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Epidemiology and Conditions Leading to Parasitic Infestations 23

Causative agents may be the well-known “amoeba,” of that are sometimes misidentified or confused with a patho-
which several species are quite prevalent. As an intro- genic organism such as Entamoeba histolytica.
duction to some of the names, which will be discussed at The one-celled amoebae also release an enzyme that
length later in this book, some of the better known para- causes ulcers or abscesses where they enter the blood-
sites will be listed as follows. Of the amoebae that inhabit stream. They can eventually reach other organs including
the human body, a species called Entamoeba histolytica is the brain or liver. Cryptosporidial infections are usually
possibly the most common pathogenic amoeba encoun- associated with waterborne outbreaks. Victims of this ail-
tered. Another of the amoebae is Endolimax nana, which ment often experience diarrhea and abdominal pain lasting
is also found frequently even in the more advanced parts of for about 10 days. The protozoal organism called Giardia
the world. Several different species of these organisms may is the most prevalent intestinal parasite in humans and is
infect the human body simultaneously. Another category found in drinking water throughout the world. Organisms
that includes common parasites is that of the flagellate of the genus Giardia reside in the smaller intestine and at
class that chiefly includes Giardia lamblia and Trichomo- times in the gallbladder and millions of these organisms
nas hominis. In addition to the flagellates Trichomonas will coat the intestinal walls, preventing the absorption
and Giardia, there is another general class called a ciliate, of nutrients and later causing illness. Symptoms include
but the most common type of parasitic infestation in some mild to moderate abdominal cramps, intestinal gas, light-
parts of the world are the heminths, which are worms. colored stools, poor absorption of nutrients, weakness,
Other causative agents commonly found world-wide other chills, stomach bloating, flatulence, and diarrhea.
than worms and protozoa include those of Blastocystis Trichomonas vaginalis organisms are a class of
hominis, Microsporidium, and Cryptosporidium. pathogens that commonly reside in the vagina of females
The term protozoa refers to single-celled parasites that and the urethra, epididymis, and the prostate gland in
inhabit various parts of the body, some of which are capable males. In women there is sometimes a yellowish dis-
of causing serious damage, whereas others may be scarcely charge accompanied by itching and burning. The or-
noticed by carriers of the organisms (Figure 2-9). These ganism is almost exclusively transmitted sexually and is
forms of parasitic organisms include amoebae, protozoa quite common in those with multiple sex partners. The
infections, toxoplasmosis, cryptosporidiosis, giardiasis, female will most commonly become aware of the illness
sarcocystosis, and Trichomonas vaginalis. Amoebae are first and it is medically necessary to treat both the male
irregularly shaped microorganisms that infect chiefly the and the female who are in a sexual relationship when one
end of the smaller intestine and colon. Amebiasis is the most is diagnosed with the infection.
common protozoal infection by a pathogenic species and is
caused by the species Entamoeba histolytica. It is necessary Bloodborne Protozoa
to identify nonpathogenic protozoa such as Entaboeba coli
Malaria is the most prevalent and debilitating disease
on a world-wide scope and is also caused by a protozoa
category of parasite. Malaria is caused by one or perhaps
Source: Centers for Disease Control and Prevention (CDC) parum. These organisms are found in various geographic
several of the four strains of the genus Plasmodium, those
of Plasmodium vivax, P. ovale, P. malaria, and P. falci-

areas throuhout the world, and the potentially most malig-
nant species is P. falciparum. Some of these and in some
cases all four strains may be found in one area of the world,
particularly in Southeast Asia. As many as half a billion
people worldwide may be infected by malaria and about

There are a number of organisms that may cause malaria
in animals. Several species other than the four known
human strains are known to infect animals and birds, but
FIGURE 2-9 Cyst of the protozoan parasite, two million people worldwide die annually from malaria.
Entamoeba coli these strains are not yet known to be passed on to people.