Cats are considered lucky in Japan, and owners of the popular pets spend big on their care. But how do you know when they're feline down?

A tech firm and university in Tokyo have teamed up to produce an app trained on thousands of cat photos that they say can tell you when puss is in pain.

Since its release last month, "Cat Pain Detector" has racked up 43,000 users, mostly in Japan but also in Europe and South America, said Go Sakioka, head of developer Carelogy.

The app is part of a growing array of tech for pet owners concerned for their furry friends' wellbeing, including similar mood and pain trackers made in Canada and Israel.

Carelogy teamed up with Nihon University's College of Bioresource Sciences to gather 6,000 cat photos, in which they carefully studied the positions of the animals' ears, noses, whiskers and eyelids.

They then used a scoring system designed by the University of Montreal to measure minute differences between healthy cats and those suffering pain due to hard-to-spot illnesses.

Next, the app developers fed the information into an AI detection system, which has further refined its skills thanks to around 600,000 photos uploaded by users, Sakioka said.

Now the app "has an accuracy level of more than 90 percent", he told AFP.

According to the Japan Pet Food Association, 60 percent of owners take their cat to a veterinarian at most once a year.

"We want to help cat owners judge more easily at home whether to see a vet or not," Sakioka said.

"Cat Pain Detector" is already being used by some vets in Japan, the land of Hello Kitty, where tourists flock to cat cafes and some small islands are overrun by stray felines.

But "the AI system still needs to be more precise before it's used as a standardised tool", he cautioned.