Low level of respiratory virus shedding found among sporthorses at a long summer show

The shedding of common respiratory viruses by a selection of 20 sporthorses attending a multi-week show was found to be low, researchers report.

Nicola Pusterla and his fellow researchers, writing in the journal Viruses, noted that with increased concerns about respiratory outbreaks, especially equine herpesvirus-1 (EHV-1), many equine showgrounds are struggling to institute compliant protocols with the goal to reduce the transmission risk.

Vaccination requirements, health certificates, biosecurity protocols, daily physical monitoring and pre-show testing are all measures that have been used in the past to reduce the risk of disease transmission at equestrian events.

“Unfortunately, there is very little contemporary information on the detection frequency of circulating respiratory pathogens in healthy adult horses,” the study team noted.

Various studies have shown that the detection frequency of selected respiratory pathogens ranges from 0 to 4% depending on the time of the year, and age and use of the population tested.

The research team set out to investigate the detection frequency of selected respiratory pathogens in nasal secretions and environmental stall samples of sport horses attending a multi-week equestrian event in Michigan during the summer.

Six out of fifteen tents were randomly selected for the study with approximately 20 horse/stall pairs from within them being sampled on a weekly basis over 11 weeks.

Following weekly collection for a total of 11 weeks, all samples were tested for the presence of common respiratory pathogens – equine influenza virus (EIV), EHV-1, EHV-4, equine rhinitis A virus (ERAV), equine rhinitis B virus (ERBV), and Streptococcus equi subspecies equi (S. equi)

A total of 682 nasal swabs and 1288 environmental stall sponges were collected throughout the study period.

In all, 19 of the nasal swabs (2.8%) and 28 of the environmental stall sponges (2.2%) tested positive using qPCR molecular-based testing.

ERBV was the most common respiratory virus detected (it was found on 17 nasal swabs and 28 stall sponges), followed by EHV-4 (1 nasal swab) and S. equi (1 nasal swab).

The other viruses were not detected in any of the study horses or stalls. Only one horse and one stall tested qPCR-positive for ERBV on two consecutive weeks. All the other positive sample results were related to individual time points.

Furthermore, only one horse/stall pair tested qPCR-positive for ERBV at a single time point.

“In conclusion, the study results showed that in a selected population of sport horses attending a multi-week equestrian event during the summer, the shedding frequency of respiratory viruses was low and primarily restricted to ERBV with little evidence of active transmission and environmental contamination.”

The authors noted that the majority of outbreaks associated with respiratory pathogens in performance horses attending large equestrian events have often been reported during the colder months.

“Outbreaks with respiratory pathogens seldom occur during the summer months, an observation that may relate to environmental factors (high ambient temperature and low humidity) reducing pathogen viability and to a low frequency of respiratory viruses and bacteria circulating amongst horses.”

The study team said their findings support previous observations that respiratory pathogens circulate at a low rate amongst sport horses during the summer months, making an occurrence of outbreaks less likely.

“However, one needs to keep in mind that the present results are specific to the studied horse population and reflect environmental factors specific to the study period.”

It is also important to highlight, they said, that while nasal swabs and environmental samples allow an assessment of the presence of respiratory pathogens circulating amongst at-risk horses, such a strategy is not intended to replace good biosecurity protocols.

The study team comprised Pusterla, Samantha Barnum and Kaila Lawton, with the University of California, Davis; Madalyn Kalscheur, Duncan Peters, Lori Bidwell and Sara Holtz, with East–West Equine Sports Medicine in Lexington, Kentucky; Matt Morrissey, with Morrissey Management Group in Wellington, Florida; and Stephen Schumacher, with the US Equestrian Federation.

Pusterla, N.; Kalscheur, M.; Peters, D.; Bidwell, L.; Holtz, S.; Barnum, S.; Lawton, K.; Morrissey, M.; Schumacher, S. Investigation of the Frequency of Detection of Common Respiratory Pathogens in Nasal Secretions and Environment of Healthy Sport Horses Attending a Multi-Week Show Event during the Summer Months. Viruses 2023, 15, 1225. https://doi.org/10.3390/v15061225

The study, published under a Creative Commons License, can be read here.