Question: Two days ago, I came home to find my 29-year-old Standardbred mare hot and sweaty with blood trickling from one of her nostrils. My mule (who was out with her) was also a little sweaty and warm to the touch. Shortly afterwards, my mare cooled down and has seemed fine since. What would have made her nose bleed? I don’t believe my mule kicked her, as she is definitely the alpha of the two. She had no visible bites nor was any swelling evident on her face, neck or anywhere else. She has never bled before but I wondered whether I should be worried about this episode.

Answer: Keep an eye on your mare but don’t worry: Most nasal trickles are harmless, isolated occurrences, not harbingers of horrors to come. A routine case of bleeding from one nostril, especially after exertion, could have any of a dozen causes, the most innocent being a poke inside the nose with a sharp twig or hay stem. Exercise can also cause an equine nosebleed by raising the blood pressure enough to rupture already-weakened vessels near the surface.

On the other hand, recurrent bleeding from the same nostril is rarely innocent. Such bleeding has three primary causes:

• a tumor in the nasal cavity or sinus
• an ethmoid hematoma, a pocket of blood in the mucosal lining of the nasal passages that develops for unknown reasons
• a fungal infection in the guttural pouch that has weakened a wall of the carotid artery. Endoscopic examinations of all these chambers and passageways, together with radiographs of any suspicious areas, are necessary for a definitive diagnosis.
  
  However, from your description, I doubt any of these more serious conditions caused your mare’s nosebleed. The more time that passes before a second episode, the less likely that anything serious has happened.