Question of the Day

A commercial pig farm experiences an explosive outbreak of respiratory disease with high mortality, primarily in young pigs under six months of age.

Affected pigs show severe respiratory distress, fever up to 107°F (41.5°C), anorexia, and reluctance to move. Some animals display open-mouth breathing with a blood-stained, frothy nasal and oral discharge.

On necropsy, the lungs are bilaterally dark and swollen and ooze bloody fluid from the cut surface, as shown in the image below.

Which one of the following choices is the most likely diagnosis?