A horrible complaint , GSDS especially are prone (so I found out) to this and it can prove fatal. However so I eventually found out, it can be managed if caught and treated early, with tlc and medication.
If your dog keeps worrying at his/her anal area and or, there is blood ensure that you check for open holes appearing around the anus, which can weep with pus. Basically it is like an internal spot that makes tracks in the anus area erupting in holes.
Veterinary care is essential, if only for diagnosis and medication. as is regular bathing or use of a damp wipe to keep clean. Around the area will need to be shaved to allow for good air circulation. Sudo cream is a great soother using only a very thin layer .
Change of diet may be required, medication is immune system supressent, and thus cause may have been due to allergens.
There is a forum on yahoo for pf dogs, which were my rock with their support, also my vets has a healthcare page which includes this problem http://www.merialvetsite.com/sites/prcooper/home.html
Hope this proves of some use as I had never heard of it until diagnosis[u]
Sam, Shadows mum now at Rainbow Bridge