Giardia symptoms persist after second treatment

[Xentik]

My friend 2 chinchillas have been diagnosed with severe giardia infection about three and a half weeks ago. They have been put on Tiabendazole (every 24 hours for 5 days, 2 courses with a 9 days break). A day after the first course some of the symptoms went away (irregular poops, mild diarhae, smelly poop, bloating and constipation) but later they came back once again, especially bloating, constipation and smelly poop, so the chins got treated with a second course of the cure. Yesterday they got the last dose and their condition is OK, they eat and drink fairly well, the poops arent perfect, but not that soft, neither that much smelly as they used to be, but some of the symptoms still persist, like: small holes in the fecals, excessive stretching (maybe due to stomach pain/bloating, I can't really tell, but they developed the habbit to chew their fur on the stomach). They feel noticeable better than they used to, but we are afraid the treatment was not completely effective and the chins will get worse once again. In a 3 days the chinchillas will get their second poop exam. The cage and the environment is being cleaned daily with a combination of boiling water and vingear, they drink bootle got replaced too.

I have a question for people who have been dealing with giardia, can the symptoms of infection persist even after successful treatment?

This is honestly super frustrating for us.