicola Pusteria). Infectious Upper Respiratory Disease Surveillance Program. Ongoing research 2008-present. 2. Vaala, W. et al. Prevalence Factors Associated with Equine Influenza Virus Infection in Equids with Upper Respiratory Tract Infection from 2008 to 2019. AAEP Proceedings, Vol. 65, 2019.

Client Education Infographics

The ABC's and D's of Biosecurity

The ABC and D’s of Biosecurity

In addition to vaccination, these practical equine biosecurity measures are critical to managing highly contagious diseases like equine herpesvirus, influenza and strangles.

Equine Influenza Quick Facts

Equine Influenza

An overview of clinical signs, prevention, and treatment of this highly contagious and common disease, plus the importance of updated vaccine strains.

Equine Hepevirus Type 4 (EHV-4) Quick Facts

Equine Herpesvirus type 4 (EHV-4)

Important facts on EHV-4. Including clinical signs, prevention, and diagnosis.

Equine Herpesvirus Myeloencephalopathy (EHM) quick facts

Equine Herpesvirus Myeloencephalopathy

Unique risk factors of the neurologic form of EHV-1 and what to expect upon diagnosis.

Strangles quick facts infographics


Get the facts on Streptococcus equi, including clinical signs, diagnosis, treatment, and recovery.

Equine Herpesvirus Type 1 (EHV-1) Respiratory and Reproductive Disease Quick Facts

Equine Herpesvirus type 1 (EHV-1)

Learn more about management and prevention of EHV-1 respiratory and reproductive disease.


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Equine veterinarian examining horse with stethoscope

How You Can Learn More

Talk to your Merck Animal Health sales representative for more information about the Merck Animal Health Equine Biosurveillance Program, or call (800) 521-5767. Once enrolled, you’ll receive collection materials, shipping labels, submission forms, sample collection instructions. After obtaining nasal swabs and blood from suspect cases, samples are shipped to UC Davis, with results provided within 24 hours of receipt. Shipping and laboratory diagnostic charges as well as collection material costs are supported by Merck Animal Health.

If you have questions about the program, call the Merck Animal Health Equine Professional Services Team as (866) 349-3497 or email