An 8-year-old dog in the team of back-of-the-pack musher Terry Williams of Fairbanks died Wednesday night in the Yukon Quest International Sled Dog Race.

Williams scratched in the Carmacks checkpoint at 8:21 p.m. Wednesday after the dog named Bags died. Williams and fellow rookie musher Jocelyne LeBlanc had been running well behind the rest of the field.

The dog apparently had heart disease undetected by veterinarians, Yukon Quest head veterinarian Kathleen McGill said on Thursday. A precise cause won't be know until the dog is inspected by a pathologist, she said.

A dog has not died on the Quest trail since 2007, when three perished. Another dog died in 2002, with none in between.

Bags died before Williams reached the Carmacks checkpoint, 177 miles from the finish of the 1,000-mile race in Whitehorse.

McGill completed a necropsy early Thursday morning. She said the heart problem couldn't be detected in the animal's heartbeat.

Dogs are checked before the race and during mandatory layovers at the Mile 101 Steese Highway, Eagle, Dawson City and Braeburn checkpoints.

"All of the checks along the way indicated a healthy dog," McGill said, adding that the condition was uncommon and could be a birth defect.

Bags was the first dog to die during McGill's tenure as head veterinarian from 2004-06 and the last two races.

Williams, 37, was a rookie. Race marshal Hans Oettli said his dog care was exceptional.

"He was very caring person, took extremely good care of his dog team," Oettli said. "He definitely did not push the dog team."

Sled dogs can live as long as 14 years, McGill said, and they typically retire from racing around 11 years old or earlier.

The final pathology report will be given to Williams in about two weeks.

Williams was the second musher to scratch. Gerry Willomitzer of Shallow Bay, Yukon, scratched last week in Dawson City.