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Protozoal Microorganisms as Intestinal Parasites 53

into the small intestine where excystation (“hatching”) found in the infected tissues of the intestines but cysts
takes place. The “freed” cyst then develops into a tro- are never found in these conditions.
phozoite form and proceeds to multiply through binary
fission in the lumen (tubelike opening of the colon). Tro- Laboratory Diagnosis
phozoites may then initiate a process called encystations,
and immature cysts are excreted where fecal contamina- It may be difficult, both for the beginning student of par-
tion begins the infective cycle that would impact other asitology as well as for an experienced laboratory techni-
animals. Both immature and mature cysts are found in cian or technologist, to identify trophozoites of amoebae.
the feces along with trophozoites, a stage that is most Features of single organisms, including observation of
prevalent in watery stools. Some trophozoites may in- stained specimens for nuclei and the overall cytoplasmic
vade the colon’s wall where they multiply and pass into appearance, cannot often be made without examining a
the vascular system to infect organs outside the colon. representative number of organisms before arriving at
a conclusion. Several different features and several dif-
Disease Transmission ferent individual organisms must be examined before a
presumptive identification may be made. Cyst forms of
E. histolytica is one of the most important and most widely amoebae are usually less variable and can be more easily
distributed human protozoans to infect humans. E. dispar identified under normal conditions. Certain features that
is considered by some medical practitioners a nonpatho- are peculiar to only one species may be helpful. In addi-
genic strain of E. histolytica. Food and water contaminated tion, other findings such as Charcot-Leyden crystals for
with amoebic cyst and sexual intercourse involving anal disintegrated white blood cells called eosinophils may
penetration are the sources of most cases of amebiasis. provide clues that parasites may be present.
Asymptomatic carriers, particularly food handlers, may Features that distinguish the differences between
transmit the disease to significant numbers of victims. The trophozoite and cyst stages of intestinal amoebae are
incubation period before symptoms arise may range from used by experienced parasitologists to aid in identifying
several days to several months. Symptomatic patients often the various organisms. Remember, there are even cases
have diarrhea and abdominal pain. With the progression of where some parasitic organisms do not exhibit both
the infection leading to dysentery, blood may be contained stages of development, and this is also used as a valuable
in the feces. Ulcers sometimes occur in the appendix and piece of information for finding and identifying some
all parts of the colon. Symptoms may mimic ulcerative of these organisms. In addition, a great many artifacts
colitis and diverticulitis, leading to erroneous diagnoses. may appear as parasites, but on further examination
When E. histolytica trophozoites pass through the colon will turn out to be harmless elements or artifacts from
walls and enter the circulatory system, the development of the diet and from the environment. The following
abscesses may occur in diverse tissues and organs of the table, Table 3-3, will identify features of both cyst and
body, including the liver and the brain or lungs. trophozoite stages of amoebae found in the intestine.
Fulminant colitis due to amoebal infection is a Descriptions of the features for differentiation are used
condition indicated by a severe and sudden onset. This comparatively.
condition is characterized by high fever and intestinal In active dysentery, no cysts of E. histolytica are
bleeding and sometimes perforation of the colon. The found in the feces but if there is little diarrhea that could
transfer of intestinal contents would release a variety of quickly result in the organisms being quickly expelled,
bacterial organisms into the peritoneum that would re- the parasites have time to encyst in the tissues of the
sult in the patient becoming seriously ill. This condition digestive system. Since excretion of the parasites is in-
will often be manifested by a distended abdomen and a termittent, it is best to carry out three different stool
form of intestinal paralysis where peristalsis is halted. analyses before deciding upon a negative result. Some-
A fulminant course may occur when patients are misdi- times it is easier to reveal the parasites in a stool ob-
agnosed as suffering from Crohn’s disease or ulcerative tained by means of a purgative medication or a laxative.
colitis in which the patient is treated with steroids. An- Other tests for identification of Entamoeba histolytica
other condition that occurs in rare cases is a condition antigen in the feces have been developed, but further
called amoeboma in which a mass may occur, resulting in evaluation may be needed in order to determine the
a diagnosis of colon cancer. Countless trophozoites are validity of these tests. These tests may, however, permit