A striking MRI scan of a pug’s head which has charmed sections of the internet is actually a reminder of the problems the dogs suffer from because of human breeding, a leading vet has said.

A tweet showing the black and white image of the pug went viral this week, garnering almost 130,000 likes.

The picture is dominated by the pug’s huge bulging eyes, while the animal almost seems to have a quizzical expression.

Many of those commenting on and sharing the image described it as adorable and cute, but Rory Cowlam, a vet based in London, said it was actually concerning.

“It is quite striking,” he told The Independent. The image is not an X-ray, as some news outlets suggested, but in fact one part of an MRI scan.

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Magnetic resonance image scanners produces multiple scans, effectively slicing up the subject cross-ways to see inside their skull.

“If you were to get a pug head cake and cut it to take a look down the middle, that is what you are looking at,” Dr Cowlam said. “This is a slice at the level of the eyes, looking head on at the pug, just two or three inches further back.”

Although all MRI images, including those taken of humans, will look unusual to an untrained eye, Dr Cowlam conceded the viral pug snapshot was “pretty odd-looking”.

But rather than simply something to gawk at, the bulging eyes and bunched up nose visible in the scan was a sign of how pugs have been unnaturally bred over many years by humans.

Dr Cowlam said: “Their faces have been shortened due to intensive breeding by humans, unfortunately. They have these massive eyes, bunched up noses… The condition is called brachycephalism.”

As well as pugs, other similar breeds including French bulldogs and Shih Tzus also suffer from bracycephalism.

“We have, through human selection, bred them to look more like a human baby because we find that cute,” Dr Cowlam explained. “But unfortunately that cute look is not very good for the animal.”

His surgery regularly sees brachycephalic dogs which suffer from a range of medical complaints because of their distorted physiology, including spinal problems and breathing difficulties.

Because pugs and bulldogs have squashed noses, their breathing becomes inefficient which means they can not only pass out during over-exertion but are also unable to cool themselves down properly, making them at risk of heatstroke.

However, Dr Cowlam speculated the MRI was probably taken of the pug for different reasons.

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“We would do that for a number of reasons, usually looking for a neurological disease," he said.

"What I imagine they were doing is looking at the brain.”

Andy Richter, who tweeted the initial image, said it was of a friend’s pug who has since been given a clean bill of health. He then tweeted a screenshot of the pug looking “nonplussed” after going viral online.

Pugs in particular have exploded in popularity in recent years. Data from the Kennel Club shows the number of pedigree puppies born each year has increased from about 1,000 twenty years ago to more than 9,000 last year.

However, there are indications the warnings from vets is now starting to sink in with the public; the numbers of pug registrations with the Kennel Club fell last year for the first time in over a decade, while Pets4Home reported a fall in the number of adverts selling pugs of 17 per cent.

Dr Cowlam said he would “actively encourage” anyone considering buying a pug or similar brachycephalic dog to think again.

If they are intent on it, “do real research into the breeding” because there are now some specialist breeders intentionally trying to undo some of the dogs’ health problems by lengthening the snout.