Posted on : June 21, 2008
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Introduction
Amebiasis (am-e-BI-a-sis) is an infection that is caused by a one-celled parasite called Entamoeba histolytica.
The infection, which produces an inflammation of the cells lining the
intestinal tract, is also referred to as amebic (or amoebic) dysentery.
Amebiasis often results in relatively mild illness, producing
diarrhea and abdominal pain. However, the infection can be quite
severe, with inflammation being so extensive that the intestinal wall
in the colon can become perforated, and damage can occur to both the
liver and the brain. As well, diarrhea can be copious and often
accompanied by vomiting, which can lead to dehydration if fluids are
not replaced.
Disease History, Characteristics, and Transmission
E. histolytica can occur in two forms. One form is known as a
cyst. This form is very tough, and can survive harsh conditions of
temperature and lack of moisture that would kill the other, growing
form of the organism called the trophozoite. This hardiness makes a
cyst similar to a bacterial spore. The parasite is excreted in feces as
a cyst. It can survive for a long time until it finds itself in a more
favorable environment, such as the intestinal tract of another person.
There, the cyst can resume growth. The trophozoite is the form that
causes amebiasis. Some trophozoites will form cysts and can be
excreted, beginning another cycle of infection.
The cysts can also invade the walls of the intestine, where they can
germinate into the trophozoite forms. Then, ulcers and diarrhea can be
produced. Or, much more seriously, the cysts may enter the bloodstream
and can be carried all over the body. Damage to tissues such as the
brain and liver can result.
When symptoms develop, they tend to begin about 2 to 4 weeks after
the parasite has entered the body, although some people develop
symptoms in only a few days.
Amebiasis has been known since the early years of the twentieth
century. Despite this, the diagnosis of amebiasis has not changed in
over a century, still relying on the visual detection of the cyst in
feces from the person suspected of having the infection. This can be a
tedious and lengthy process, often requiring days of examination.
Complicating diagnosis, the cysts of E. histolytica resemble that of other amoeba called Entamoeba coli and Entamoeba dispar, which are normal and harmless residents of the intestinal tract of warm-blooded animals, including humans. Indeed, E. histolytica and E. dipar are virtually identical in appearance. This means that many cases of amebiasis are likely diagnosed incorrectly.
Scope and Distribution
Some people who are infected carry E. histolytica in their
intestinal tract without displaying symptoms. Since the parasite can be
excreted along with feces, a person can unknowingly pass the parasite
to someone else by handling food with unwashed hands after going to the
bathroom, by person-to-person contact (including sexual intercourse),
or by contaminating drinking water with feces. This route of
transmission can persist for years after a person has been exposed to
the parasite. The persons who subsequently become infected might become
ill.
Amebiasis affects about 50 million people world-wide each year,
making it one of the two most common causes of intestinal inflammation;
the other is caused by Shigella. Approximately 100,000 people
die of the infection each year. Those most often affected are in poorer
health; thus, amebiasis tends to be more common in developing
countries, where sanitation is inadequate and where people live in
crowded conditions, making the spread of the parasite much easier.
However, anyone is susceptible; several hundred cases are reported each
year in the United States, for example. In developed countries, those
who become infected tend to be pregnant women, the young and the
elderly, and those whose immunesystems have become compromised due to
malnourishment or disease (such as acquired immunodeficiency syndrome
[AIDS]).
Treatment and Prevention
Amebiasis is treatable using a combination of drug therapies. Some
drugs generically called amebicides kill the organisms that are growing
in the intestinal tract, while other drugs can lessen the chance that
the infection will spread to tissues such as the liver.
Impacts and Issues
Persons who travel to high-risk countries such as parts of Africa,
India, Latin America, and Southeast Asia, where the infection is
commonly prevalent in some regions (such an infection is described as
being endemic) should take precautions against contracting amebiasis.
Precautions include drinking bottled water or boiling drinking water
for at least one minute, peeling the skins off fresh fruits and
vegetables before eating them, and proper handwashing using soap.
An important issue concerning amebiasis is that the parasite can be
excreted in the feces of someone who has no symptoms of the infection.
In fact, this is true for the majority of people; estimates are that
only one in ten people who are infected actually become sick. While
this is a small percent, the fact that millions of people become
infected each year still means that a great many people become ill,
with many more remaining capable of spreading the infection to others.
Research is ongoing to find more definitive ways of treating
amebiasis, and in preventing the infection in the first place. As of
2007, there is no vaccine for the infection. A blood test is available
that can detect the presence of the parasite. However, because the test
detects the presence of antibodies—molecules produced by the immune
system that are targeted against the particular invading organism—the
test only reveals if someone has ever had an infection, not necessarily
an ongoing infection.
The World Health Organization (WHO) recommends that if the presence
of amoeba in the feces is confirmed microscopically but the person is
not experiencing any symptoms, then it should not be assumed that the
person has amebiasis.
On a larger scale, the WHO is building an international network, now
totaling over 100 organizations, that together aim to reduce worldwide
deaths from diseases such as amebiasis. The group, called the
International Network to Promote Household Water Treatment and Safe
Storage, plans to implement sustainable and affordable methods of
purifying drinking water supplies in communities without access to
sanitation or treated water, or with water that is improved but from
unsafe sources. Although large waterborne outbreaks of amebiasis are
uncommon, water treatment and sanitation measures are complimentary and
are developed together when possible.
In the era of molecular biology, procedures have been developed that can detect the genetic material of E. histolytica
in feces. However, the test is relatively expensive and requires
specialized equipment and training that may not be part of a clinic,
especially in an underdeveloped region.
