Which of the following is most likely to be equine influenza?

• A. A mild respiratory disease affecting a single newborn foal on a farm
• B. A respiratory disease affecting large numbers of young horses
• C. A severe respiratory disease, with abortions, that affects mares
• D. A severe respiratory disease, with neurologic signs, that affects older mares

Answer: (B)

Equine influenza is a highly contagious respiratory infection of horses that spreads rapidly through groups, especially among young, naïve animals kept in close quarters. The hallmark is outbreaks that involve many animals, rather than a single case, with signs like fever, cough, and nasal discharge that can affect large numbers. This pattern fits the description of a respiratory disease affecting large numbers of young horses.

The other scenarios don’t match influenza’s typical picture: a single mild case in a newborn foal suggests a localized or non-epidemic issue; severe respiratory disease with abortions points to pathogens like equine herpesvirus that cause reproductive losses; and a severe respiratory disease with neurologic signs, especially in older mares, is not characteristic of influenza, which usually lacks such neurologic involvement or abortion associations.