
The problem with giardia is that the dogs reinfect themselves very quickly, so by the time they are on the last day of treatment of Panacur they are already infected again. Metronidazole is the preferred treatment usually for 10 days, it alters the DNA of the protozoa so that they cannot reproduce, so even if they swallow the protozoa it does not get the chance to multiply, you only need 5 protoza to cause symptoms. You could then follow that with a course of Panacur (speak to your vet first)
I am sure that you are practising strict hygiene, but it is such an opportunist protozoa, it could be on their feet, it could be passed from one to the other when they lick around their bottoms and then lick each other, on their toys, it could be on the grass, or they could be picking it up if they are eating rabbit, bird, sheep droppings etc.
Some schools of thought think that once a dog is infected it can lie dormant in the system until the dog is under stress.
It is possible to clear your dogs of the condition but you must clear any faeces up immediately, and what you are doing with the Superkill will ensure that the home/kennel environment is safe. Just sounds like you need metronidazole and a longer course of treatment.
Good luck with it and let us know how it goes. Have had it in my kennels and it nearly drove me mad, took ages to get on top of it, I became quite paranoid about it. Now all is fine and we have not had a recurrence in the last few years. But I still pick all faeces up immediately, our breed is renowned for eating undesirable but tasty morsels!!!