The first confirmed diagnosis of chronic traumatic encephalopathy (CTE) in a fully professional rugby union player has been made in New Zealand. Billy Guyton, who represented the New Zealand Māori, played scrum-half in Super Rugby for the Blues and had stints with the Hurricanes and Crusaders, died in May at the age of 33.

Guyton’s brain was donated to the brain bank at the University of Auckland. On Monday, his family received a summary of the results, the first item of which details “changes consistent with chronic traumatic encephalopathy”. The diagnosis was confirmed by Associate Professor Michael Buckland, founder and director of the Australian Sports Brain Bank, as stage 2 CTE, which sits between mild (stage 1) and severe (stage 4) in the strata of this degenerative disease.

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The co-director of the Auckland brain bank, Prof Maurice Curtis, told RNZ: “The diagnosis of a young person with CTE is significant as it indicates how early in life the brain has been affected by head knocks.”

The only known cause of CTE is traumatic brain injury, be it from one catastrophic incident, such as a car crash, or through repeated blows to the brain over several years, as in a collision sport. Length of time in such a sport is the most powerful association with CTE, but Guyton suffered from multiple concussions and took the decision to retire because of the symptoms he was experiencing in 2018 at the age of 28.

Related: AFL faces years of litigation over ‘messy’ concussion lawsuits, court hears

The summary of the report, seen by the Guardian, also describes the condition of cavum septum pellucidum, a fissure in the middle of the brain associated with traumatic brain injury, and age-related tau deposits, which are not diagnostic of CTE, although often associated, but irregular in a young brain.

“We share the family’s concern at his diagnosis,” said New Zealand Rugby. “Any time the rugby community loses a member, especially someone as young and talented as Billy, it is felt deeply. NZR is concerned about the possibility that repeated head impacts during participation in rugby may contribute to neurodegenerative diseases in later life.”

CTE can currently be diagnosed only postmortem, although science is moving closer to a diagnostic method for the living. Dozens of former professional rugby players, including the England World Cup winner Steve Thompson, the Wales international Alix Popham and the All Black Carl Hayman, have been diagnosed with suspected CTE.

They have joined hundreds of other former players in taking action against the governing bodies of World Rugby, the Rugby Football Union and the Welsh Rugby Union, claiming they were neither looked after properly during their playing days nor informed of what was known about the links between repeated head injury and CTE.

Guyton never played rugby professionally in England or Wales, so his family is not eligible to join the action for now. Reports in New Zealand said Guyton is suspected to have taken his own life. His death was referred to the coroner at the time.

“All who care about collision sports have to accept we will witness over the next few decades many more players dying prematurely from their neurodegeneration,” said Richard Boardman of Ryland’s Garth, who is representing the players in their lawsuit.

“Many more will be found to have, postmortem, CTE. There is an urgent need for these sports to limit the amount of contact players are sustaining over 30-game, 11-month seasons. Playing elite rugby is as dangerous for the brain as ever.”

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