What can I do to help a sick dog?

O.K. Here's the scoop. This is not my dog. I'm looking after it for my in laws while they are on vacation. They failed to mention to me that the dog is sick, and by sick I mean it's literally spraying liquid crap everywhere it goes. It's a big dog too. A chow chow. I have to bathe it frequently. Every time it goes it gets liquid crap soaked into it's fur all over it's backside. It's house trained, but obviously can't hold it in. I cant leave it outside because it's -25 out where I am. I have to keep the poor thing locked in a room of the house which I've cleared out. I'll have to scrub down the room, and disinfect it once this is over. I have no information about their vet. I'm trying to get hold of the owners, but so far no dice. Someone told me they fed it something different than it usually eats, and now all hell is breaking loose. It would be a slight understatement to mention that I am unhappy. Any suggestions?