Giardia lamblia ( synonymous with Lamblia intestinalis and Giardia duodenalis ) is a flagellated protozoan parasite that infects the gastrointestinal tract and causes giardiasis . Giardia lamblia ( synonymous with Lamblia intestinalis and Giardia duodenalis ) is a flagellated protozoan parasite that colonizes and reproduces in the small intestine , causing giardiasis .