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Protozoal Microorganisms as Intestinal Parasites 69

material have also been implicated in widespread out-
breaks of diarrhea in recent years. Municipal water sup-
plies may sometimes become contaminated with many
organisms, including E. dispar and other organisms
such as Coccidia, that are not filtered out in the treatment
plants. Standard chlorination levels by water treatment
plants do not control this organism and levels of up to
Source: Centers for Disease Control and Prevention (CDC) est waterborne outbreak most likely in U.S. history. This
30 times what is normal are necessary in water supplies
to destroy the organisms.
In 1993, Milwaukee, Wisconsin, suffered the larg-

outbreak of Cryptosporidium occurred when one of the
water purification plants and the associated water tower
became contaminated and treated water showed tur-
bidity levels well above normal. Most of the coccidian
infections in man are zoonoses, a term indicating that
a distinct possibility of contracting the disease from in-
fectious animals exists. Immunocompetent individuals
usually suffer from mild, self-limiting infections that are
FIGURE 3-20 Cryptosporidium parvum oocysts
sometimes not even noticed to an appreciable extent,
(i.e., encapsulated zygotes), which have been stained
but for those with an immunocompromised defense
using the modified acid-fast method
system, the symptoms may progress to a much more
severe condition.
Through fertilization, facilitated by the union
of the macrogametes by the microgametes, a zygote is Laboratory Diagnosis
formed, which develops a cell wall and then becomes
an oocyst. Ingestion of this oocyst by another host ani- Examination of fecal samples and biopsy specimens to
mal starts a new life cycle. These oocysts can survive detect oocysts are the most common laboratory prac-
in the environment for several months until ingested, tices for identification. Specialty stains and Sheather’s
followed by incubation in the gut of the host for sugar flotation procedure for oocysts that are capable of
approximately a week. I. belli develops only in humans being seen microscopically, are commonly used. Fluores-
but differs from C. parvum, as it is capable of infecting cent antibody detection methods are now available, as
a variety of hosts. However, reproduction for the two a more sensitive and specific method than microscopic
organisms is similar except that for I. belli, unsporu- examination.
lated oocysts must develop for roughly a week in feces
before they reach the infective stage. After oocysts are Treatment and Prevention
formed in the cytoplasm of the enterocytes (specialized
nutrient-absorbing cells of the small intestine), they are A number of antiparasitic agents are ineffective against
completely developed and are excreted in feces, a form C. parvum, but those who are immunocompetent often
that will infect other hosts. are free of symptoms within a short period of time after
the appearance of symptoms. Some medications in cur-
Disease Transmission rent use are showing limited success in AIDS victims.
Water treatment plants with filtration procedures and
The infective form from animals and humans is the sanitary practices could prevent most infections by
resistant oocyst that is passed in feces. Ingestion of con- C. parvum, although filtration of water is not a standard
taminated food and water as well as person-to-person practice. Most infections could be eliminated or reduced
transmission are the main routes of infection. Munici- by the control or elimination of runoff of water from ani-
pal water systems that become contaminated with fecal mal pens into water supplies.