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70 CHAPTER 3

ISOSPORA BELLI and further schizogony occurs, producing microgametes
and macrogametes, a form of sexual reproduction. The
Isosporiasis is a human intestinal disease caused by the fusion of these two gametes results in a zygote that devel-
parasite Isospora belli. This organism is found world- ops a cell wall and becomes an oocyst. These oocysts are
wide, especially in tropical and subtropical areas. Infec- excreted in feces and for I. belli, these oocysts can survive
tion often occurs in immunocompromised individuals, in the environment for several months until ingested and
particularly those who are HIV-infected, and outbreaks incubated in the gut for about a week. After oocysts de-
have been documented in those who are institutional- velop in the cytoplasm of enterocytes (infected nutrient-
ized, such as in prisons. The condition has been known absorbing cells of the small intestine), there is no further
since 1915. The coccidian parasite Isospora belli infects development until excreted in feces.
the mucosal epithelial cells of the small intestine. I. belli is
less common than two other intestinal coccidia that infect Disease Transmission
humans, those of Toxoplasma and Cryptosporidium.
The resistant oocyst is passed in feces as the infective
Morphology form from animals and humans. Ingestion of contami-
nated food and water as well as person-to-person trans-
Isospora is diagnosed most frequently by the study of mission are the main routes of infection. Municipal water
freshly passed stools. Oocysts are formed by the union of systems that become contaminated with fecal material
microgametes and macrogametes, resulting in infective oo- have also been implicated in widespread outbreaks of
cytes. Morphology of oocysts reveals two sporocysts with diarrhea in recent years. Municipal water supplies may
each containing four sporozoites. The size of these oocysts sometimes become contaminated with many organisms
vary greatly, measuring an average of 17 to 37 μm. including E. dispar and other organisms such as Coccidia
that are not filtered out in the treatment plants.
Symptoms The outbreak in 1993 in Milwaukee, Wisconsin,
is a case in point. Such an outbreak of a number of or-
A self-limiting diarrhea may occur in those who have a ganisms may not be effectively controlled in standard
healthy immune system, and after a few weeks of incu- water purification plants. Standard chlorination levels
bation, nausea with vomiting, fever, abdominal cramps, do not control this organism, even at higher levels than
loss of appetite, and often watery diarrhea occur. For in- normally employed, so additional measures may require
fants, morbidity and mortality are prevalent as well as in implementation such as filtration, or asking the citizens of
immunocompromised individuals (organism present in a community to boil any water used for human consump-
many AIDS victims). The respiratory tract may become tion. Because most of the coccidian infections in humans
involved, as well as the gastrointestinal tract. are zoonoses, meaning there is a distinct possibility of
contracting the disease from infectious animals, measures
Life Cycle designed to control runoff of surface water from feeding
facilities and where animals are processed for meat may
The elements of the life cycle of both C. parvum and require governmental standards to be implemented and
I. belli are similar, as both are single-host pathogens enforced. Most immunocompetent individuals will suffer
with no intermediate host involved. I. belli differs from from mild, self-limiting infections that are sometimes not
C. parvum as it develops only in humans, but reproduc- even noticed. In those who are immunocompromised,
tion in the two organisms is similar except that for I. belli, the individual’s defense system may result in much more
unsporulated oocysts must develop for roughly a week in serious symptoms and progression to a critical medical
feces before they reach the infective stage (Figure 3-21). condition.
When infective oocytes are ingested by persons or ani-
mals; sporozoites emerge from the oocyst, containing Laboratory Diagnosis
four sporozoites. The sporozoites enter the microvilli
of the intestine and develop into trophozoites, an asex- Examination of fecal samples and biopsy specimens to
ual means of reproduction. The trophozoites multiply detect oocysts are the most common laboratory practices.
(schizogony) by forming merozoites that enter other cells Specialty stains and Sheather’s sugar flotation procedure