The owner of a four-year-old service dog who fell ill after a stay at a Toronto PetSmart says the dog was put down on Sunday because veterinarians were unable to successfully treat her ailments.

Corey Dixon, 23, said Aspen went peacefully, surrounded by her human family, at an emergency animal hospital in Oakville where she had been undergoing treatment for several days.

Dixon and PetSmart have offered differing accounts of what ultimately killed Aspen. According to Dixon, the black Labrador Retriever was suffering from severe pneumonia that was making it nearly impossible for her to breathe.

"We got a call this morning and they said, 'We've prepared for the worst,'" Dixon told CBC Toronto in a phone interview from the animal hospital. The call was preceded by an episode during which Aspen nearly stopped breathing altogether.

"She was an incredible animal," an emotional Dixon said.

Dixon became Aspen's caretaker in January. He lives with a heart condition that can cause him to become unconscious, and Aspen was specially trained to alert passerbys when he fainted.

"She was my service dog, and she saved my life more than once," Dixon said.

On July 1, Dixon dropped Aspen at a PetSmart location in north Etobicoke for a week-long stay while he was on a trip in cottage country. He thought that the trip might offer her an opportunity for some down time and rest without the responsibility of watching over him.

Aspen was suffering from severe pneumonia in both of her lungs when she was put down on Sunday, Dixon said. (@CoreyDixon33/Twitter)

When he returned to collect Aspen on July 8, she was 20 pounds thinner and shaking in a pool of vomit and saliva, according to Dixon. He could tell immediately that she was "extremely sick" and required medical attention, he says.

A vet concluded the underlying cause of her poor condition was not clear, but suggested that it could be "systemic illness/infection" to a toxin or foreign object she may have eaten.

"We know that she was sick in PetSmart's care, we know that much. And that they didn't bother to call the vet when she should have been seen a lot earlier," Dixon said.

"If she'd seen the vet when she started showing signs of being sick, maybe she would have had a better chance."

A spokesperson for the company said last week that it is investigating, but that "our initial findings indicate that our associates followed all policies and procedures."

PetSmart contributed $5,000 to veterinary expenses for Aspen's care, something which Dixon said he appreciates.

In an email statement on Sunday, PetSmart spokesperson Erin Gray said the company is "deeply saddened to learn of Aspen's unfortunate passing."

She added, however, that at least one veterinarian "reported the cause of death as a pre-existing autoimmune disease ... completely unrelated to Aspen's stay at PetSmart."

The condition in question is Myasthenia gravis, a disease that affects skeletal muscles and can impede breathing.

Dixon maintains that while a vet took a blood sample from Aspen to test for the disease, the results of that test have not been returned.

He adds that even if Aspen had a pre-existing condition, he doesn't understand why PetSmart did not get her medical help once it became clear that something was wrong.

Corey Dixon said he believes that Aspen may have had a better of surviving if she had seen a veterinarian earlier. (@CoreyDixon33/Twitter)

Given the significant costs of an emergency veterinary stay, Dixon has also launched a GoFundMe campaign that raised about $2,700 in just a few days. He added that he was extremely grateful for the donations he has received.

Dixon said he's too devastated to think about finding a new service animal, and that for now, his mind is on Aspen.

"She was so much more than just a dog to me," he continued.

He plans to have Aspen cremated and will host a memorial for her for family and friends in the coming weeks.