Also notable is the presence of hydrogenosomes, which are also a characteristic of other trichomonads. Leukorrhea is also caused by trichomonads, a group of parasitic protozoan, specifically "Trichomonas vaginalis". Some related trichomonads do form cysts resistant to drying. A hydrogenosome is a membrane-enclosed organelle of some anaerobic ciliates, trichomonads, fungi, and animals. Generally parasitic trichomonads cannot survive drying out. Members of this order are referred to as trichomonads. Because the trichomonads do not survive for long outside the bird, transmission must occur rapidly.