Page 126 - Medical Parasitology_ A Textbook ( PDFDrive )

Pseudophyllidean Tapeworms 119

a b

c

Fig. 11.1 Diphyllobothrium latum. (a) Scolex, (b) Gravid proglottids, (c) Egg

There are 3 stages of larval development:

1. First stage larva (coracidium)
2. Second stage larva (procercoid)
3. Third stage larva (plerocercoid)

Life Cycle (Fig. 11.2)
(1) Unembryonated eggs are passed in faeces of infected human. (2) Eggs embryo-
nate in water. (3) Coracidia hatch from eggs and are ingested by crustaceans. (4)
Procercoid larvae develop in body cavity of crustaceans. (5) The infected crusta-
ceans are ingested by small freshwater fish and the procercoid larvae develop into
plerocercoid larvae. (6) Predator fish eats the infected small fish. (7) Human
acquires infection via ingesting raw or undercooked infected fish containing plero-
cercoid larvae. (8) Adults develop in the small intestine. (9) Proglottids release
immature eggs which are passed in the faeces.

Pathogenesis and Clinical Features
Infection may be asymptomatic, while some patients may present with intestinal
obstruction. Abdominal discomfort, diarrhoea, nausea, weakness, weight loss and
anaemia are the usual manifestations. The worm lives in the ileum where vitamin
B 12 absorption takes place. It competes with the host for vitamin B 12 and may cause
vitamin B 12 deficiency anaemia.