Of course gastroenteritis can be caused by other protozoa, bacteria, viruses, other microorganisms, and various chemical toxins. There's more than just fish swimming in there. Giardia lamblia occurs in two forms, trophozoites and cysts. The active trophozoite looks like a flagellated amoeba, which attaches to the lining of the small intestine, causing damage. This form is very susceptible to environmental damage.
Adverse conditions convert it into an infectious capsulated cyst form.
Gastric acidity converts the cyst back into the active form. Cysts are larger than bacteria and can be excluded by most filters. Extensive tests have revealed how cysts are killed by heat and iodine. Cysts can remain viable in cold water for two months. Unfortunately, unless there is very recent data, we do not know the answers, but we do know that infection only occurs if the cysts enter the mouth and then the intestine.