The British Horseracing Authority is planning a full assessment after the deaths of seven horses over the four days of the Cheltenham Festival.
Long Dog became the sixth fatality after he broke down in between obstacles in the Albert Bartlett Novices' Hurdle on the final day of the meeting, while Jonjo O'Neill's Montdragon was pulled up in the Martin Pipe Conditional Jockeys' Hurdle and later collapsed and died.
Long Dog was the second horse trainer Willie Mullins and owner Rich Ricci lost during the week after Pont Alexandre was injured in a fall in the National Hunt Chase on Tuesday.
The Govaness and Rezorbi also suffered fatal injuries on the opening day of the meeting, while it was confirmed on Thursday morning that No More Heroes had been put down after damaging a tendon in the RSA Chase on Wednesday.
The death of Niceonefrankie after a late fall in the Brown Advisory & Merriebelle Stable Plate took the tally to five on Thursday.
Quincy Des Pictons was also injured in that race and taken to a veterinary hospital for further treatment.
Trainer Alan Jones said: "As far as I know he's comfortable. There's no negative news anyway."
Jenny Hall, chief veterinary officer of the BHA, highlighted that four of the fatal injuries, including Long Dog and Montdragon, were not caused by a fall at a fence or hurdle.
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[/raw]Sad News
— HorseRacing Memorial (@HorseRacingRIP) March 18, 2016
LONG DOG Age6
Trainer Willie Mullins
Owner Mrs S Ricci
Sadly Lost 1450 Cheltenham
Sincere Condolences
RIP pic.twitter.com/vzsKkd0RuA
She said: "First and foremost our thoughts today are with the connections of Long Dog and Montdragon, in particular all of the staff who will have loved and cared for the horses during their lives.
"It is notable that four of the fatal injuries incurred this week were not related to a fall over a hurdle or fence. Horses are at risk of serious injury throughout their lives, regardless of the type of equestrian activity they participate in, even when turned out in a field, exercising at home or, in the case of thoroughbreds, doing what they were bred to do, namely racing on the track.
"It is not possible to eradicate risk completely from any activity in which horses are involved.
"A recent study by Liverpool University found that 62 per cent of 'traumatic injuries' (ranging from grazes to fractures) suffered by a sample leisure and competition horses occurred when turned out in the field, compared to only 13 per cent during ridden exercise. Meanwhile the fatality rate on British racecourses continues to decrease, it has reduced by a third in the last 20 years to 0.18% of all runners in 2015, the lowest on record, and 0.39 per cent for Jump racing.
"We will work with Cheltenham to assess all of the incidents that took place this week. We have a good relationship with the RSPCA and World Horse Welfare, and, as always, we will work with them to ensure we continue we do all we can to make racing as safe as possible."
Speaking earlier in the week, RSPCA equine consultant David Muir admitted the deaths were "deeply concerning" but was keen to assess each case individually, rather than make a knee-jerk reaction to the incidents.
He said: "Obviously the deaths are deeply concerning to us, but we need to consider each fatality individually before making an informed comment.
"I look at the fatalities once the meeting is over and the first thing I do is satisfy myself that the course was prepared in the right manner and from there we look at the issues relating to each death."
