Feral honeybees were responsible for the death of a Montana horse in June, according to an entomologist from that state’s Department of Agriculture.

Last month the horse’s owners observed the animal scratching its back on a tree when a swarm of bees exited the tree and stung the horse repeatedly. The animal received veterinary treatment, but was later euthanized due to severe anaphylactic shock, which occurs when horses experience a severe reaction to an allergen such as bee venom. The condition causes hives, sweating, difficulty breathing, irregular heartbeat, drop in blood pressure, and fluid accumulation (edema). Severe cases can cause death in horses.

The horse’s owners later photographed the bees that delivered the stings and sent the photos to Montana State Entomologist Cam Lay to examine. Lay identified the insects as feral honeybees. Lay said the horse’s severe allergy to the bees’ venom caused the fatal reaction.

Feral honeybees frequently establish hives in hollow trees, but the insects are not normally aggressive unless they think their hive is under threat, Lay said. The horse had probably been using the tree as a scratching post for years before bees established a hive in it, he added