Treat all of your dogs this is contagious to any dog that goes near and may possibly come in contact or ingest the infected stool. You can also get infected with it as well, so wear gloves when handling any stool. I suggest going to the store for some Lysol disinfecting concentrate liquid and washing all your floor with this. If you have carpets steam clean them adding this concentrate in the water too. Heres a link with information on it:
http://www.marvistavet.com/html/giardia.html
This is not easy to get rid of because it can lay dormant and because cysts can stick to the fur of the infected patient and be a source for re-infection, the positive animal should receive a bath at least once in the course of treatment.
Wash everything and Bathe them well!!!