Giardia lamblia protozoan, coloured scanning electron micrograph (SEM). This is a single-celled parasite of the intestinal tract. It is most common in tropical regions. This is the trophozoite (active, feeding) life stage of the protozoan. The cell has flagella (hair-like structures), which are used for locomotion. Giardia lamblia is the causative agent of giardiasis, an intestinal disease that causes abdominal pain, diarrhoea and nausea. It is spread through contaminated food and water. Treatment is with antibiotics or antiprotozoal drugs, such as metronidazole.