Anchorage - For the first time in the history of the world's most famous sled dog race, several of the high-performance animals have tested positive for a prohibited drug. But race officials have refused to name the musher involved.Several dogs tested positive for the opioid pain reliever Tramadol, the governing board of the nearly 1 600km Iditarod Trail Sled Dog Race said in a statement. The team was tested six hours after finishing the race in Nome in March, officials said. They estimate the drug could have been administered between 15 hours before the test and right before it.Officials say they likely could not legally prove intent on the affected musher's part. Officials did not immediately respond to requests for comment, and to say whether the musher faces discipline.Iditarod board member and musher Aaron Burmeister said on Wednesday he doesn't know the musher's identity. However, only the first 20 teams to reach Nome are tested, he noted.Everyone seems guilty as long as the affected team remains unknown, said Burmeister, an Iditarod contender who sat out the past two races."It's not a good situation," he said on Wednesday. "I'm hoping that we can turn a positive light on it and the musher steps forward."The Iditarod began testing sled dogs for prohibited substances in 1994. Dogs on all teams are subject to random testing between pre-race examinations and along the race trail. Testing in Nome for top finishing teams, however, is not random but expected.As a result of the positive test findings, the race rule dealing with canine drug use was revised earlier this month to hold mushers liable for any positive tests in future races unless they can prove the results occurred outside their control. Previously, the rule could be interpreted to require that race officials provide proof that a musher intended to administer the prohibited substance.Hot topicThe doping is a hot topic among fellow mushers, Burmeister said. As to why a musher would give dogs prohibited drugs when testing is expected in Nome, there has been speculation that perhaps a competitor could have administered the drug. Mushers fly their team's dog food to checkpoints along the trail up to two weeks in advance, and it sits there until mushers arrive at the checkpoint and use it.But Burmeister isn't buying that theory. "As a musher, why would another musher give their competitor a performance-enhancing drug?" he said.Burmeister said several times in a phone interview that no other dogs have ever tested positive in the race."I just hope that people look at the big picture and realise that mushers out there are not doping their dogs," he said. "This is an isolated incident."