Equine Practice: Seasonality, Age, Stress Play Roles In Susceptibility To IURD Infections

The stress of training could lead to an increased potential for a respiratory outbreak.

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A cold winter followed by rainy spring weather can create the perfect storm for an outbreak of infectious upper respiratory disease, or IURD.

Many horse owners will be moving young horses to indoor facilities for training and sale preparations. Subsequently, horses that are already more susceptible to IURDs due to their age are exposed to environments with a lower air quality and more irritants, such as dust and ammonia. In addition, the stress of training could lead to an increased potential for a respiratory outbreak.

The Latest Research

A recent study at the University of California, Davis, funded by Merck Animal Health of Summit, N.J., revealed that this perfect storm plays a role in the risk of an IURD outbreak. In this study, veterinarians throughout the U.S. were enrolled in a voluntary surveillance program.

They were asked to collect blood and nasal secretions from their equine patients that presented with signs of acute IURD and/or acute onset of neurologic disease. In all, 761 horses, mules and donkeys were enrolled and tested via real-time PCR testing for the four major IURD pathogens: equine herpesvirus-1 (EHV-1); equine herpesvirus-4 (EHV-4); equine Influenza virus (EIV) and Streptococcus equi subspecies equi.

“The initial results of the surveillance study were very helpful to better understand the prevalence of infectious upper respiratory diseases. But [the results] also helped the participating veterinarians provide a more timely and accurate diagnosis for their patients, which can be especially helpful during an IURD outbreak,” said Nicola Pusterla, DVM, Dipl. ACVIM, of the UC Davis Department of Medicine and Epidemiology. He is lead investigator and author on the biosurveillance study.


During the study’s 24-month duration, most of the IURD cases were seen during the fall, winter and spring. Of the four pathogens, the one detected most often was EHV-4, primarily seen in fall and winter. EIV was seen second most often, mainly during the winter and spring. S. equi, the third most-detected pathogen, was primarily seen during the winter and spring, and EHV-1 was mainly seen in fall and spring.

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“While the seasonality of IURDs has been reported in other studies, this study reminds veterinarians to advise their clients on the importance of biosecurity and appropriate vaccination recommendations during the fall, winter and spring months,” Dr. Pusterla said.

Age Matters

Much like children, younger horses are more susceptible to contracting a respiratory disease and should be considered at moderate to high risk for IURDs and be vaccinated and managed accordingly.

Of the 761 horses enrolled in the study, 385 (50.6 percent) were younger than 5 years old, with 130 (17.1 percent) less than 1 year old. The highest frequency of EHV-4 was seen in horses 1 year of age or less. The second-highest frequency of EIV was seen in horses 1 to 5 years of age.

One vaccine option veterinarians should consider for all horses at higher risk for contracting equine influenza, including older foals, yearlings and young horses, is Flu Avert I.N. This modified-live intranasal EIV vaccine by Merck Animal Health is the only influenza vaccine that requires only one dose for primary immunization.

Challenge studies conducted by HGG Townsend support an onset of immunity of five to seven days in naïve horses1. Considering cell-mediated and serologic anamnestic responses as well as mucosal responses, the onset of immunity with this vaccine in primed horses would be expected to be even sooner.

This evidence provides for the use of this vaccine in an outbreak situation to help reduce occurrence, severity and duration of disease. The vaccine can also be especially useful for the older foal’s first EIV vaccine because all other EIV vaccines require a two-dose series over a 4- to 6-week period to achieve optimum immune response. 

Additionally, EIV is one of the seven diseases included in Merck’s Equine Vaccine Performance Guarantee, which reimburses reasonable diagnostic and treatment costs up to $5,500, for a specific lack of vaccine efficacy to any properly vaccinated horse.

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Stress is a Factor

While stress can be caused by a multitude of factors in the horse, one of the most significant stressors is transportation. In the initial study, recent transportation history was available for 667 horses and revealed that 192 horses (28.8 percent) had been transported seven to 10 days before the onset of clinical signs.

It is important to remind clients about the immunosuppressive effect travel can have on their horses. Trailers should be well-ventilated and long-distance transportation should be broken into segments to help reduce the horse’s chance of contracting an IURD as a result of travel.

Unknown Pathogens

Of the 761 animals enrolled in the biosurveillance study, 201 (26.4 percent) tested PCR-positive for one or more of the four major IURD pathogens. However, for the remaining 560 horses the type of infection causing clinical signs of IURD was not determined.

“The first 24 months of data from the biosurveillance program have provided great insight to the frequency and prevalence of the four major IURD pathogens,” Pusterla said.

Ongoing Research

The Merck Animal Health biosurveillance study is in its fourth year. Since the initial data were published this past June, Merck and UC Davis have received data for more than 1,300 additional horses, mules and donkeys.

“The updated data have been relatively consistent with the data initially published,” Pusterla said. “However, the key take-away is that we are still seeing cases of all four primary respiratory pathogens. Veterinarians should encourage their clients to remain vigilant on their horses’ vaccinations, as well as biosecurity measures.”

Veterinarians interested in enrolling in the voluntary biosurveillance program or receiving the complete data may call 1-800-521-5767. 

Dr. Barnett is a senior equine technical services veterinarian for Merck Animal Health.


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