Another possible source of amoebic dysentery is keeping tropical fish. Several years ago I was diagnosed with this, and the physician was puzzled how I could contract a disease that's normally associated with the tropics in Maryland... in the winter. What I remember most is losing seven lbs. in five days.
We traced it to the blackworms I was feeding to my fish. These water-dwelling annelids are cultured under not-so-sanitary conditions and could easily have been carrying something nasty. What likely happened was that I failed to wash my hands completely after feeding some to the fish.
I was put on a drug called flagyl (Metronidazole), a harsh antiprotozoal medicine noted for inducing nausea and leaving a constant metallic taste in ones mouth. I also experienced a less common side-effect, depression. If I remember right, it cleared up in about ten days.