A Brooklyn dog who ended up at the pound when her owners lost everything in a fire languished from a treatable-but-life-threatening condition doctors failed to promptly diagnose — then was euthanized following an ineffective emergency surgery, The Post has learned.

Hennessy, a 7-year-old un-spayed pit bull with a goofy smile and no behavior issues, was brought to the Brooklyn Animal Care Center on July 25th with another dog while their owners searched for a new home, records obtained by The Post show.

By Aug. 11, Hennessy’s condition had deteriorated, and she was showing signs of a life-threatening uterine infection called pyometra, including vomiting and not eating, the dog’s medical notes show.

Dr. Steven T. Brigande, who’s been working as a veterinarian for nearly three decades, said pyometra is common among older, “intact” — or unspayed — female dogs and it would’ve been one of the first things he’d test for after noting the vomiting and lack of appetite.

“If they’re vomiting and it’s an intact animal, the first thing you would do is get the full blood work and then get an abdominal X-ray or ultrasound immediately. That’s going to give you a diagnosis in minutes,” Brigande told The Post.

“Any third-year vet student could diagnose [pyometra]. If you have a 7-year-old pit bull that’s intact, it should be one of the top five differentials on your list.”

Instead, Hennessy was given medicine to treat nausea and was never given an abdominal x-ray, the medical notes show.

Brigande said pyometra is treatable and can be fixed — as long as it’s caught early.

“In vet school, we were told to ‘never let the sun go down’” on a pyometra case, because the more time that passes, “the risk of a dog having a complication is much higher,” Brigande explained.

ACC, which is under contract with the city’s Department of Health to operate city shelters, failed to diagnose Hennessy with pyometra until Aug. 15 when a rescue group was waiting to foster her, despite examining her two more times after the symptoms began.

By then, Hennessy’s blood work, reviewed by Brigande, showed the infection was already in her bloodstream and had progressed significantly. Shelter vets scheduled her for an emergency spay surgery, which treats pyometra, but were forced to euthanize her a day later when the dog was overtaken by sepsis, the agency said.

“Even with our best efforts to save her life, she was not able to recover due to sepsis,” the agency said in a statement.

Sepsis, which is a massive infection in the body, was likely caused by the pyometra, Brigande said.

Animal activist Abby Bogen, who oversees a Facebook group that connects at-risk shelter animals with adopters, said Hennessy deserved a better outcome.

“They knew this dog was sick…. Why wasn’t she sent to the emergency room?” Bogen questioned.

“She survived a fire. She lost everything… she deserved a second chance.”

This is just the latest issue to arise within the scandal-scarred organization that’s currently in the midst of an Attorney General probe into their spay-and-neuter practices, which The Post exclusively reported on last year.