The answers seem sort of trivial: using the same ethical framework that informs your decision-making process on other ethical questions. Regardless of the unique flavor of one's moral tradition, most seek to limit the suffering of sentient beings.

Most mammals have a similar brain architecture. Humans share the limbic system with animals like dogs, cows and pigs. This means these animals have extremely similar subjective experiences of pain, fear, joy and other emotions.

For a consistent framework of ethics, these animals should be treated essentially as lobotomized humans.