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Washington (AFP)

As a new American football season is set to begin, researchers examining the brains of deceased NFL players have found that 99 percent of them showed signs of degenerative disease -- believed to be caused by repeated blows to the head.

Researchers found striking evidence of chronic traumatic encephalopathy in 110 of the 111 donated brains of players who played in the National Football League, according to the study published Tuesday in the Journal of the American Medical Association.

CTE causes symptoms including memory loss, vertigo, depression and dementia. Problems can crop up years after a player's career has ended.

The NFL has faced growing scrutiny in recent years linked to the issue of concussions and head trauma, with the league agreeing in 2015 on a $1 billion settlement to resolve thousands of lawsuits by former players suffering from neurological problems.

Along with NFL players, researchers also examined the brains of those who had played in high school, college, semi-professionally and in the Canadian Football League.

Of the total 202 players examined, the authors of the study from Boston University found that 87 percent of the players -- whose median age at death was 66 years old -- showed signs of CTE.

"These findings suggest that CTE may be related to prior participation in football and that a high level of play may be related to substantial disease burden," the study authors wrote.

The most acute evidence of the degenerative condition, which currently can only be diagnosed post-mortem, was found among those who played at the highest levels, with 86 percent of professional players having severe forms of CTE.

Though the research -- the largest CTE study published to date -- suggests the disease may be related to prior participation in football, researchers cautioned against extrapolating the results to the general population.

Because the brains studied were for the most part donated by concerned families, they do not necessarily represent all people who have played the rough contact sport.

- Traumatic impact -

Head injury risks in the US sport have become a major concern as former players speak out about long-term health effects, including erratic behavior and mood disorders.

The issue sprang to the fore after Junior Seau -- considered one of the greatest linebackers of all time -- shot himself in 2012 at the age of 43.

A post-mortem study of his brain showed that he had been suffering from CTE.

Another former player, Dave Duerson -- who killed himself in 2011 -- was also found to have the condition, a possible explanation for his mood swings and depression.

Last year, an NFL official acknowledged for the first time a clear link between football and CTE.

In recent years, the league has donated money to concussion research and stepped up regulations in an attempt at minimizing the potentially traumatic physical impact of the game.

In a statement widely published in US media, the league said it appreciated the study "for the value it adds in the ongoing quest for a better understanding of CTE."

"The NFL will continue to work with a wide range of experts to improve the health of current and former NFL athletes," the statement said, noting that "there are still many unanswered questions relating to the cause, incidence and prevalence of long-term effects of head trauma such as CTE."

"The NFL is committed to supporting scientific research into CTE and advancing progress in the prevention and treatment of head injuries."

© 2017 AFP