Page 137 - Medical Parasitology_ A Textbook ( PDFDrive )

130 11 Cestodes: Tapeworms

Echinococcus multilocularis

This parasite causes rare but serious condition of alveolar or multilocular hydatid
disease in humans. Its distribution is from Siberia to Canada. The adult worm of
Echinococcus multilocularis is smaller than E. granulosus and lives in the intestines
of foxes, dogs and cats which are the definitive hosts. Rodents are the main interme-
diate hosts. Human acquires infection from ingesting eggs of E. multilocularis from
faeces of definitive host. The multilocular infiltrating lesion appears like an invasive
growth, without any fluid or brood capsules and can be mistaken for a malignant
tumour. The liver is the most commonly affected organ. Patients present with right
upper quadrant and epigastric pain, hepatomegaly and obstructive jaundice. It may
metastasize to other organs. The prognosis is poor and can be fatal if not treated.
Treatment is by surgical resection when possible. Albendazole is recommended
after surgery or in cases where surgery is not an option.

Hymenolepis nana

Common name Dwarf tapeworm

Distribution
It is cosmopolitan in distribution but is more common in tropical countries. Infection
is most common in children.

Habitat
The adult worm lives in the ileum of human.

Morphology
Hymenolopis nana is the smallest intestinal cestode that infects human. The adult
worm is 5–45 mm in length and less than 1 mm thick. The scolex has 4 suckers and
a retractile rostellum with a single row of hooklets. It has a long slender neck and a
strobila consisting of 200 or more proglottids, which are much broader than long.
The uterus is lobulated with 3 round testes. Eggs are released in the intestine by
disintegration of the gravid segments.
The egg is roughly spherical or ovoid, 30–40 μm in size (Fig. 11.10). It has a thin
outer membrane and an inner embryophore enclosing the hexacanth oncosphere.
The space between the 2 membranes contains 4–8 thread-like polar filaments aris-
ing from 2 knobs on the embryophore. Eggs are immediately infective when passed
in faeces.

Life Cycle (Fig. 11.11)
(1) Embryonated eggs are passed out in faeces of infected human. (2) The eggs are
ingested by insects (flea, beetle) and develop into cysticercoid larvae. (3) Humans
and rodents are infected when they ingest cysticercoid infected arthropods.