Timmy, who was 15, became the first dog in the world to survive a stem cell transplant into the brain in 2015 by researchers at the University of Sydney’s Mind and Brain Centre in a bid to reverse his canine form of dementia.

Team leader, Professor Michael Valenzuela, only this month declared that after nearly two years Timmy was free of all symptoms. He described Timmy’s results as “the first step in providing powerful evidence to support a human trial hopefully within three years’’.

 “We considered Timmy functionally cured … we could not have wished for a better outcome,’’ he added.

Timmy died peacefully of old age on Wednesday. On hearing the news, Professor Valenzuela declared Timmy’s brain was “at this point the most important brain in veterinary history’’.

His owners, Tony and Michele Leeder-Smith from Horsley, were saddened but said they were happy to have had their mate back to his old self, even if only for a short time.

“Michele and I don’t have children together so Timmy was pretty much our boy,’’ he said. “We adopted him at four months when his owners couldn’t cope with his congenital deafness and he proved to be a great dog.”

They made the decision to include Timmy in the trial in 2014 when he grew distant and started to behave strangely.

“He slept at the end of our bed but he was waking every hour on the hour barking at nothing, and sometimes he walked into the kitchen and just stared at the wall,’’ Mr Leeder-Smith said. “Our vet diagnosed him with Canine Cognitive Disorder that mirrors human dementia and she suggested we apply to have him included in the university trial rather than have him put to sleep.

“We didn’t know at that stage if it would be too invasive … but we knew it was for something so important.’’

In 2015 Timmy was fitted with a collar to gather base-line information on activity, sleep and barking. Next the team transplanted stem cells grown from Timmy’s belly directly into the hippocampus using MRI guidance. 

The team used a combination of scientific measurements over the past two years to determine success. “Our main measure is the Canine Cognitive Dysfunction Scale, a clinical tool that we developed, in which owners rate the frequency and severity of abnormal behaviours,’’ said Prof Valenzuela. “Using this, Timmy’s score almost halved, from categorically demented to normal.’’

“Timmy started to improve almost from the word go after the transplant,’’ said Mr Leeder-Smith.