Fish Microbiology and Quality Assurance

Unit 6 - Biological hazards in foods

6.5.2. Pathogenic flatworms and round worms

Occurrence of flatworms and round worms is common in fish and shellfish and over 50 species of helminth parasites from fish and shellfish are known to cause disease in human beings. These parasitic helminths have complicated life cycles during their development involving a number of intermediate hosts. Generally, sea snails or crustaceans serve as first intermediate host, marine fish as second intermediate host, and mammals as the final host for sexually mature parasites. In between these hosts, one or more free living stages may occur. Infection of human may be part of this life cycle or it may be a side track causing disruption of the life cycle.

The pathogenic flatworms and round worms transmitted by fish and shellfish are

Nemotodes or Round worms	Fish/shellfish involved
Anisakis simplex	Herring
Pseudoterranova dicipiens	Cod
Gnathostoma sp.	Fresh water fish, frog
Capillaria sp.	Freshwater fish
Angiostrongylus sp.	Freshwater prawns, snails, fish

Cestodes or tapeworms
Diphyllobothrium latum	Freshwater fish
D. pacificum	Marine fish

Trematodes or flukes
Clonorchis sp.	Freshwater fish, snails
Opisthorchis sp.	Freshwater fish Paragonimus sp. Snails, Crustaceans, Fishes
Echinostoma sp.	Clams, Freshwater fishes, Snails
Heterophyes sp.	Snails, Freshwater fish, Brackishwater fish
Metagonimus yokagawai	Freshwater fish

A. Nematodes or round worms

• Round worms or nematodes are common and found in marine fish all over the world.
  
• Anisakis simplex is commonly known as “herring worm” and Pseudoterranova dicipiens is known as “Cod worm”.
  
• Live worms when ingested by humans penetrate into the wall of the gastrointestinal tract and cause an acute inflammation (“herring worm disease”).
• Gnathostoma sp is another nematode found in freshwater fish in Asia. The ingested larvae migrate from the stomach to other body regions (subcutaneous sites in the thorax, arms, head and neck) and induce a creeping sensation and edema.
• Capillaria philippinensis infection in humans causes severe diarrhea and possible death due to fluid loss.
• Angiostrongylus cantonensis is an common nematode in Asia and is associated with freshwater fish, snails and prawns, and known to cause meningitis in humans.
  

B. Cestodes

• Very few cestodes or tapeworms are transmitted to humans through fish.
  
• The broad fish tapeworm, Diphyllobothrium latum is a common human parasite affecting the intestinal tract of humans.
  
• The related species (D. pacificum) is transmitted by marine fish.
  
• These parasites are transmitted through the consumption of raw or semi processed fish.

C. Trematodes

• Some of the trematodes or flukes are common in Asia. The liver fluke (Clonorchis sinensis) is a common parasite infecting bile-ducts in the liver of humas in Asia.
• Flukes such as Metagonimus yokogawai and Heterophyes heterophies infect the intestines of the final host (humans) causing inflammation, symptoms of diarrhea and abdominal pain. The adult oriental lung fluke, Paragonimus sp. lives in cysts in the lungs of final hosts.