Ways To Die: Amoebic Dysentery and Amoebiasis
First off, let’s get this out of the way - dysentery is not always amoebic, and the term “dysentery” refers only to the condition of having bloody diarrhea (bloody flux), caused by an infectious pathogen in the large intestine.
There are viral, protozoan, and bacillary dysenteries, in addition to the amoebic dysentery that shows up so often in tropical medicine. As a side-note, the dysentery that reared its ugly head in Oregon Trail (and on the real Oregon trail) would have been either bacillary (Shigellosis) or amoebic, depending upon the season and the location.
Amoebic dysentery and amoebiasis are caused by the amoebic pathogen Entamoeba histolytica. This single-celled organism is hardy, and able to spread easily, especially in environments where proper sanitation is not practiced.
Infection occurs by fecal-oral transmission, and begins when an encysted parasite is ingested by an individual, due to improper handling of food, water, fecal matter, or poor hand-washing practices. When the organism reaches the stomach, the acid dissolves the tough cyst surrounding the amoeba, and the now-active trophozoite (bottom) moves into the small intestine.
There are several paths that E. histolytica is known to take from here; for 90% of those infected, they’ll experience no symptoms, but will still spread encysted amoebas in their feces, possibly infecting others. Of course, acute amoebic dysentery (center left) is a possibility, and occurs in approximately 10% of infected patients. Many times, the amoebas will lie dormant in the mucosal wall of the small intestine, and not cause dysenteric symptoms until months or years later. Sometimes, chronic, long-term infection can occur (center right), especially when there is no or inadequate treatment. However, that is much more common in bacillary dysentery than amoebic dysentery. Of the 10% who become symptomatic, only 16% will experience severe ulceration and long-term damage of the intestine, and that number is much lower with proper anti-amoebic treatment.
In some people, the amoeba makes its way through the intestinal wall, and into the bloodstream. From there, it can cause amoebic liver abscesses (top), compromising liver function, and sometimes mistaken for cancers. With some Latin American strains of E. histolytica, the semi-dormant amoebae will burrow into the lining of the ascending colon or rectum, and cause a long-lasting cellular response, which eventually can end up forming a large granulomatous mass. Other strains of this amoeba can cause severe swelling and flask-shaped ulcers in the lining of the large intestine.
The most important part of treating amoebic dysentery is continuous rehydration therapy - the rapid loss of fluids severely hinders the activity of the immune system and the body at large. Amoebicides (anti-amoebic medications) are also used these days, to speed recovery. In a typical amoebic dysentery case, the patient will recover within one week, assuming a basic standard of care.
Top: Large amoebic liver abscess protruding from the epigastrum.
Center left: Intestines affected by acute amoebic dysentery.
Bottom: Entamoeba histolytica in the mucosa of the small intestine.
Center right: Intestines affected by chronic amoebic dysentery.
Source: Dysenteries; their differentiation and treatment. Leonard Rogers, 1913.
Diseases of the Oregon Trail at APHL, by Michelle Forman
Dysentery in the Bad Bug Book, United States FDA