Korinne Mortensen of Salt Lake City, Utah, was relaxing with her dog Niko in November 2022 when suddenly he lunged at her and mauled her face.

He had been 'the world's perfect dog' for four years before the incident, which left Ms Mortensen with a gaping bloody hole on the bridge of her nose.

It wasn't until after the attack that the Mortensens learned that their beloved pet had a severe and aggressive form of cancer in his brain.

They believe the tumor is to blame for the drastic personality change.

The family felt they had no other choice but to humanely euthanize Niko, wanting to both spare him from pain and protect their children. 

Ms Mortensen, 30, does not fault her dog for the incident. 

The dog that attacked her, she said, was not the same dog they adopted years ago.

The cancer had hijacked his personality.

Tumors in dogs' forebrains have been known to cause abnormalities in their behavior, including to become more aggressive.

The forebrain is responsible for 'thinking,' behavior, and final integration of sensory information.

Tumors in the forebrain may therefore cause dogs to be irritable, confused and aggressive.

After Niko lunged at her face, Ms Mortensen did not understand immediately what had happened in those brief few seconds. 

She went to the bathroom and saw her face covered in blood. The pain had not yet set in.

Ms Mortensen said: 'It was half a second. It was so fast and one bite. It all happened so fast and I was in shock as I never imagined my sweet little dog would do this.'

When she got to the hospital, she began to feel the throbbing, stinging pain. Her nose had been completely broken in half, and a quarter-sized hole had been ripped off the bridge.

She said: 'I was worried about if my face was going to be messed up long term and what we were going to do with our dog following this.

'In the surgery, he re-broke my whole nose and reconstructed it so it was functional and also filled in the big huge gap.

'He did a zigzag skin layer to cover up the scarring. My surgeon told me at my follow-up that he said he had to jigsaw it [my nose] back together.'

Before the attack, Niko had been suffering seizures. The family had an appointment to take him to the vet in the next few weeks.

Given that Niko had never been violent before, the Mortensens were not concerned about getting in to see the doctor sooner.

In the year following the freak attack, Ms Mortensen struggled to look at herself in the mirror. She was afraid she would never look like herself again.

And she wasn't sure how Niko would fit into her expanding family.

She had just given birth to her second son in September 2023, one month before he lunged at her face.

She said: 'He then started getting aggressive with our other dog and we had to have him in a muzzle 24/7 and we felt so bad about this.

'How fast he was going downhill, there wasn't a lot they could do for him so we decided to put him down as this was the best option as it wasn't fair on him.'

Since going into emergency surgery in November last year, Ms Mortensen has had two rounds of injectable fillers shot into her nose to restore its normal shape.

She said: 'There is still scarring and I am always going to have this but the structural integrity looks like a normal nose now.

'I now feel confident to go out and it's not the first thing people notice when they speak to me.'

The Mortensens have come to peace with the event since then and hold cherished memories of their time with Niko before his uncharacteristically savage behavior.

She added: 'I don't blame our dog at all for the attack. When I think about it, it wasn't our dog that did this.

'We got closure and can now remember the good memories and the four years of the perfect dog we had.'