Airport authorities in the US got a big shock this week when an X-ray machine detected a highly unusual find in a passenger’s backpack.

The high-tech machine at the Wisconsin airport picked up the outline of a small brown pup curled up in the bag.

“A dog was accidentally sent through the X-ray,” the local Transportation Security Administration branch tweeted on Tuesday.

An image posted by the TSA’s Great Lake branch clearly shows the outline of the dog in the carry-on luggage. The dog’s owner had apparently neglected to tell screeners about her pet before she put the bag onto the conveyor belt at Dane County Regional Airport in Madison.

Tweet from @TSA_GreatLakes

TSA spokesperson Jessica Mayle later confirmed the small dog – a dachshund-chihuahua cross, was unharmed after its unexpected journey through the machine – “just a little skittish”.

“After [the dog] was discovered by the X-ray operator, the officer explained the proper process to the passenger and confirmed she had disclosed she was traveling with the pet to the airline,” Ms Mayle told USA Today on Wednesday.

“After her bags were cleared, she proceeded to her gate.”

In the US, small pets are allowed on domestic flights. But they must be in approved containers or restraints.

Major US airlines will also charge for travelling with a pet on board – sometimes more than the passenger’s airfare.

“When traveling with any animal, notify your airline and know their rules,” the TSA said in its tweet.

Tweet from @TSA

This week’s discovery comes after a live cat was found trapped in a suitacase at John F. Kennedy Airport in New York in November.

That cat – named Smells – was stuffed inside a suitcase that had been checked in. He was also picked up on an X-ray by staff, who were “shocked” to open the case and find the unharmed, but decidedly unhappy, cat.

TSA spokesperson Lisa Farbstein told CNN at the time that the owner of the luggage – who was travelling on Delta Air Lines – said “it was not his cat – it belonged to someone else in the household”.

The traveller said the cat – which was on top of the luggage, and even had its fur sticking out of the case between the zippers – had got into the case unseen.

Smells was treated to a human-sized Thanksgiving feast after his ordeal.