Important respiratory disease findings revealed in surveillance program

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A road advisory during the 2007 equine influenza outbreak in Australia. Photo: File
A road advisory during the 2007 equine influenza outbreak in Australia. Photo: File

Ten years of monitoring respiratory diseases in horses reveals the ongoing challenges presented by equine influenza.

The ongoing Equine Respiratory Biosurveillance Program run by Merck Animal Health has tracked an uptick in equine 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 Craig Barnett, Merck Animal Health’s 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 equine influenza virus circulating – it was the most common disease in November 2018 and again in January 2019.”

The biosurveillance program led to the isolation and identification of a new influenza strain, Florida ’13, which resulted in Merck updating its Prestige vaccine line.

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

“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 continued: “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.

It is now recognised as the largest equine infectious respiratory biosurveillance study ever compiled. It now comprises 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 have shaped industry management and preventative strategies.

The work, carried out in partnership with the University of California, Davis, has monitored equine herpesvirus types 1 and 4 (EHV-1, EHV-4), Streptococcus equi (which causes strangles) and the equine flu virus since the start of the program. Equine rhinitis A/B viruses (ERAV/ERBV) were added in 2012.

Nicola Pusterla, who is integral to the surveillance program’s design and implementation, and leads the UC Davis Equine Infectious Disease Research Laboratory where program samples are submitted for analysis, says the work 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,” Pusterla 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 equine influenza virus isolates to monitor how the virus is changing in the field and to evaluate and improve the efficacy of vaccines.”

More than 8200 samples have been collected over more than a decade.

Up until December last year, EHV-4 was the most commonly diagnosed infectious upper respiratory disease, comprising 33 percent of all positive samples, followed closely by the equine flu virus at 28 percent and S. equi at 22%.

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 US, 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); 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.

Christine Cocquyt, an internal medicine associate with the Tennessee Equine Hospital and a longtime participant in the program, said the biosurveillance initiative has been invaluable in improving the hospital’s diagnostic capability.

“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 program are reported bi-weekly through the Equine Disease Communication Center.

In addition, Merck Animal Health publishes a bi-annual newsletter highlighting cumulative and six-month disease trends.

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