Outrage grew online today in response to news that a third horse had died during filming of HBO's critically acclaimed horse racing drama Luck.

The horse died yesterday morning while being walked to a stable after it reared up, fell backward and suffered a serious head injury, the American Humane Association said in a statement. The animal was euthanized at the track. The other two horses died during filming of Season 1 in 2010 and 2011 after being injured during racing scenes. 

HBO has agreed to AHA's demand that all production involving horses shut down immediately while an investigation is conducted.

Animal lovers were out in force on Twitter, some going so far as to call for the show's cancellation. 

"Another horse was killed today on the set of Luck, bringing the total to 3. Can't believe animals are dying for the sake of a dumb TV show," Ari Solomon@VeganAri tweeted. "They should shut it down for good," he added.

"Shame on you!" tweeted another user.

"Dear HBO it is unacceptable that you have allowed 3 horse deaths on Luck. AnimalAbuse Shame on you!" tweeted Stephanie@stefsstuff.

On TMZ, where the news was first reported, users filled the comments section with negative views.

"OMG a third horse dead, awful ...This show is bad luck for horses ... Imma cancel HBO, right after Game of Thrones," a person with the username "The Truth hurts" wrote. 

Though vastly outnumbered by enraged viewers, a few people took the death of the horses in stride. 

"I love horses and all, but what the hell should they have done ... had a memorial service?" user Larry wrote in TMZ's article comments.

Luck is currently airing its first season and filming its second. The TV drama, produced by Dustin Hoffman and starring Hoffman and Nick Nolte, takes a look at the world of horse racing from owners to gamblers, jockeys and industry players.

Tags: Dustin Hoffman