PHILADELPHIA (CBS) — Carbon monoxide is often referred to as a silent killer because people can’t see it or smell it as it poisons its victims.

But a family in Wisconsin survived a carbon monoxide leak in their home not because of a detector, but because of their cat.

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It happened in Reedsburg, northeast of Madison.

It was the middle of the night when their cat “Gracie” suddenly started pounding on the door.

Kevin Shanahan says, “I got out of bed to stop her from pounding on the door and then I looked to my left and Annette was there in the chair.”

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Annette Shanahan says, ”I was hanging on to the arm of the chair and I thought I was dying.”

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Firefighters arrived and found dangerous levels of carbon monoxide inside of their home.

A broken water heater is to blame.

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The family has since replaced their carbon monoxide detectors.