Lesser-known respiratory virus in horses on the rise in the US

A lesser-known respiratory pathogen in horses is being detected with greater frequency in the United States under a biosurveillance program, researchers report.

Chrissie Schneider and her colleagues, reporting in the journal Pathogens, have described the rise of equine rhinitis B virus (ERBV) under a voluntary upper respiratory biosurveillance program that has been running in the US since 2012.

ERBV has been detected worldwide in horses over the past 50 years. There are currently three known serotypes, designated 1, 2 and 3.

Previous studies have reported ERBV detection rates of 1.5% to 30.4% using either viral isolation or molecular-based methods, and up to 86% using serology.

The clinical relevance of ERBV has yet to be fully determined, the study team said, but it has been detected in horses with clinical signs of acute respiratory disease, including fever, nasal discharge, poor appetite, cough, enlarged lymph nodes, and limb swelling.

Previous investigations into ERBV detection reported that it was frequently found along with other viral and bacterial pathogens as coinfections.

The presence of the virus as part of a coinfection may contribute to the disease process by lengthening the disease course and/or increasing the severity of clinical signs.

The study team, with Merck Animal Health and the veterinary school at the University of California, Davis, used data from the surveillance program to delve into the demographics, and observed clinical signs and coinfection status of horses with respiratory illness who tested positive for the virus via quantitative polymerase chain reaction (qPCR).

The program received 8684 nasal swab submissions between 2012 and 2023. During this time frame, as many as 324 clinics were enrolled in the program across 45 states.

The nasal swabs were submitted for qPCR testing for six common upper respiratory pathogens: Streptococcus equi subspecies equi (S. equi), equine influenza virus (EIV), equine herpesvirus type 1 (EHV-1), equine herpesvirus type 4 (EHV-4), equine rhinitis A virus (ERAV), and ERBV.

The overall ERBV qPCR-positivity rate was 5.08%, representing 441 of the 8684 swab submissions.

ERBV was detected as a single pathogen in 291 of the 441 cases (65.99%), and was detected as a coinfection with at least one other respiratory pathogen in 150 of the 441 cases (34.01%).

Horses less than a year of age with an acute onset of fever and respiratory signs, as well as horses used for competition, were found to be more likely to test positive for ERBV.

“Horses with ERBV may present with fever, nasal discharge, ocular discharge, and/or cough,” the study team said.

Coinfection is common with such infections, with S. equi, EHV-4 and EIV proving to be the most common pathogens coinfected with ERBV.

The researchers, discussing their findings, said the increasing frequency of ERBV detection identified over the study period indicates that clinicians are more likely to be presented with an ERBV clinically infected case than in the past.

“This fact makes it even more important to characterize ERBV infection in horses and help practitioners interpret ERBV diagnostics.”

ERBV infections were found to be less common in summer compared to other times of the year. “The reason for this is speculative and may relate to the age, husbandry, and use of sampled horses.”

“While there were ERBV cases in every age category in this study, horses less than one year of age were over-represented. This may be due to young horses’ immature immune systems.

“Perhaps these young horses show more obvious clinical signs the first time their immune system is exposed to ERBV, leading to an examination and diagnostic testing.”

The authors said the greater likelihood of infection in competition horses may be due to increased exposure to ERBV, along with other respiratory pathogens, when mixing with horses from other farms at events.

Ponies tended to have a lower frequency of ERBV-positive samples. “We believe this is not necessarily due to genetics but potentially a factor of use distribution and management strategies.”

Historically, the clinical significance of an ERBV qPCR-positive result has remained poorly characterized, the study team said. “Therefore, clinicians presented with a horse displaying acute signs of respiratory disease may choose to ignore an ERBV qPCR-positive result, concluding that the detection of ERBV in their case isn’t clinically relevant.

“The findings of the present study do not support that conclusion as multiple clinical signs were associated with an ERBV qPCR-positive nasal swab.”

Horses with ERBV infection, as a sole pathogen or as part of a coinfection, displayed fever, nasal discharge, a discharge from the eyes, and a cough. Eye discharges may be a clinical sign of respiratory disease that is sometimes overlooked by practitioners, they said.

The fact that ERBV is often part of a coinfection has been reported previously, they noted. “It has been hypothesized that, as part of a coinfection, ERBV may contribute to worsened severity of disease.

“This theory was not supported by the results of the present study. There was not a significant difference in the severity of any of the reported clinical signs between ERBV qPCR-positive–sole pathogen cases and ERBV qPCR-positive–coinfection cases.”

The researchers said there is much left to learn regarding ERBV and its role in equine health. “Future studies could investigate the length of disease course, persistence of ERBV infection, tissues infected by ERBV, reservoirs of ERBV in the equine population, and methods of prevention.

“There is more to learn regarding the three known serotypes of ERBV as well.” Serotypes were not determined in the study, they said.

The virulence, prevalence, and clinical role of the ERBV serotypes have yet to be fully detailed.

The authors said their study provides important information regarding the clinical relevance of ERBV in the horse and begins investigating the impact of coinfection on clinical disease.

“In conclusion, ERBV frequency is increasing overall, and ERBV plays a clinically relevant role in equine respiratory disease.”

The study team comprised Schneider, Kaitlyn James, Bryant Craig, Duane Chappell, Wendy Vaala, Philip van Harreveld and Cara Wright, all with Merck Animal Health in the US; and Samantha Barnum and Nicola Pusterla, with the Department of Medicine and Epidemiology, part of the School of Veterinary Medicine at the University of California, Davis.

Schneider, C.; James, K.; Craig, B.W.; Chappell, D.E.; Vaala, W.; van Harreveld, P.D.; Wright, C.A.; Barnum, S.; Pusterla, N. Characterization of Equine Rhinitis B Virus Infection in Clinically Ill Horses in the United States during the Period 2012–2023. Pathogens 2023, 12, 1324. https://doi.org/10.3390/pathogens12111324

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