Next time you go in(and I hope you do), ask for the most senior manager in charge.
Politely explain your position; That you've been coming in for weeks, seeing this thing get worse and worse. Point out the obvious health problems that need to be addressed let them know that you only want to save an animal from what seems to be certain death.
If the managers shrugs you off or continues to make excuses, I would ask to speak to their manager, or for their manager's number.
If they are unwilling to give you that information, then I wold politely warn them that they will be recieving a call from their "animal care coordinator". Record the names of all employees who gave you trouble and include them in a well-written(polite) complaint letter.
Trust me; Everyone has a boss, and nobody likes it when an angry customer goes over their heads and complains directly to their boss... If you aren't getting what you need from that person, move up the ladder to his boss.
You might end up speaking to some district branch manager who has 16 stores under his supervision; Imagine the bad day that incompetent petstore manager will be having when they get chewed out because they let too many things slide, and some angry customer called his boss's boss's boss over in Michigan... Ooohoohooh.... The trickle-down effect can be a great tool when you come up against a roadblock in the form of a troublesome employee...