Giardia symptoms persist after second treatment

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...

Giardia symptoms persist after second treatment