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INTRODUCTION
Isospora belli is a spore forming coccidian
protozoan that infects humans and some
primates.
It has been studied as the causal agent in
the intestinal disease Isosporiasis.
Isospora belli has been reported to cause
travelers diarrhea in visitors to endemic
areas.
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MORPHOLOGY
Oocysts (the stage of I.belli that exists in
the external environment) are ovoid in
shape and measure 20-33 x 10-19 µm.
Immature Oocyst Containing One Sporoblast
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CLASSIFICATION
Higher order taxa
Domain: Eukaryota
Phylum: Apicomplexa
Class: Coccidia
Order: Eucoccidiorida
Family: Eimeriidae
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EPIDEMIOLOGY
It is found worldwide especially in tropical
and subtropical areas.
This occurs more frequently under poor
sanitation conditions.
There is a 3-14 day incubation period.
There are no known organism vectors or
reservoirs of Isosporiasis.
Infection occurs in immunodepressed
individuals.
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TRANSMISSION
Transmission occurs by fecal oral route.
It is primarily through ingestion of contaminated
food or water
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LIFE CYCLE
As with other coccidian, Isospora belli has both a
sexual and an asexual cycle.
Oocysts are passed in the feces unsporulated.
Sporulation takes approximately 24 hours and
the oocyst becomes infective
At time of excretion, the immature oocyst
contains usually one sporoblast (more rarely
two).In further maturation after excretion, the
sporoblast divides in two, so the oocyst now
contains two sporoblasts.
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The sporoblasts secrete a cyst wall, thus becoming
sporocysts; and the sporocysts divide twice to
produce four sporozoites each.
Infection occurs by ingestion of sporocystscontaining oocysts: the sporocysts excyst in the
small intestine and release their sporozoites,
which invade the epithelial cells and initiate
schizogony.
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Upon rupture of the schizonts, the merozoites
are released, invade new epithelial cells, and
continue the cycle of asexual multiplication.
Trophozoites develop into schizonts which contain
multiple merozoites.
After a minimum of one week the sexual stage
begins with the development of male and female
gametocytes.
Fertilization results in the development of
oocysts that are excreted in the stool.
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THE SUMMARY OF THE LIFE
CYCLE
1) Immature, unsporulated oocyst is excreted
through feces.
Sporoblast divides into two.
Each sporoblast develops into a sporocyst with 4
sporozoites, resulting in mature oocysts.
2) Mature oocyst is ingested.
3) Oocyst bursts. Sporozoites are released and lodge
into the intestinal lining.
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Sporozoites undergo asexual reproduction to
form merozoites.
(The time spent in stages 1 through 3 is 2-3
days).
4)The merozoites mature into gametes
5)The gametes undergo fertilization to produce a
new oocyst.
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CLINICAL FEATURES
In healthy individuals I. belli is usually
asymptomatic or produces self limiting
diarrhoea.
If the host is young or immunodepressed the
symptoms can include:
 profuse watery non-bloody diarrhoea with
fluid loss of 2-20L/day (may contain mucous).
abdominal pain and cramping.
 Anorexia.
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Weight loss .
CONTINUATION OF CLINICAL SYMPTOMS
general malaise.
low fever.
Vomiting.
steatorrhea with prolonged illness.
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PATHOGENESIS
The sporozoites invade epithelial cells in
the small intestine which eventually
destroys these cells.
It has been suggested that the cause of
Isosporiasis symptoms is toxin related
but no toxin has been identified.
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LABORATORY DIAGNOSIS
I. belli infection can be diagnosed by stool
specimen.
The I. belli oocysts are easily dyed with modified
acid-fast stains
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IMMATURE OOCYST OF I. BELLI STAINED WITH
ACID-FAST, SHOWING A SINGLE SPOROBLAST
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If stool examinations are negative,
examination of duodenal specimens by
biopsy or string test (Enterotest®) may be
needed.
The oocysts can be visualized on wet mounts
by microscopy with bright-field,
differential interference contrast (DIC),
and epifluorescence.
They can also be stained by modified acidfast stain.
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TREATMENT
Most treatment for Isosporiasis is simply to
relieve symptoms as they appear.
It may be necessary to treat patients for
malabsorption if severe diarrhoea persists.
I. belli infections can also be avoided with
trimethoprim-sulfamethoxazole and even
patients who are already infected usually
respond well to this treatment
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PREVENTION
The key to prevention is
proper sanitation to ensure that the environment
is free of feces. Once the Isospora are passed
in the feces, they can quickly develop into the
infective stage, so rapid removal of the feces is
very important.
Mature oocysts of Isospora are resistant to most
cleaning products and they can survive for
months to years in the environment. However,
the use of strong ammonia-containing compounds
may be helpful in disinfection, and steam
cleaning also helps kill the infectious oocysts. 19
PREVENTION
Be sure to allow for adequate ventilation while
cleaning the infected areas, as fumes from
cleaning products can be harmful to animals and
people.
Dogs and cats should not be permitted to ingest
rodents, since rodents may be carriers of the
parasites.
The treatment of infected canine and feline
mothers soon after parturition may help prevent
the spread to the young.
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REFERENCES
Textbook Of Medical Parasitology;5th Edition by
CK Jayaram Paniker;MD.
Medical microbiology,Thieme 2005.
www.wikipedia.org/wiki/isosporiasis
www.practicalscience.com/isospora.htm
Center for disease control and prevention (CDC)
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