its because the anal glands are full and not emptying when your dog takes a shit,it was happening to my dog and the vet tolled me its because i was changing her diet to offten and that i should stick to one brand of food.
Here is my 1/4 pit, 3/4 pat, Burt.What do use people think.
hey earthdog that photo is off google images
http://images.google.co.uk/images?gbv=2&hl=en&q=patterdale+terrier&sa=N&start=&ndsp=20