What is Cryptosporidia? Describe its life cycle.
- Cryptosporidia are sporozoan protozoan parasites that
infect the intestinal tract of a wide range of mammals including humans.
Life Cycle.
- The fully mature, infective oocysts are excreted in the
stool of the parasitized animal.
- Following ingestion by another animal,sporozoites are
released from the oocyst and attach to the microvilli of the small bowel
epithelial cells where they are transformed into trophozoites.
- They divide asexually by multiple fission (schizogony) to
form schizonts containing eight daughter cells known as type 1 merozoites.
- Upon release from the schizont, each daughter cell attaches
itself to another epithelial cell, where it repeats the schizogony cycle,
producing another generation of type I merozoites.
- Eventually, schizonts containing four type 2 merozoites are
seen.
- Incapable of continued asexual reproduction, these develop
into male (microgamete) and female (macrogamete) sexual forms.
- Following fertilization, the resulting zygote develops into
an oocyst that is shed into the lumen of the bowel.
- The majority possess a thick protective cell wall
that ensures their intact passage in the feces and survival in the external
environment.
- Approximately 20% fail to develop the thick wall.
- The cell membrane ruptures releasing infective sporozoites
into the intestinal lumen and initiating a new "autoinfective"
cycle within the original host.
- In the normal host, acquired immunity dampens both
the cyclic production of type 1 merozoites and the formation of thin walled
oocysts, halting further parasite multiplication and terminating the acute
infection.
- In the immunocompromised (AIDS) both processes
continue resulting in chronic infection.
How is cryptosporidia acquired?
- Domestic animals are the major reservoir in this country.
- Transmission occurs by person to person, animal to person,
and from the environment, particularly water.
- The principle route of transmission is by direct fecal-oral
spread.
- Waterborne outbreaks, including one in Milwaukee affecting 403,000
persons, have occurred.
How does cryptosporidia cause disease?
- Organisms apear as spherical structures arranged in rows
along the microvilli of epithelial cells.
- They are covered by a double membrane derived from
reflection, fusion and attenuation of the microvilli and are thus considered
intracellular.
- There may be villous atrophy and blunting, crypt
hyperplasia and lengthening, and infiltration of the lamina propria with
inflammatory cells.
- Mechanism of diarrhea is not known.
- Studies in AIDS patients have shown secretory mechanisms
unaffected by fasting and malabsorptive mechanisms with positive D-xylose
and decreased absorption of vitamin B12 as well as steatorrhea.
How is it treated?
- There is no palliative or curative treatment for
cryptosporidiosis.
- The disease is self-limited in the immunocompetent.
-
Currently paromomycin (a luminal antimicrobic) or azithromycin to treat
infection or octreotide (a somatostatin analogue) to control the diarrhea
are used in persons with AIDS.
- Bovine transfer factor and hyperimmune bovine
colostrum have been experimentally shown to ameliorate the symptoms.
- Recent publications have documented clinical
resolution of cryptosporidiosis in patients who responded to highly active
antiretroviral therapy.