After a mild week-long respiratory illness, 95% of the affected horses in a stable recover. Which disease is least likely to be the cause of the illness?

• A. EHV-4
• B. Hendra
• C. EHV-1
• D. Equine influenza

Answer: (B)

A mild, week-long respiratory illness with a high recovery rate in a group of horses points to common, relatively low-virulence equine respiratory viruses rather than pathogens that cause severe disease. Hendra virus fits this pattern least well because it typically produces serious, often fatal respiratory or neurologic disease in horses and carries a risk of severe illness for humans who are exposed. In a stable with a self-limited outbreak, you would not expect the pronounced severity and mortality associated with Hendra.

The other possibilities align better with the scenario. EHV-4 usually causes mild upper respiratory infections, especially in young horses. Equine influenza is highly contagious and can cause fever, coughing, and nasal discharge but many horses recover completely. EHV-1 can cause respiratory disease as well, though it has the potential for abortion or neurologic signs; it can still present as a mild respiratory illness in some outbreaks.