The moment a family dog was found alive and well after its owners thought it perished in California’s deadliest-ever fires has been caught on video.

At least 40 people have died in the fires and thousands of Californians have been displaced or left homeless by nearly 20 blazes that burned across the state.

The scale of devastation meant when Jack Weaver and his brother-in-law Patrick Widen returned to Mr Weaver’s mother’s northern California home, they were sure they would find the body of the family’s beloved dog Izzy.

They walked around police barricades, through a creek and up treacherous hilly roads to save Mr Weaver’s mother from the grim task of recovering the body.

While some parts of the property survived, the family home was completely destroyed.

But then Izzy suddenly came bounding out for a joyous reunion.

Mr Weaver, who was filming the scene for his parents, captured the moment on his phone in a video that has gone viral on Facebook, providing a rare bit of good news amid endless scenes of severe destruction.

“She was very happy to see us,” Mr Weaver said of Izzy, a nine-year-old Bernese mountain dog.

“It was one of the greatest moments of my life,” he said.

“She’s such a brave dog. She was panting a lot and clearly stressed. But she was not frantic or anything.”

In the race to escape in the middle of the night, the Weavers and many others were forced to leave behind their pets because they could not be found or there was simply no time to get them.

Thousands of people remained evacuated as fires continued to rage in California’s wine country and authorities said it was still too dangerous to return to burned-out neighbourhoods of Santa Rosa.

Entire neighbourhoods have been decimated by the fires. Photo: AP

Mobile phone service was spotty, but Mr Weaver was eventually able to reach his mother, who was staying with relatives in the San Francisco Bay Area.

“She just lost it,” Mr Weaver said. “She went from being devastated about losing her home to the being the happiest person I’ve ever seen.

“I couldn’t get home fast enough. She was really, really happy … She’s still shaken up by the whole thing, but she’s in much better spirits now that Izzy is at our house.”

Animal care workers were working overtime to feed and provide medical care for the pets who survived the inferno.

Vets said Izzy did not show signs of damage from the fire. Photo: AP

A veterinarian said Izzy was fine, likely insulated from the fire’s intense heat by its thick fur coat.

At Sonoma County Animal Services, vets and assistants were caring for 64 cats and 44 dogs, nearly all of them brought in from fire zones.

Cats generally have the most severe injuries, including smoke inhalation, burned paws or singed fur and whiskers.

“The severity is often terrible,” lead vet Dr Katie McKenzie said.

Shelter workers update their website every hour with photos of the pets they have rescued, and they are fielding frantic calls from worried pet owners searching for their furry companions.

So far 25 animals have been reunited with their owners, Monica Argenti, a spokeswoman for the shelter, said.