Major study highlights nature of respiratory infections in US horses

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Commonly recognized pathogens were responsible for just under a quarter of cases among 10,296 equids in the American study.
Commonly recognized pathogens were responsible for just under a quarter of cases among 10,296 equids in the American study. Image by Hanne Hasu

A 13-year American study of more than 10,000 horses affected by acute onset of fever or respiratory problems linked 23.8% of the cases to common viral and bacterial pathogens.

The cause of clinical signs of upper respiratory tract infections in the remaining 76.2% of animals was not further investigated.

Nicola Pusterla and his fellow researchers, writing in the journal Pathogens, said that while their study focused on well-known pathogens, unknown or poorly investigated viruses could have been responsible for respiratory disease in some of the cases.

Recent studies, they noted, have reported on novel equine viruses (picornavirus, protoparvovirus and copiparvovirus) being detected in horses with respiratory disease. The study team said there is a great need to investigate the nature of novel viruses and their association with clinical disease.

The research, backed by Merck Animal Health, provides important insights into the nature of respiratory-related infections across the United States.

Infectious respiratory diseases are considered one of the most prevalent medical problems in young performance horses, and a common reason for their temporary removal from training and competitions.

“With the increase in national and international movement of equids for breeding and sporting events, the likelihood of respiratory pathogen transmission has increased, as evidenced by recent outbreaks worldwide,” the researchers said.

Investigating risk factors for developing upper respiratory tract disease, and determining factors for each respiratory pathogen, can shed light on the nature of the infections and help implement practices to control their spread.

A voluntary US biosurveillance program was established in 2008 to determine the shedding frequency and prevalence factors for common respiratory pathogens associated with acute respiratory signs in equids.

Over 13 years, a total of 10,296 equids affected by acute onset of fever and/or respiratory problems were enrolled. Nasal secretions taken at the time of their illness were analyzed using molecular-based quantitative polymerase chain reaction (qPCR) testing.

The researchers checked for the presence of equine influenza virus, equine herpesvirus-1, equine herpesvirus-4, equine rhinitis A and B viruses, and Streptococcus equi subspecies equi (S. equi), which causes strangles.

Single infections with respiratory pathogens were detected in 21.1% of the submissions, with equine influenza virus (6.8%) and equine herpesvirus-4 (6.6%) being the two most commons viruses, followed by S. equi (4.7%), equine rhinitis viruses (2.3%), and equine herpesvirus-1 (0.7%).

Multiple pathogens were detected in a further 274 horses (2.7%), while none of the major respiratory pathogens were detected in 7836 of the unwell horses (76.2%).

The findings showed that the important infectious respiratory pathogens were found in a total of 23.8% of cases, normally alone but occasionally with other pathogens.

Most of the respiratory pathogens were detected during the colder months, reflecting seasonal differences in population dynamics, they said.

“In general, viral infections were associated with young performance horses, while infections with S. equi were seen in older pleasure horses.

Clinical signs were fairly consistent between viral and bacterial respiratory infections, they wrote. However, coughing was a clinical hallmark of equine flu.

The most frequently reported clinical signs in the entire study population were, in decreasing frequency, fever (73.0%), nasal discharge (67.8%), lethargy (65.3%), poor appetite (53.8%), and coughing (43.5%).

About a third of all cases had a reported vaccination history against equine flu and/or equine herpesvirus-1 and 4, while vaccination against S. equi was seldom reported.

The researchers said they considered their study unique as it included a very large equine population across 44 US states, focusing on a panel of well-characterized respiratory pathogens.

“While upper respiratory tract infections can affect horses of any age, breed, sex, and use, the study data showed that young performance and pleasure horses, and Quarter horses were over-represented as index cases.”

Quarter horses, they noted, represent the highest percentage of resident horses by breed in the US.

“Further, young age and population dynamics, such as the congregation of large horse groups during equestrian events, are well-recognized factors predisposing the transmission of respiratory pathogens.”

They noted that equine veterinarians often struggle to characterize respiratory infections into viral or bacterial – an important distinction as the treatments differ.

The study data showed only subtle differences between the two groups, mostly regarding demographics, and clinical signs. Viral infections were more common in performance horses aged four or younger, while S. equi infections were more common in pleasure horses aged five or more.

The differences in demographics and use likely relate to susceptibility and spread of the different pathogen groups.

Horses infected with S. equi displayed a greater frequency of lethargy and poor appetite compared to viral infections, which may relate to the magnitude of systemic inflammation triggered by each of these two pathogen groups.

A recent study showed that the inflammatory response measured via serum levels of amyloid A was often stronger in horses with S. equi compared to horses infected with equine influenza virus and the equine herpesvirus.

From a practical standpoint, the findings do not offer conclusive information aimed at differentiating viral from bacterial respiratory infection, the study team said. Laboratory diagnostic testing is needed to help differentiate between the two infectious groups.

The study team comprised Pusterla and Samantha Barnum, with the School of Veterinary Medicine at the University of California, Davis; Kaitlyn James, with Massachusetts General Hospital in Boston; and Fairfield Bain, Craig Barnett, Duane Chappell, Earl Gaughan, Bryant Craig, Chrissie Schneider and Wendy Vaala, all with Merck Animal Health in Madison, New Jersey.

Pusterla, N.; James, K.; Barnum, S.; Bain, F.; Barnett, D.C.; Chappell, D.; Gaughan, E.; Craig, B.; Schneider, C.; Vaala, W. Frequency of Detection and Prevalence Factors Associated with Common Respiratory Pathogens in Equids with Acute Onset of Fever and/or Respiratory Signs (2008–2021). Pathogens 2022, 11, 759. https://doi.org/10.3390/pathogens11070759

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

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