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.

By Day 3, the ponies' submandibular (under the jaw) lymph nodes were enlarged and they had nasal discharge and tracheal mucus that was confirmed with endoscopy; the tracheal mucus persisted until Day 21. Lower airway samples revealed viral shedding Days 5 to 7. In the infected ponies an antibody response appeared by Day 7 that persisted to Day 21, along with airway inflammation lasting for three weeks. By Day 21, the lymph nodes were still enlarged but nasal discharge had abated.

Equine rhinitis virus can have a marked impact on health and performance by its effects on both the upper and lower respiratory tracts persisting for three weeks. Diaz-Mendez noted that infection with ERAV might be considerably underestimated in the equine population and, thus, practitioners should consider it when presented with a horse with acute respiratory viral infection. In addition, he pointed out that equine rhinitis virus might be associated with episodes of other respiratory conditions, such as IAD and heaves. At this time there is no vaccine available to protect against equine rhinitis virus.

Diaz-Mendez continues to research ERAV and received one of three inaugural $25,000 Boehringer Ingelheim Vetmedica Equine Research Awards at the convention for a study proposal. In a study to be completed in 2012 he'll examine the response of equine airway epithelium to ERAV infection and he'll perform viral surveillance.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.