Q. An acquaintance of mine was returning her 14-year-old stallion to his stall after a trail ride, when he bit into her bicep and literally dragged her out of the stall into the aisle. All told, the attack exposed the muscle in the bicep, broke several of her fingers, and did enough other damage to warrant several stitches. After the episode the animal backed off and became docile. Any idea what caused this seemingly unprovoked attack? She now intends to geld (and keep) him.

Pat, Colorado

A. It is difficult to say much about what brought on a sudden attack of this type in a 14-year-old stallion, assuming he has not shown similar aggressive tendencies in the past. An attack of this type can have both physical/­medical as well as social and handling causes. In rare instances, something as simple as a sudden startle can evoke fear sufficient to provoke a sudden severe outburst, but the owner likely would have recognized a tendency for that type of reaction before this age.

So first, I would recommend a detailed veterinary workup including a neurologic exam. Some medical conditions can affect the brain and should be considered when uncharacteristic and severe aggression occurs. For example, liver disease can affect the brain enough to cause aggressive outbursts. Also, a sudden sharp pain or startling stimulus (e.g., static electricity spark) can elicit such a