A morning run in Elk Island National Park left Craig Neilson with about six stitches in his butt and an incredible story about being attacked by a bison.

"How does that even happen? You got charged by a bison?" said Neilson, who admitted he has been subjected to "a lot of butt jokes" since sharing his story.

The Edmonton man and his pregnant wife, Amberly, had been camping in the Astotin Lake Campground on May 19.

Neilson told CBC News he went for a morning run, leaving his sleeping wife in the tent.

'He started to charge at me'

On his way back, Neilson said he encountered a lone bison near the road that led to the campground.

"He looked up at me, and he started to charge at me," said Neilson, who made the snap decision to head for the safety of nearby trees.

That's when he lost his balance.

"When I fell, I was like, 'Are you kidding me?' "said the ultra-marathon runner. "I was like, 'This is going to cost me,' and it did."

"As I looked behind me, I could see him coming up on me, and basically watched him hit me from behind," said Neilson. "He hit me right on my butt, got me with his horns and his head, and he lifted me up and forward."

The impact propelled Neilson closer to the trees, which he scrambled to reach. Once safe, he assessed his injuries.

Craig Neilson snapped this picture while on a morning run in Elk Island National Park. He was attacked by a bison on his way back to the campground. (Craig Neilson )

"I could feel a hole in my compression shorts, and I could feel this flap of skin with all this blood rushing out," said Neilson.

"At that point, I was like, 'Oh man, this is serious. I'm actually going to need help, I can't just walk back to the campground.' "

Neilson flagged down motorists, who headed to the main gate of the park to call for help.

He lost his cellphone during the attack, but found it once the bison moved away.

Neilson said he first called his wife, but she was still sleeping. He then called an ambulance.

A pain in the butt

By then, the motorists had located a sheriff, who came to Neilson's aid and helped stop the bleeding with some gauze.

He finally reached his wife, who went with him to the hospital. Neilson received about six stitches to close the wound.

"Considering what happened, it could have easily been so much worse," he said.

Neilson hopes his injury will be healed in time for him to participate in the Sinister 7 Ultra, a gruelling race happening July 7-8 in the Rocky Mountains.

Despite his injury, Neilson doesn't think the bison at Elk Island National Park need to be kept behind a fence.

"I just got stuck in a really bad situation, and it happened to turn out really poorly for me," he said. "I would have never gone toward him if I had known he was there."

'Unpredictable' animals

Bison are unpredictable creatures, said Dale Kirkland, superintendent of Elk Island National Park.

"They may react to any perceived threat, perhaps when they're startled," he said.

Bison are not naturally aggressive. There have been only two reported bison attacks in the park in the last six years, he said.

Park visitors should stay at least 100 metres from a bison, and make their presence known if they do encounter one in close proximity.

"Be alert, be aware, and give them space," said Kirkland.