Your Guide to Equine Health Care

Sudden Attack: What Could Cause a Stallion to Become Aggressive?

A reader wants to know about possible reasons behind a 14-year-old stallion’s seemingly unprovoked aggressive behavior.

Aggressive horse
When a horse is threatened by another horse, but not at liberty to attack that animal, he might lash out at a person or another animal that is easily within reach, especially if positioned between him and the other horse. | Photo: iStock
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

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Written by:

Sue M. McDonnell, PhD, is a certified applied animal behaviorist and the founding head of the equine behavior program at the University of Pennsylvania’s School of Veterinary Medicine. She is also the author of numerous books and articles about horse behavior and management.

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