Dennis Farina, left, and Dustin Hoffman in a scene from the series "Luck". © HBO
"The show's theme is showcasing the dark side of racing, and while it does acknowledge how many thoroughbreds suffer catastrophic breakdowns and how horses are routinely doped, two dead horses in a handful of episodes exemplify the dark side of using animals in television, movies, and ads.
"We refrained from telling the show's producers "we tried to tell you so" and are now in discussions with HBO about how to prevent even more deaths on the show."
The animal sequences in the acclaimed series, starring Dustin Hoffman and Nick Nolte, were filmed under the supervision of the American Humane Association (AHA).
It provides consultants to television and movie production companies, with the aim of carrying its certification that no animals were harmed during production.
The association, in a detailed account on its website, confirmed that two fatal accidents occurred several months apart - one in the filming of the pilot and one during the filming of the seventh episode.
"The two racehorses stumbled and fell during short racing sequences," the association said.
"The horses were checked immediately afterwards by the onsite veterinarians and in each case a severe fracture deemed the condition inoperable.
"The decision was that the most humane course of action was euthanasia.
"An American Humane certified animal safety representative was monitoring the animal action on the set when the incidents occurred and observed the veterinarian on the set perform the soundness checks and approve the horses, prior to racing them.
"A full investigation and necropsy was conducted for each accident immediately afterwards."
The association said it was deeply saddened by the deaths.
"Because of these accidents, the two episodes in question do not carry the full certification, 'No Animals Were Harmed'. The other episodes did obtain the certification.
"No such incidents occurred in other episodes, which did allow them to achieve certification. Following the second incident, American Humane Association insisted that production be suspended and imposed additional stringent soundness protocols.
"These included, but were not limited to, daily training and care records, microchips in all of the horses, hiring an additional veterinarian to do the comprehensive soundness checks at the top of the day, and radiographs of the legs of all horses being considered for use on the show.
"We insisted that these protocols be in place before any filming could resume.
"HBO agreed to all our requests and worked collaboratively regarding these many additional safety guidelines and precautions, and resumed filming once all of the horses in the show stable were radiographed and those deemed at risk were pulled."
The association said the series had many racing sequences as well as milder action with horses in barns, being walked, groomed, bathed, and so on.
"Some of the racing scenes appear to be very intense; horses are seen running at fast speeds, and at times close to one another and the railing that surrounds the track."
An American Humane Association certified representative was present when the vet checked each horse before filming began, after each racing scene, and at the end of each day. All grounds were inspected before each race and cast, crew and takes were limited.
Mandatory daily safety meetings were held each morning before filming began, it said.
"The horses that appeared in the race sequences were racehorses that were conditioned to this racing type of environment. Some sequences required trained movie horses to work with specially designed camera cars.
"A total of up to nine horses were run on the track in any given racing scene. Each horse was limited to three runs per day and was rested in between those runs.
"Each race was shot in separate takes and the horses only ran halfway around the track, or less, in any given take.
"Before each racing sequence, each horse was placed inside the starting gate one at a time. Action would begin immediately after being placed in the gate so the horses would not have to stand still for too long.
"An experienced gate man opened the gate each time. Only experienced jockeys and/or stunt riders rode the horses during the racing sequences. An ambulance and a vet were present at all times, and the vet would follow the horses in a truck during each race. Each horse was accompanied by its own groomer. Wranglers on foot and also on horseback surrounded the area for safety. Each horse had a pony horse assigned to it as a safety precaution.
"All animals were introduced to each other prior to filming. Horses were acclimated to stalls and barn areas and were adequately fed and watered.
"Any injuries, exams or procedures depicted on the screen were simulated with the help of trainers, props, make-up and well-trained animals."
The association said in over 70 years of oversight on thousands of film and television productions, countless animal injuries and deaths have been prevented by its presence on set.
"Sadly, despite all precautions, accidents do occasionally happen, but as long as animals continue to be used in film and television entertainment, the American Humane Association will continue to monitor their treatment and work to ensure their safety.
"We also research ways to continually improve and expand guidelines and safety protocols in order to proactively prevent harm and keep pace with technology and animal science."
"Luck" has already been renewed for a second season.