Diphyllobothrium spp

From Fish
A plerocercoid larva of D. nihonkaiense in masu salmon
Plerocercoid larvae of D. nihonkaiense
Adult worm of D. nihonkaiense

Diphyllobothrium spp are a common parasite in fish.

Larval forms encyst in visceral organs and muscle, while adults usually are found in the intestinal tract. Aquatic Crustacea are the most common intermediate host for fish; accordingly, wild and cultured pond fish may be heavily infected.

D. latum

Diphyllobothrium latum, the broad fish tapeworm infection of humans, is acquired by eating larval tapeworms in the flesh of food fish. Aquarium fish may be purchased with heavy cestode infections but have limited exposure once in the aquarium (unless fed infected intermediate hosts). There is no safe, effective treatment for larval tapeworm infections[1].

D. dendriticum

Wild Atlantic salmon Salmo salar caught by anglers in Maine are sometimes infested by the salmon tapeworm.

This cestode parasite lives in the pyloric caeca and small intestine of adult salmon. The number of these segmented worms found in an individual fish can vary from a few to hundreds. This parasite can be fatal to salmon and brook trout if infestations overwhelm the fish. The adult D. dendriticum release eggs in the gut of the fish which are excreted into the water with the fish’s feces. The eggs hatch and are eaten by zooplankton which are in turn eaten by small fish. The small fish are eaten by gulls. When gulls defecate into the water they spread the salmon infective life stage of the parasite.

Unlike many fish parasites, D. dendriticum can infect other animals including dogs, cats, guinea pigs, fox, rat, squirrel, terns, other salmonids, and humans. Most animals are paratenic hosts, which means that the parasite lives in the animal’s body, but cannot complete its lifecycle[2].

Clinical signs

It is considered that the parasite is not pathogenic to fish.

This parasite infects to human and causes diphyllobothriosis. Therefore, be careful when consume the host fishes raw. Heating or freezing to -18 C for 48 h is an effective method of killing the parasite. In general, the pathogenicity to human is low though the parasite reaches 10 m in length in the small intestine. Diseased patients exhibit diarrhea and stomachache. However, D. latum infection sometimes causes B12 deficiency anaemia.

Treatment

Praziquantel is an effective anthelminthic.

References

  1. Merck Veterinary Manual
  2. Awakura, T., S. Sakaguchi and T. Hara (1985) Studies on parasites of masu salmon Oncorhynchus masou ? VIII. Observations on the seasonal occurrence of Diphyllobothrium latum plerocercoid. Sci Rep Hokkaido Fish Hatchery 40:57-67