Respiratory health directly affects how well a horse can perform, and viral infections in the lungs or airways can seriously interfere with a sport horse’s abilities. Scientists have long known that acute respiratory infections reduce equine performance, but the impact of subclinical (inapparent) viral infections remains less clear, said Thibault Frippiat, DVM, Dipl. ECVSMR, of the University of Liege’s Faculty of Veterinary Medicine, in Belgium. In a recent literature review he and his coauthors, including Dominique-Marie Votion, DVM, Dipl. ECVSMR, evaluated the prevalence of respiratory viruses, how they affected horses clinically, and management strategies.

The Most Common Respiratory Viruses Found in Sport Horses

The research team pointed out the high prevalence of respiratory viruses in both clinically healthy horses and horses exhibiting clinical signs of respiratory illness. When analyzing data from studies that collectively included over 45,000 horses, they found that:

• Equid gammaherpesviruses (EHV-2 and EHV-5) were extremely prevalent, detected in about one-third of all horses, regardless of health status.
• Equid alphaherpesvirus-4 (EHV-4) and equine influenza virus (EIV) were significantly more common in horses with acute respiratory disease than clinically healthy animals.
• Equine adenovirus 1 (EAdV-1), equine arteritis virus (EAV), and equine rhinitis A virus (ERAV) were rarely detected in either healthy horses or those showing clinical signs of illness.
• Equine rhinitis B virus (ERBV) was frequently found in horses displaying respiratory illness and might contribute to coinfections with other pathogens.

So while certain viruses lead to clinical disease, others might cause only subclinical infections. These silent infections could contribute to airway inflammation and reduced respiratory efficiency in sport horses, the researchers said, potentially impairing performance without manifesting as signs of illness.

What Does This Mean for Veterinarians and Horse Owners?

For practitioners and horse owners, these results highlight the importance of biosecurity efforts at equine events. Events pose a heightened risk for viral transmission because large groups of horses from different farms congregate in one location, said Frippiat. For certain diseases vaccination remains a cornerstone of prevention, helping mitigate clinical severity and viral spread.

Horse owners should remember that even without visible signs of illness, certain viruses can still hinder a horse’s ability to train or compete. And when horses do show clinical signs, such as fever, coughing, nasal discharge, or other signs of illness such as poor performance, owners need to consult their veterinarians, Frippiat added. Subtle signs of poor performance due to respiratory disease might include tiring easily or having poor results in the show ring (i.e., a show jumper knocking down more rails than normal).

Future Respiratory Research in Horses

While the researchers consolidated existing knowledge, they also highlighted gaps that call for further investigation, said Frippiat. He noted that researchers should further examine how subclinical viral infections affect performance limitations in sport horses.

Advancements in rapid on-site diagnostic tools, such as PCR-based systems, will enhance early viral detection and containment efforts. These systems continue to emerge, he said, but veterinarians need to validate them scientifically before putting them into practice.

Take-Home Message

Respiratory viruses pose ongoing challenges for equine athletes, with potential effects ranging from acute illness to subtle declines in performance. Through strategic vaccination, enhanced biosecurity, and improved diagnostics, veterinarians and horse owners can work together to mitigate these threats and safeguard the health, welfare, and performance of sport horses.

The study, “Respiratory viruses affecting health and performance in equine athletes,” appeared in Virology in December 2024.