"There is no longer a debate that when an athlete is diagnosed with a concussion caused by a sharp blow or a fall, there is a chance it may contribute to brain changes that could either be temporary or permanent," said Dr Ravi Menon, of Western University, Canada.
"But what are the effects of the smaller jolts and impacts that come with playing a contact sport? Our study found they may lead to subtle changes in the brains of otherwise healthy, symptom-free athletes."
They looked at 101 female college athletes aged between 18 and 23 over a number of years, some of whom played rugby, while others were rowers or swimmers.
MRI scans on the rugby players found abnormalities in the white matter of their brains that contained nerve fibres linking areas that governed emotions, becoming more severe over time in some cases. The brain stem, which sends messages to the rest of the body, changed in some athletes between the in and off-seasons of the sport.
Memory and visual processing were also seen to experience change, while the brain scans of the swimmers and rowers stayed normal.
"Even with no concussions, the repetitive impacts experienced by the rugby players clearly had effects on the brain," said Dr Menon."
More research is needed to understand what these changes may mean and to what extent they reflect how the brain compensates for the injuries, repairs itself or degenerates so we can better understand the long-term health effects of playing a contact sport.
"Links between brain damage and contact sport have become more accepted in the UK in recent years.
In January, a foundation set up in the name of Jeff Astle, the former England and West Bromwich Albion player, 18 years after his death, took steps to lobby for dementia to be recognised as an industrial disease in football. His death was ruled by the coroner to have stemmed from a career of heading footballs, with hundreds of families of ex-players with dementia coming forward in the years since.
Though much was now accepted about the dangers of a career in contact sports, the study raises concerns over how quickly damage can set in. It found that 70 per cent of the rugby players who took part saw changes during just two practice sessions and a pre-season game.
Dr Menon said: "While we only looked at these impacts during a few events during the season, previous research has shown that these kinds of sub-clinical impacts may accumulate over years of participation in contact sports."
