parasite, Isospora belli, infects the epithelial cells of the
small intestine, and is the least common of the three intestinal
coccidia that infect humans.
At time of
excretion, the immature oocyst contains usually one sporoblast (more
In further maturation after excretion, the sporoblast divides in two
(the oocyst now contains two sporoblasts); the sporoblasts secrete a
cyst wall, thus becoming sporocysts; and the sporocysts divide twice to
produce four sporozoites each
Infection occurs by ingestion of sporocysts-containing oocysts: the
sporocysts excyst in the small intestine and release their sporozoites,
which invade the epithelial cells and initiate schizogony
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
Isospora belli infects both humans and animals.
especially in tropical and subtropical areas. Infection occurs in
immunodepressed individuals, and outbreaks have been reported in
institutionalized groups in the United States.
acute, non bloody diarrhea with crampy abdominal pain, which can last
for weeks and result in malabsorption and weight loss. In
immunodepressed patients, and in infants and children, the diarrhea can
be severe. Eosinophilia may be present (differently from other
demonstration of the large, typically shaped oocysts, is the basis for
diagnosis. Because the oocysts may be passed in small amounts and
intermittently, repeated stool examinations and concentration procedures
are recommended. 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 UV fluorescence. They can also be stained by modified acid-fast
comparison with other intestinal parasites
Bench aids for
Trimethoprim-sulfamethoxazole is the drug of choice.