WATCH: Dog attacks deer in downtown Banff

This is why you keep your dogs on a leash — especially in a national park.

Parks Canada is investigating after a video went viral this week showing a dog attacking a deer along a busy Banff thoroughfare.

The Feb. 18 video shows the dog clamping on to the deer's horns as its handler tries to bring the canine under control.

"The town has designated the dog a vicious animal under the Animal Services Bylaw and the owner has fully complied with the requirements, including a special licence and muzzling to ensure public and wildlife safety," the town posted on its Facebook page Thursday.