You are not going to believe this but I am actually having the same problem MYSELF, not my dog. It is related though so bear with me. I have had diarrhea for 5 weeks now and have lost 20 lbs (THe good part of all of this as I am very overweight). Dr.s do not know what it is and I am scheduled for a colonoscopy on May 1st. It started with a fever and chills on March 12 and though the fever went away in 8 hrs, the diarrhea has stayed with me ever since. Now here is where it is related. The dr thinks I may have giardia which is a parasite that dogs get. My dog could have licked me on the mouth (he is a poop eater) and transmitted the giardia to me. I try to not let him lick me on the mouth, but sometimes they lick your hands, you forget to wash before eating and then you can get germs or parasites. Now Giardia can be difficult to detect. Fecal samples can come up negative several times before they find it. It has to do with the life cycle of the parasite. It seems it is easier to detect the cysts (eggs) than it is to detect the actual parasite and if they are not in an egg state when the fecal sample is tested, you will not see them. I am now being treated with Flagyl in case it is giardia and my vet is treating Mai Tai. Some dogs get the parasite without having any symptoms so Mai Tai is being treated to prevent a recurrence in me or my family. So my point is your pet may have giardia. Your vet may have given your dog an antibiotic that kills giardia but sometimes it takes 2 to 3 courses of antibiotics to kill it. On the other hand, my dr. says it could be a virus and that some of them can last 6 weeks. Anyway, sorry for the long gross story, but I think it is worth asking your vet about. I wish you the best of luck. Now I have to sign off and go to the bathroom...LOL