[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 7 Miscellaneous Protozoa
167
Life Cycle Notes
I. belli was initially thought to be a typical coccidial
parasite. Pigs and cattle appeared to be
intermediate hosts for this organism. It has now
been determined that I. belli is the only known
coccidial parasite that does not have intermediate
hosts. Humans serve as the definitive host, in
whom both sexual and asexual reproduction
take place.
It is presumed that infection with I. belli is
initiated following the ingestion of infective
mature (also known as sporulated) oocysts in
contaminated food or water. The sporozoites
emerge after excystation of the oocyst in the
small intestine. Asexual reproduction (schizogony),
resulting in merozoites, occurs in the cells
of the intestinal mucosa. The formation of macrogametocytes
and microgametocytes (gametogony)
takes place in the same intestinal area. The
resulting gametocytes develop and ultimately
unite to form oocysts, the form that is excreted
in the stool. Immature oocysts typically complete
their development in the outside environment. It
is the mature sporulated oocyst that is capable
of initiating another infection. When this occurs,
the life cycle repeats itself. There is evidence suggesting
that I. belli may also be transmitted
through oral-anal sexual contact.
Epidemiology
The frequency of contracting I. belli has traditionally
been considered rare, even though the
organism has worldwide geographic distribution.
The difficulty often experienced in organism
recognition may have led to potentially
false-negative results, which thus may have been
the major contributing factor to the documented
rare frequency of infection. An increase in
reported cases began to occur during and following
World War II. Specifically, cases were reported
in Africa, Southeast Asia, and Central America.
In addition, countries in South America,
particularly in Chile, have reported I. belli infections.
An increase in frequency was particularly
noted in patients suffering from AIDS. Unprotected
oral-anal sexual contact has been suggested
as the mode of parasite transmission in
these patients. The resulting infections with I.
belli are now considered opportunistic.
Clinical Symptoms
Asymptomatic. A number of infected individuals
remain asymptomatic. In such cases, the
infection is self-limited.
Isosporiasis. Infected patients may complain
of a number of symptoms, ranging from mild
gastrointestinal discomfort to severe dysentery.
The more commonly noted clinical symptoms
include weight loss, chronic diarrhea, abdominal
pain, anorexia, weakness, and malaise. In addition,
eosinophilia may occur in asymptomatic
and symptomatic patients. Charcot-Leyden crystals
(Chapter 12) may form in response to the
eosinophilia and may be visible in corresponding
stool samples. Patients experiencing severe infection
typically develop a malabsorption syndrome.
In these cases, patients produce foul-smelling
stools that are pale yellow and of a loose consistency.
Fecal fat levels of these stool samples may
be increased. Infected patients may shed oocysts
in their stools for as long as 120 days. Death may
result from such severe infections.
Treatment
The treatment of choice for asymptomatic or
mild infections consists of consuming a bland
diet and obtaining plenty of rest. Patients suffering
from more severe infections respond best to
chemotherapy, consisting of a combination of
trimethoprim and sulfamethoxazole or pyrimethamine
and sulfadiazine. It is interesting to note
that chemotherapy at a lower dosage for a longer
period may be necessary for AIDS patients
infected with I. belli.
Prevention and Control
The prevention and control measures for I. belli
are similar to those of E. histolytica. They include