Diphyllobothrium latum (Broad Fish Tapeworm)

TABLE OF CONTENTS

Diphyllobothrium latum (Broad Fish Tapeworm): Morphology, Life Cycle, Pathogenesis & Treatment

Diphyllobothrium latum, commonly known as the broad fish tapeworm, is one of the largest cestodes infecting humans and other fish-eating mammals. It has an indirect life cycle involving copepods and freshwater fish as intermediate hosts. This zoonotic parasite is of significant importance in veterinary and medical parasitology due to its association with vitamin B₁₂ deficiency and megaloblastic anemia in infected individuals.

Parasite Overview

Common Name: Broad fish tapeworm
Host: Humans, dogs, cats, pigs, and other fish-eating mammals.
Location: Small intestine
1^st Intermediate Host: Cyclops (Diaptomus gracilis – copepod crustacean)
2^nd Intermediate Host: Freshwater fish (pike, trout, and perch)

Taxonomical Classification

Kingdom: Animalia
Phylum: Platyhelminthes
Class: Cestoda
Subclass: Eucestoda
Order: Diphyllobothriidea
Family: Diphyllobothriidae
Genus: Diphyllobothrium
Species: Diphyllobothrium latum
Common Name: Broad fish tapeworm, fish tapeworm

Diphyllobothrium latum is also referred to as Adenocephalus latus in modern taxonomy.

Morphology

Worms are medium to large in size. The scolex has a narrow neck and deep muscular grooves known as bothria (holdfast organs) situated on both the dorsal and ventral sides.
The scolex is unarmed and almond-shaped.
Each segment contains a single set of reproductive organs.
The genital pore and uterine pore open separately on the ventral aspect, whereas in Mesocestoides, there is no separate uterine pore.
The ovary is bilobed. The vitelline glands and testes are distributed along the lateral margins of each segment.
In gravid segments, the uterus is spiral in shape.

Life Cycle

Eggs are passed in the feces of the host. They are light brown in color, operculated, and unembryonated when laid. Egg development occurs in the environment and takes several weeks for the coracidium to develop. The coracidium is a six-hooked oncosphere covered with a ciliated embryophore.
The fully developed coracidium hatches and swims in water for a short period. It is then ingested by the 1^st intermediate host (Cyclops), where it develops into a procercoid within about 3 weeks. These infected 1^st intermediate hosts are then ingested by fish (2^nd intermediate host).
Within the 2^nd intermediate host, it develops into a plerocercoid in the viscera and musculature.
The definitive host acquires the infection by consuming infected raw or undercooked fish. The prepatent period is approximately 4 weeks.

Pathogenesis

In Humans: It causes non-specific abdominal symptoms and vitamin B₁₂ deficiency, which may lead to megaloblastic anemia due to competition between the host and the parasite for vitamin B₁₂.

Diagnosis

Based on clinical signs.
Microscopic fecal examination for the presence of characteristic operculated eggs.

Treatment

Praziquantel: 25 mg/kg body weight.
Niclosamide: 75–150 mg/kg body weight.
Quinacrine: 7–10 mg/kg body weight.

Control

Avoid consuming raw or undercooked fish.