Paragonimus westermani

Geographical distribution

Geographical distribution

Geaographical distribution

and

Habitat

P. westermani is endemic in eastern and southern Asia, western and central Africa and Central and South America. In India, it has been reported from Assam, Bengal, Tamil Nadu and Kerala.

Habitat

The adult worms reside usually in pairs, in the cystic cavities in the lungs of man and other definitive hosts.

Eggs

Egg

The eggs of P. westermani are oval, yellowish-brown, measure 90 x 50 μm and have a prominent 'shouldered' operculum. The shoulder serves to distinguish the Paragonimus ova from those of Diphyllobothrium latum, the opercula of which are devoid of shoulders. P. westermani eggs also do not possess knoblike protrusion at the other end which is characteristic of Diphyllobothrium eggs. P. westermani eggs are unembryonated when laid.

Metacercariae encysted in flesh of various crustaceans (crayfish, crab).

Adult worm

It is thick, fleshy, oval-shaped and reddish-brown in colour with an integument covered with scale-like spines (Fig. 10.8). Its anterior end is slightly broader than the posterior end. It measures up to 16 mm in length, 8 mm in width and 5 mm in thickness. The ventral sucker is located towards the middle of the body and is of similar size to the oral sucker on the anterior end. The fluke possesses a large excretory bladder extending from the posterior extremity to the level of pharynx in the anterior region. The two blind intestinal caeca are unbranched and extend to the caudal region.

Life span of adult worm is about 6-7 years.

Adult Worm

Life cycle diagram

Life cycle in detail

The adult worms reside, usually in pairs, in the cystic cavities in the lungs and lay eggs. Eggs escape into the bronchi and are coughed up and voided in sputum, or swallowed and passed in faeces. Freshwater bodies (lakes, canals, small ponds, rivers, flooded rice fields and other reservoirs of water) become contaminated with eggs from sputum and faeces. They develop and hatch in water after approximately 3 weeks, releasing free-swimming miracidium, which burrows into the flesh of an appropriate snail.

Inside the snail, the miracidium passes through the stages of sporocyst, first-generation rediae, second generation rediae, finally giving birth to cercariae. The mature cercariae escape from the snail into water and penetrate gills and muscles of its second intermediate host, a freshwater crab or a crayfish. Crabs and crayfish can also become infected by eating the infected snail. Inside the crustacean host, the cercariae transform into metacercariae in the viscera, muscles and gills.

Humans become infected following ingestion of raw or poorly cooked or salted crab or crayfish, flesh of which contains encysted metacercariae. Ingested metacercariae excyst in the duodenum releasing larvae that attach to the duodenal mucosa. These larvae penetrate the intestine and enter into abdominal cavity. Then they migrate upwards, piercing through or around the diaphragm to the pleural cavity and lungs, finally arriving in the vicinity of the bronchioles, where they develop into adult worms usually in pairs encapsulated by host's inflammatory response and produce eggs. The time required for completion of their development in the definitive host is 65-90 days. Eggs are expelled in the sputum or may be dislodged by coughing, swallowed and excreted in facces. The cycle is thus repeated.

In detail

Leakage of fluid into the bronchioles causes paroxysmal coughing and haemoptysis. Up to 50 ml of gelatinous, rusty-brown sputum containing traces of blood and yellowish-brown parasite eggs may be expectorated daily during paroxysmal coughing. It is often misdiagnosed as tuberculosis because of overlapping clinical manifestations including chest pain, cough and haemoptysis and confusing radiological findings in chest X-ray.

In addition to the usual migratory routes, P. westermani may become lodged in ectopic sites. These include liver, intestinal wall, peritoneum, pleura, mesenteric lymph nodes, brain, heart, muscles and subcutaneous tissue of the groin. When this occurs completion of the life cycle is not achieved, because the eggs laid cannot exit from these sites. Flukes which lodge in ectopic sites invoke inflammatory response, similar to that seen in lungs, leading to ulcerations and abscesses. The clinical manifestations in these cases depend on the organ involved. Dull abdominal tenderness and bloody diarrhoea are the chief complaints of abdominal paragonimiasis. If parasites lodge in brain or spinal cord, patient develops headache, fever, paralysis, visual disturbances and convulsive seizures.

In Detail

Laboratory diagnosis of paragonimiasis:

1. Demonstration of eggs: Characteristic, yellowish-brown, operculated eggs in the sputum, aspirated pleural effusion or faeces. The sputum is blood-tinged and is peppered with rusty-brown flecks, consisting of clumps of yellowish-brown eggs.

2. Serological tests: A variety of serological tests, of which ELISA is sensitive and practical, are available.

3. Skin test: Skin test using extracts of adult P. westermani as antigen.

4. Biopsy: Parasite fragments and eggs can sometimes be seen in biopsy.

5. X-ray chest: It shows patchy foci of fibrotic change, with a

characteristic 'ring shadow'.