The first was in 2013, a year after Herfel moved from California to Wisconsin.
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"She put her nose on my lower belly and sniffed so intently that I thought I spilled something on my clothes. She did it a second time and then a third time. After the third time, Sierra went and hid. I mean hid!" Herfel told the Journal Sentinel.
Herfel had been feeling pain in the area, but an emergency room doctor said it was an ovarian cyst and gave her pain medication.
But the dog knew better than the ER doctor. The dog acted oddly, got herself in a tight ball and hid deep in a closet.
A few weeks later, after taking the signals from Sierra, Herfel saw her doctor and got the diagnosis of stage 3C ovarian cancer, the Journal Sentinel reported.
She underwent a full hysterectomy and spleen removal. She was also on chemotherapy until April 2014.
But that wasn’t the only time Sierra signaled her owner that something was wrong.
In 2015 and again 2016, Sierra hid again and both times Herfel took her dog's behavior as a signal to contact doctors, according to the Journal Sentinel.
In 2015, she was diagnosed with liver cancer and in 2016, it was cancer in her pelvic area.
Herfel's oncologist, David Kusher, told the Journal Sentinel it isn't odd for some dog breeds to be able to sniff out cancer. Some can find it 98 percent of the time and it can be a variety of cancers.