Giardia lamblia ( synonymous with Lamblia intestinalis and Giardia duodenalis ) is a flagellated protozoan parasite that infects the gastrointestinal tract and causes giardiasis . Giardia affects humans , but is also one of the most common parasites infecting cats , dogs and birds . The names for the human parasite Giardia duodenalis , Giardia lamblia and Giardia intestinalis are all in common current use despite the potential for confusion that this has created .