They walk the streets and parks of major American cities with their camera phones ready, hopeful of a significant sighting.

Unlike other tourists, who come to admire the scenery, their eyes are cast down. Some carry notebooks and printouts of the rules. These are the dogspotters, devotees of a game that now has more than half a million active players and an online fanbase admiring their work. All hope to encounter an “unexpected dog”, record it in a photograph and gain points and the respect of fellow spotters.

John Savoia, 31, a photographer from Boston, said that the hobby bore resemblances to trainspotting, though it had been supercharged by the age of social media and the fascination with photographs of dogs.

“Free-range” dogs being athletic also earn dogspotters a bonus

“It’s the specifics that make a train or a dog interesting,” he said. “Train spotters might ask themselves, what cargo does the train have? Dogspotters would think, is this pug wearing a really funny hat? Is it standing on its hind legs?’ ”

Mr Savoia is the founding father of dogspotting, which now has a mobile phone app, a leaderboard recording the achievements of the highest-scoring spotters and a range of merchandise.

It began as a joke with friends on Facebook, he said. “I’m a photographer and I wander the streets looking for shots. I was really into video games as a kid, and I like the concept of giving myself points for arbitrary tasks.” He shared a Facebook page setting out these thoughts with a few friends in February 2008, but it remained a niche interest until 2014, when the membership began to grow. “It spiralled from there,” Mr Savoia said. “It’s grown into a beautiful monster.”

Rules were laid out. Spotters could not know the dog in advance, nor would they be permitted to photograph the “low hanging fruit” of dogs in dog parks or at a vet or grooming shop. Extra points would be awarded for “free-range” dogs spotted off the leash, “action dogs” jumping or “displaying extreme athletics”, and dogs riding in the back of a car or on a motorcycle.

Celebrities’ dogs are among the most valuable. Eric Kmetz, 31, recently caught what was said to be a 12-point dog: the Obamas’ Bo. Top of the leaderboard is Bo Quintana, 18, from Texas, who photographed Louboutina, a golden retriever in Manhattan who has become known for hugging passers-by and has 168,000 followers on Instagram. “She’s really my crown jewel spot,” he told The Wall Street Journal.