GIARDIASIS -- Giardiasis is caused by a protozoan of the giardia species.  Giardia are one-celled  organisms that live in the small intestines of dogs and cats.  Dogs get the infection from drinking water from streams or other sources contaminated with infective oocysts.  Young dogs can develop diarrhea that may be acute or chronic, intermittent or persistent, and may be accompanied by weight loss.