The Walking Dead star Jon Bernthal has criticised officials at a U.S. university for permitting medical research on dogs.

The actor has sent a letter to M. Roy Wilson, the president of Wayne State University in Detroit, Michigan, to voice his anger at the "cruel" and "unnecessary" experiments.

Bernthal even attached a picture of his own rescued dogs, Venice and Boss, to the letter.

He writes, "As a pit bull rescuer and owner of two (Boss and Venice), I am deeply saddened to learn that Wayne State University is performing cruel, unnecessary heart failure experiment on dogs-experiments which have produced no benefits to human health throughout their two decade history... Dogs serve us all better as companions and as family members rather than experiments."

Wayne State University officials have come under fire for carrying out a series of experiments on canines to research heart failure in humans.