When Greg Manteufel is frustrated or feeling down, Ellie sits by him. At night, she sleeps under his covers and at dinner, she's there next to him, knowing he'll throw something her way. She belies the stereotype of the vicious pit bull.

And yet, Ellie may be the reason he nearly died.

"We love her like she's our daughter," Mr Manteufel, 49, who lives in West Bend, in the US state of Wisconsin, said of the dog.

Mr Manteufel has lost parts of his arms and legs, as well as the skin off his nose and part of his upper lip. The cause was capnocytophaga, a germ from Ellie's mouth, or otherwise perhaps from another dog he encountered.

The germ is commonly found in the saliva of cats and dogs and almost never leads to people getting sick, unless the person has a compromised immune system.

But Mr Manteufel was perfectly healthy and doesn't think he'd ever used his health insurance before he suddenly fell ill.

The case is extremely rare and doctors at his hospital, Froedtert and the Medical College of Wisconsin, had no explanation for why he got so sick.

View photos Greg Manteufel had both legs and arms amputated after being severely affected by a germ, possibly from his dog, Ellie. Source: AP More

But over the last 10 years there have been at least five other healthy people who have had severe reactions to the germ. A team of researchers connected with Harvard Medical School has developed a theory on why — a gene change in all the victims.

Their finding means doctors can't rule out the capnocytophaga bacteria could strike Mr Manteufel and other victims again.

Mr Manteufel had the tell-tale signs he was getting the flu in June of 2018. He had a fever, vomiting and diarrhoea. But when he started getting confused, his family took him to hospital.

Doctors found capnocytophaga, which caused sepsis, a severe blood infection that led to his blood pressure dropping and many of his organs shutting down.

"Do what you have to, to keep me alive," he told the doctors.

He had so much to live for — his wife of 16 years, Dawn, and 26-year-old son, Mike. He was just starting to get really good at his day job, painting houses, and he cherished his Harley Davidson Electric Glide. He was also in the middle of fixing up his '66 El Camino. And of course there was Ellie, the pup.

View photos Mr Manteufel never considered giving Ellie up, as he loves her like a daughter. Source: AP More

So he persisted through more than 20 surgeries, including amputations of his left and right arms just below the elbow, and legs through the middle of the knee.

Mr Manteufel's life now includes frequent occupational therapy appointments to perfect his use of arm prosthetics — the kind with metal moveable hooks at the end. He's using a fork regularly and he's now working on picking up the TV remote, opening doorknobs, cutting vegetables and doing the dishes.

Throughout the ordeal, Ellie has remained by his side, and despite her possibly carrying a fatal germ in her mouth, the family haven’t thought twice about whether or not they will keep her.

Results of Mr Manteufel's genetic tests are expected in three to four months, which will determine if he carries the gene change that causes an extreme reaction to capnocytophaga, and will put him at greater risk in the future.

While Mr Manteufel doesn't like the sound of that, he said Ellie's accidentally scratched him since he's been home and even licked his mouth and he's been fine.

And even if he does have the gene variant, he said, it changes nothing.

"We didn't even bother testing her. We weren't going to get rid of her if it was her that caused it anyway."

"We just love her to death."

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