Cat Breed Info & Feline Health Care Questions > recently bought kitten diagnosed Giardia

I recently bought a kitten who was diagnosed with Giardia. We treated her with metronidazole and Albon liquid we did that for two weeks. Then My oldest cat which is 11yrs old caught it and we treated him for 14days on metronidazole and 7 days on Albon liquid. They seemed to have gotten better, well a couple of weeks ago I was cleaning out the litter box and I noticed that their feces look normal but still have blood in them. I also have another cat that is 2yrs old and she had shown no signs of infection when the other cats were sick. But she also has blood in her feces as well, now could they all have giardia again or could it be something else. They all act healthy and their eating habits are normal they play together and everything but they all have blood in thier feces. For the female kitten she was given 35Mg/ML of metronidazole and 50mg/ml of Albon Liquid. and the male who is 11yrs old was given 200 mg/ml of metronidazole and 1ounce of Albon liquid. I was under the impression that metronidazole is guarteed to get rid of Giardia. But I had to end up treating both animals twice now the 2yr old female was never treated because she never showed signs she didn't have diarea or vomiting her eating habits were normal so if it is not giardia then what else could it be?? That would cause all three cats to have blood in their feces. Also when I was treating the other two cats for giardia I had them in seperate rooms and they each had their own liter box to use and I constantly disinfected the liter box with Chlorox mixed in a spray bottle with water. So if it is Giardia how did the other cat catch it and not show signs of infection.