The former scrum-half has been diagnosed with a form of motor neurone disease.
The 40-year-old's publicist has released a statement revealing that the 89-cap 1995 World Cup winner, a former South Africa captain and their joint-record try scorer, was suffering with "serious" symptoms.
The diagnosis was made after Van der Westhuizen complained to his doctor of a pain in his arm, and he was sent for a series of tests. He has since seen two neurologists in order for treatment to begin.
Mr Hoskins said: "We only have the media reports to go on at this moment, but the news about Joost's health is distressing in the extreme.
"The thoughts and prayers of SARU, the Springboks and the South African rugby community are with Joost and those nearest and dearest to him at this challenging time.
"He was an outstanding Springbok and is a legend of our game - he will always have a place of honour in the history of Springbok rugby.
"He gave his all on the field for the teams he represented and - at his prime - was the best scrum-half in world rugby.
"His current health problems are the cruellest twist of fate for one who was so athletic in his prime."
Van der Westhuizen has endured some difficult times during the last few years.
In 2009 he was admitted to hospital after suffering a suspected heart attack after watching a Test between South Africa and the British and Irish Lions, while his marriage came to an end in 2010.
His former South Africa team-mate Andre Venter has also suffered with a form of motor neurone disease, which has recently seen him confined to a wheelchair, while former London Irish wing Jarrod Cunningham died from the disease aged 38 in 2007.
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