Depression is common, debilitating, and sometimes fatal, and it can certainly be seen among animal advocates, whose activism forces them to continually reflect on the immense tragedy that befalls so many helpless animals.
One question: You said, "don't ask if they are feeling better today." I can see that this phrase suggests that the questioner is hoping for an affirmative response, which would encourage depressed people to say "yes" when they really are not feeling better. It would seem to me that a more neutral question such as, "How are you feeling?" would be OK. Do you agree?