Respiratory Disease Surveillance Study: Two-Year Results

Respiratory Disease Surveillance Study: Two-Year Results

Nasal discharge was the most commonly reported clinical sign and was present in 76.2% of the 761 horses in the first two years of the respiratory disease surveillance study.

Photo: The Horse Staff

Have you ever wondered what the most common clinical signs of respiratory diseases in American horses might be? Or whether your horse's age, or simply the time of year, could make him more likely to contract an infectious respiratory pathogen? The answers you're looking for might finally be here. The first two years of results are in from an ongoing surveillance study at the University of California, Davis (UC Davis), School of Veterinary Medicine, that examines the prevalence of respiratory pathogens in U.S.-based horses. 

In the study 95 veterinary practices in 23 states collected blood and nasal secretions from equids with acute infectious upper respiratory signs. For each horse, donkey, or mule with respiratory signs (including fever, depression, nasal discharge, coughing, or anorexia), veterinarians filled out a questionnaire and sent blood and nasal samples to UC Davis for PCR (polymerase chain reaction) analysis.

Of the 761 horses with clinical signs of acute respiratory disease from March 2008 to February 2010, 26.4% (201 horses) tested positive for one or more of four common respiratory pathogens: equine herpesvirus, types 1 and 4 (EHV-1 and EHV-4, respectively), equine influenza virus (EIV), and Streptococcus equi subsp. equi (strangles). Of these four, EHV-4 had the highest detection rate (82 cases), followed by EIV (60 cases), strangles (49 cases), and EHV-1 (23 cases). Fifteen of those 201 horses tested positive for two pathogens, and one was infected with three of the four.

The remaining 76.3% (560 horses) did not test positive via PCR for any of the four pathogens included in the study.

Upon further analysis, the researchers found that confirmed cases were typically age-related, with the largest number of EHV-4 cases occurring in very young horses (less than a year in age). Equine influenza occurred most frequently in juvenile horses (1 to 5 years of age). Strangles was reported most frequently in horses 6 to 10 years of age, and EHV-1 was most commonly detected in horses 11 to 20 years of age. Respiratory disease cases varied by season, with EHV-4 cases detected mainly during the autumn and winter months, and EIV and strangles cases seen mainly in the winter and spring, the researchers noted.

Other interesting findings include:

  • Nasal discharge was the most commonly reported clinical sign (76.2%), followed by fever (56%), depression (51.3%), coughing (45.3%), and anorexia (43%);
  • Ocular discharge and lower limb swelling were reported in less than 10% of cases;
  • On average, EHV-1 was characterized by fever; EHV-4 was characterized by nasal discharge and fever; EIV was characterized by depression, nasal discharge, fever, and coughing; and strangles was characterized by depression, nasal discharge, fever, coughing, and anorexia; and
  • Neurologic signs were associated with both EHV-1 and EHV-4 infections.

"The results underline the need for continued monitoring, immunization (depending on the risk of exposure), and also the importance of using and applying basic biosecurity measures," says Nicola Pusterla, DVM, , PhD, Dipl. ACVIM, associate professor in the Department of Medicine and Epidemiology at UC Davis and lead author on the study.

Surveillance studies identify cases in the population in order to identify emerging trends. "We (also want) to look at the potential impact of less well-characterized respiratory pathogens such as equine rhinitis viruses, adenoviruses, and the gamma-herpesviruses (EHV-2/-5)," noted Pusterla.

The two-year results of the ongoing study, "Surveillance programme for important equine infectious respiratory pathogens in the USA," were published in the June 24 issue of the Veterinary Record. The abstract is available on PubMed.

About the Author

Nancy Zacks, MS

Nancy Zacks holds an M.S. in Science Journalism from the Boston University College of Communication. She grew up in suburban Philadelphia where she learned to ride over fields and fences in nearby Malvern, Pa. When not writing, she enjoys riding at an eventing barn, drawing and painting horses, volunteering at a therapeutic riding program, and walking with Lilly, her black Labrador Retriever.

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