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Every science class starting in middle school, examines the Amoeba. In this lesson, we will dig into the world of one potentially deadly amoeba, Entamoeba histolytica, and the diseases it can cause. You probably didn't learn about this in Intro Bio!
In the 1950s, a movie, called The Blob, was released about a giant gelatinous ball of goo. The goo oozed around a small town, consuming everything in its path. Sounds like a great movie premise straight out of science fiction, but not so fast, is it really fiction? Is it possible you have a blob inside of you right now? Perhaps millions of blobs, growing and thriving in your gut? They live there peacefully? for now. There is always a small chance that one of those blobs could wreak a little Hollywood-style havoc!
Entamoeba is a genus of amoeboid protozoa that live in the human intestine. Some species within this genus are harmless, while others are pathogenic. One, especially, has the potential to become as dangerous as The Blob, Entamoeba histolytica. It is estimated that up to 15% of the world's population is infected by the pathogen Entamoeba histolytica. Every year, over 100,000 people die of the disease caused by E. histolytica, amoebiasis, making it the second most common parasitic cause of death, after malaria.
The lifecycle of Entamoeba histolytica is pretty typical for a protozoan parasite. Cysts, which are the round, dormant, resistant and infectious stage, are acquired by the fecal-oral route, usually by drinking contaminated water. Once in the intestine, excystation occurs, with trophozoites emerging from the cyst. Trophozoites are the motile, feeding stage of Entamoeba histolytica. These trophozoites are like the blob from the movie.
They are amorphous cells containing prominent round vacuoles that move via oozing pseudopod motion through the length of the intestine, until they reach the large intestine. Once there, they multiply by binary fission and begin to form new cysts. These cysts are excreted in the feces, can survive outside the body for several weeks and are capable of infecting new hosts.
The new hosts are usually infected by drinking water contaminated with the feces of people shedding Entamoeba histolytica cysts. Consuming foods that might have been contaminated by infected water, prepared by people with poor personal hygiene or prepared on contaminated surfaces can also spread the disease, although this mode is far less common. Tropical countries with poor sanitation are often hit hardest by outbreaks. People traveling to these countries should take care to only drink bottled water, commercially sealed beverages, or tap water boiled for one minute or filtered through a one-micron filter. Thorough hand washing and bathing is usually enough to eliminate the threat of infection from contaminated surfaces or contact with unhygienic people.
No need to start boiling your water at home, however. In the United States, Entamoeba histolytica is not nearly as prevalent as the rest of the world. Only about two percent of the population is infected, and most of the cases are in the American Southwest, near the Mexican border.
Now, you know how to prevent acquiring Entamoeba histolytica, but on your recent trip to the Tropical Republic of Amoeba, you just couldn't resist sampling some of the local cuisine. After you hit several questionable roadside stands, you returned home and began second-guessing your brave culinary adventure. What can you expect now?
There is a 90% chance that nothing at all will happen. Only ten percent of those infected with Entamoeba histolytica ever develop amoebiasis and its associated symptoms, most of which are very mild. Within two to four weeks of exposure, you may experience brief bouts of diarrhea, accompanied by stomach cramping and abdominal pain. During this time, you can be shedding millions of cysts.
In rare cases, without treatment, the symptoms can become more severe. Entamoeba histolytica is able to survive in the intestinal tract, but can also invade the cells of the intestinal lining. The trophozoites release proteases, which are enzymes that break down protein, causing deep, painful lesions and ulcers in the intestinal lining. What results is a more severe form of amoebiasis, called amoebic dysentery. A patient with amoebic dysentery can experience intense abdominal pain, periodic loose stools with blood and mucus, periodic constipation and fever. Weight loss and fatigue can also occur.
If the amoebic dysentery goes untreated, it can progress to extraintestinal amoebiasis, although this is rare. The trophozoites can migrate out of the large intestine, travel through the blood and invade other tissues, causing the same lesions found in the gut. The most commonly infected organ is the liver, but Entamoeba abscesses have been found in the brain and lungs as well. This is a very dangerous amoeboid infection that can quickly lead to death of the host, much like The Blob!
Since treatment might be necessary to avoid dysentery or extraintestinal amoebiasis, how do doctors confirm an Entamoeba histolytica infection? The most common method is to directly examine the feces for cysts or trophozoites. Cysts are often present in formed stool, but are not shed in every movement, often requiring examination of several samples over several days.
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Trophozoites can often be found in loose, fresh stool. The problem with looking for cysts and trophozoites is that all amoebas look nearly identical. Entamoeba dispar is a close relative of Entamoeba histolytica, but is completely harmless and about ten times more common in the gut. Also, trophozoites don't survive outside the host for more than about an hour, making an accurate, timely diagnosis difficult.
There are blood tests that look for antibodies to Entamoeba histolytica, but these are not without problems. They are not specific for active infections and often pick up evidence of previous exposures. Unfortunately, natural immunity does not result from infection, so reinfection is common. The blood tests are generally only used with a severely ill patient, where extraintestinal amoebiasis is suspected and must be more accurately diagnosed.
Treatment depends on symptoms. If exposure to Entamoeba histolytica is suspected but no symptoms have developed, the antibiotic diloxanide is given. If the infection is actively causing disease, metronidazole and iodoquinol are usually effective at killing the trophozoites in the tissues of the body. Unfortunately, they don't eliminate cysts or parasites still in the intestine, so relapses can be common after treatment ends. Often, drugs that will kill parasites in the intestine, like paromomycin, are given after a course of metronidazole to more fully eliminate the infection.
Let's take a quick look back at the key points of Entamoeba histolytica. Entamoeba histolytica is a protozoan parasite that is the second leading cause of parasitic death, after malaria. The amoeba is acquired by the fecal-oral route. Cysts are commonly found in contaminated drinking water in tropical areas with poor sanitation.
The trophozoite stage is motile and causes disease in about ten percent of those infected. Amoebiasis usually manifests as a mild diarrhea, accompanied by stomach pain. Amoebic dysentery can result from untreated infections, characterized by lesions in the large intestine accompanied by bloody diarrhea and fever. A continued infection can become extraintestinal amoebiasis, in which the trophozoites migrate to other organs, usually the liver, causing more lesions and possibly death.
Entamoeba histolytica is diagnosed by identifying cysts or trophozoites in fecal material. A combination of antibiotics, usually headed by metronidazole, is able to get most infections under control. Reinfection is possible, due to the body's inability to develop natural immunity to the parasite.
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