Page 156 - Medical Parasitology_ A Textbook ( PDFDrive )

Lung Fluke 149

a b

Fig. 12.10 Paragonimus westermani. (a) Adult, (b) Egg

Morphology
The adult worm is coffee bean shaped about 10 mm long, 5 mm broad and 4 mm
thick. It is reddish brown in colour (Fig. 12.10a). It has an oral sucker and a ventral
sucker. It has a lifespan of up to 20 years in humans.
The eggs are operculated, golden brown in colour and about 100 μm by 50 μm in
size (Fig. 12.10b). Eggs are unembryonated when freshly laid.

Life Cycle (Fig. 12.11)
(1) Unembryonated eggs are passed out in sputum of infected human. (2–3)
Embryonated eggs hatch in water to release miracidia which penetrate freshwater
snail. (4) In the snail, the miracidia develop to the cercarial stage. (5) Cercariae
released by the snail invade the crustacean and encyst into metacercariae. (6) Human
ingests inadequately cooked or pickled crustaceans containing metacercariae. (7–8)
The metacercariae excyst in the duodenum and the larvae penetrate the intestinal wall
and migrate to the abdominal cavity, diaphragm and pleural cavity to the lungs. The
larvae mature in the lungs to adults. Adults in cystic cavities in lungs lay eggs which
are excreted in sputum. Eggs may be swallowed and are passed out in stool.
Pathogenesis and Clinical Features
In the lungs, the worms live in cystic spaces surrounded by fibrous capsule formed
by the host tissues. Inflammatory reaction to the worms and eggs causes peribron-
chial granulomatous lesions, cystic dilatation of the bronchi, abscesses and pneu-
monitis. Patients present with cough, chest pain, dyspnoea and haemoptysis. The
rusty sputum contains golden brown eggs. Chronic cases may mimic symptoms of
pulmonary tuberculosis.
Paragonimiasis may also be extrapulmonary. Ectopic sites include abdomen
and CNS. In the abdomen, it may cause abdominal pain and diarrhoea. In the
CNS, it may cause Jacksonian epilepsy. Some children may present with mental
retardation.