Amoebic dysentery. Amoebic dysentery is caused by the amoeba Entamoeba histolytica. This protozoan exists in nature in the cyst form and is transmitted by contaminated food and water. In patients, the amoebas revert to trophozoites (feeding forms) and invade the intestinal lining. Then they enter the bloodstream and move to distant organs, such as the liver and lung. Infected individuals pass the cysts in stools and remain carriers for long periods. A drug called metronidazole is used for therapy.
Giardiasis. Giardiasis is a protozoal disease caused by the flagellate Giardia lamblia. The organism is taken into the body in its cyst form in contaminated food and water. In the intestine, the trophozoites emerge from the cysts and multiply along the walls of the intestine. A foul‐smelling, watery discharge accompanies the infection, followed by abdominal pain and diarrhea. Hikers, backpackers, and campers are particularly susceptible, since mountain streams often contain the cysts from wild animals. Metronidazole is used in therapy.
Balantidiasis. Balantidiasis is caused by the protozoal ciliate Balantidium coli. This protozoan enters the body as a cyst, and the trophozoite form emerges in the intestine. Tissue invasion may occur, and diarrhea is accompanied by blood and pus in the stools. Symptoms tend to last for long periods. Patients become carriers. Metronidazole can be used in therapy.
Cryptosporidiosis. Cryptosporidiosis is caused by species of Cryptosporidium such as C. parvum and C. coccidi. The organism invades the intestinal epithelium and induces mild gastroenteritis with abdominal pain and watery diarrhea. Symptoms tend to be very severe in AIDS patients, and the massive diarrhea can be lethal. No treatments are known as of this writing. Water is believed to be the main mode of transmission. Many methods for purifying water permit this organism to pass, and modifications of these treatment methods are now being considered.