VANCOUVER -- The Vancouver Humane Society is calling for an investigation into a local zoo after a moose that was recently photographed looking emaciated had to be euthanized.

A statement from the society called the images posted on social media "disturbing," and suggested the BC SPCA launch an investigation into the Greater Vancouver Zoo.

The senior female moose, named Oakleaf, had been unwell for a while. Her ribs could be seen in a photo taken Tuesday, the day before she was euthanized.

A veterinarian at the zoo told CTV News it was his decision.

"When I talked to the managers, we all agreed and we decided to put her down," Bruce Burton said.

The photo posted on social media was shared several times.

Shannon Marcoux, who posted it to Facebook, wrote that she thought it was unacceptable, and an activist agreed.

"There should really be no excuse for it. You know, this is an animal that's in their care," David Isbister said.

A former zoo employee told CTV News that the animal, along with many others, wasn't getting the care it needed.

She spoke to CTV anonymously, saying she feared backlash for speaking out.

"I had worked at private zoos before, and I was so disappointed with the care and animal welfare at this facility," she said.

"It was very draining because I cared about the animals and there was only so much that I could do on a day-to-day basis."

The worker says there was no full-time veterinarian, and over the course of six months, perhaps up to 15 other animals had died.

Oakleaf was eight years old, and had been born in captivity. She had a myriad of health problems with her feet and teeth, and had trouble eating.

Her food was ground up for easier digestion.

"Sometimes we are working hard to try and improve the quality of life of an animal, a particular animal, that we think we can save," Burton said.

"You're going to have live animals, you're going to have sick animals, and unfortunately you're going to have dead animals. That happens."

Burton said unlike moose in the wild, which can live 15 to 20 years, moose tend to not live as long in captivity.

In December of last year, the Vancouver Humane Society commissioned a report on the zoo, which suggested it was failing its animals. 

The report found the facility's "small" and "barren" enclosures were detrimental to their inhabitants, and suggested some of the animals showed signs of boredom and frustration.