Trematodes (Helminths)
Intestinal flukes
Fasciolopsis buski
Fasciolopsis is a genus of trematodes. is more notable in terms of
prevalence and pathogenicity as it causes the disease fasciolopsiasis. It is
a common parasite of humans and pigs and is most prevalent in Asia,
mainly central and southeast Asia. It belongs to the class Trematoda,
family Fasciolidae.
Morphology
Fasciolopsis buski is commonly called the giant intestinal fluke, being
the largest known parasitic fluke in humans. The body can be up to
7.5 cm in length and 2.5 cm in width. The reason for its common name is
due to the fact that it is one of the largest flukes to infect humans. The
worm inhabits the upper region of the small intestine and, can also be
found in the lower areas of the intestine and the stomach. F. buski is a
large leaf-shaped, dorso-ventrally flattened worm that is characterized by
a blunt anterior end, unbranched ceca, testes, branched ovaries, and
ventral suckers to attach itself to the host. The acetabulum is larger than
the oral sucker. It has extensive vitelline follicles. It can be distinguished
from other fasciolids by a lack of cephalic cone or "shoulders" and the
unbranched ceca.
Life Cycle
Adults produce over 25,000 eggs every day which take up to seven weeks to
mature and hatch at 27-32°C. Immature, unembryonated eggs are discharged
into the intestine and stool. In two weeks, eggs become embryonated in water,
and after about seven weeks, eggs release tiny parasitic organisms called
miracidia, which invade a suitable snail intermediate host. Several species of
genera Segmentina and Hippeutis serve as intermediate hosts. In the snail the
parasite undergoes several developmental stages (sporocysts, rediae, and
cercariae). The cercariae are released from the snail and encyst as metacercariae
on aquatic plants such as water chestnut, water caltrop, and other plants. The
final host, becomes infected by ingesting metacercariae on the aquatic plants.
After ingestion, the metacercariae excyst in the duodenum in about three months
and attach to the intestinal wall. There they develop into adult flukes (20 to
75 mm by 8 to 20 mm) in approximately 3 months, attached to the intestinal
wall of the mammalian hosts (humans and pigs). The adults have a life span of
about one year.
Pathogenesis
Infection known as fasciolopsiasis . and the damage which live attached
th the mucosa of duodenum may be traumatic, obstructive, and toxic . at
the site of attachment the worm cause inflammation and ulceration of the
mucosa. Large number of worms provoke increased secretions of mucus
and may lead to partial obstruction of the bowel. In massive infections
patient develops profound intoxication and sensitization that result from
absorption of the worms metabolites into the system. Occasionally
vitamin B12 absorption is impaired .
Symptoms
Most infections are light and asymptomatic. In heavy infections,
symptoms can include abdominal pain, chronic diarrhea, anemia, allergic
responses, sensitization caused by the absorption of the worms allergenic
metabolites (may eventually cause death of patient), and intestinal
obstruction.
Laboratory Diagnosis
Microscopic identification of eggs, or more rarely of the adult flukes, in
the stool or vomitus is the basis of specific diagnosis. The eggs are
indistinguishable from those of Fasciola hepatica.
Fasciolopsis buski egg
Treatment
Praziquantel is the drug of choice for treatment. Treatment is effective in
early or light infections. Heavy infections are more difficult to treat.
Studies of the effectiveness of various drugs for treatment of children
with F. buski have shown tetrachloroethylene as capable of reducing
faecal egg counts by up to 99%. Other antihelmintics that can be used
include mebendazole, and nitroxynil are also highly effective.
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Heterophyes heterophyes
Morphology
small flukes found in the small intestines of fish-eating birds and mammals.
The adult flukes range from 1.0mm to 1.7mm long and about 0.35mm at their
greatest width. The body of the fluke is covered in scales mostly concentrated
at the anterior end. Also at the anterior end is an oral sucker. Located in the
medioanterior of the body is the acetabulum. At the posterior end of the fluke
are two oval testes. The vas deferens leading from the testes expands to form
a seminal vesicle. The fluke also has female reproductive organs. the fluke's
has one ovary and leading away from the ovary is the vitellaria. The uterus is a
long tube like structure that also leads away from the ovary. The genital pore
which is lined with 60-90 toothed spines. The genital pore is where the fluke
releases its eggs.
Life cycle
The adult flukes live burrowed between the villi of the host's small intestine.
The eggs that are laid contain a miracidium but do not hatch until they are
ingested by a snail (Pirenella conica in Egypt or Cerithidia cingula in
Japan). Inside the snails gut, the miracidium becomes a sporocyst which then
begin to produce rediae. The rediae produce cercariae which then exit the
snail, swim toward the surface of the water, and slowly fall back down. On
their way down, they contact a fish and penetrate into the epithelium of the
fish. Here, the cercariae encyst in the muscle tissue. The definitive host, such
as humans or birds, eats the undercooked or raw meat of a fish and ingest the
parasite.
Pathogenesis
Each worm causes a mild inflammatory reaction at its site of contact with
the intestine. In heavy infections which are common cause damage to the
mucosa and produce intestinal pain and mucous diarrhea. Sometimes
eggs can enter the blood and lymph vascular systems through mucosa go
into the ectopic sites in the body. The heart can be affected with tissue
reaction in the valves and myocardium that cause heart failure. Eggs can
also get into the brain or spinal cord and cause neurological disorders and
sometimes fatalities.
Diagnosis
Diagnosis done by stool examination is difficult when adult worms are
not present because the eggs are hard to distinguish from C.sinensis.
Treatment
Praziquantel, a quinolone derivative.