I had worse in the house we moved into 6mths ago. We had poo but of the human variety, smeared on the bowl AND the toilet seat. Just vile, i ended up ripping out the toilet seats. The downstairs toilet had carpet which was actually wet with piss, my lovely BIL ripped that up for me as I couldn't face it.
We also found black mould all inside the integrated fridge freezer and the kitchen which was only a few years old had never been cleaned. We found the empty silicon tubes on the top of the cupboards from when it was put in. The tops were also thick with grease.
I did cry that first night at the filth.
We had to scrub everything before painting as it was minging, it was like the previous owners thought sod it i'm not cleaning anything again til we move.
oh and they had painted round the furniture as well, lovely yellow skirting boards were the sofas etc. had been.
As it was just cleanliness we couldn't really do anything but I think it should be in the contracts to get the house professionally cleaned before hand over.
Discovered though that both neighbours on either side didn't really like the previous owners, who had been here 20 years. They like us though, So i've had a good bitch about her to them !