When considering viral respiratory infections in horses, a common assumption among veterinarians is animals are infected with either influenza or rhinopneumonitis (a respiratory condition–mostly of young horses–caused by equine herpesvirus). However, another viral infection–equine rhinitis–is commonly responsible for respiratory disease outbreaks. Andres Diaz-Mendez, Med Vet, MSc, of the University of Guelph presented information about equine rhinitis virus A (ERAV) at the 2011 American Association of Equine Practitioners Convention, held Nov. 18-22 in San Antonio, Texas.

Diaz-Mendez described clinical signs of ERAV, which mimic those seen with equine influenza or rhinopneumonitis: nasal discharge ranging from serous (clear, runny) to mucopurulent (thick, greenish-white), fever, and cough. In one study of nasal swabs from horses with acute respiratory disease, researchers found the prevalence of influenza was 56%, ERAV was 42%, and 24% of the horses were infected with both viruses. He explained that ERAV affects both the upper and lower airways with moderate inflammation that exacerbates inflammatory airway disease (IAD) and recurrent airway obstruction (heaves).

Genome sequencing of ERAV has shown that any changes since it was first isolated in 1962 have been very minor, as the currently circulating virus is 96% similar to when it was first identified. This is in contrast to equine influenza virus, which mutates frequently.

In Diaz-Mendez’s study he induced experimental infection in a dozen 8- to 12-month-old healthy ponies by suppressing their immune systems with the corticosteroid dexamethasone three days prior to viral exposure. Within 24 hours of exposure, clinical signs began to appear as fever and increased wheezing lung sounds