Sushi Worms (Part 2)

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The Fish Tapeworm!!!
30 feet long!!
Nearly an inch wide!!!
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Causal Agents: The cestode Diphyllobothrium latum (the fish or broad tapeworm), the largest human tapeworm. Several other Diphyllobothrium species have been reported to infect humans, but less frequently; they include D. pacificum, D. cordatum, D. ursi, D. dendriticum, D. lanceolatum, D. dalliae, and D. yonagoensis.

Life Cycle: Immature eggs are passed in feces [1]. Under appropriate conditions, the eggs mature (approximately 18 to 20 days) [2] and yield oncospheres which develop into a coracidia [3]. After ingestion by a suitable freshwater crustacean (the copepod first intermediate host) the coracidia develop into procercoid larvae [4]. Following ingestion of the copepod by a suitable second intermediate host, typically minnows and other small freshwater fish, the procercoid larvae are released from the crustacean and migrate into the fish flesh where they develop into a plerocercoid larvae (sparganum) [5]. The plerocercoid larvae are the infective stage for humans. Because humans do not generally eat undercooked minnows and similar small freshwater fish, these do not represent an important source of infection. Nevertheless, these small second intermediate hosts can be eaten by larger predator species, e.g., trout, perch, walleyed pike [6]. In this case, the sparganum can migrate to the musculature of the larger predator fish and humans can acquire the disease by eating these later intermediate infected host fish raw or undercooked [7] . After ingestion of the infected fish, the plerocercoid develop into immature adults and then into mature adult tapeworms which will reside in the small intestine. The adults of D. latum attach to the intestinal mucosa by means of the two bilateral groves (bothria) of their scolex [8]. The adults can reach more than 10 m in length, with more than 3,000 proglottids. Immature eggs are discharged from the proglottids (up to 1,000,000 eggs per day per worm) [9] and are passed in the feces . Eggs appear in the feces 5 to 6 weeks after infection. In addition to humans, many other mammals can also serve as definitive hosts for D. latum.

Geographic Distribution: Diphyllobothriasis occurs in the Northern Hemisphere (Europe, newly independent states of the former Soviet Union [NIS], North America, Asia) and in Uganda and Chile. Freshwater fish infected with Diphyllobothrium sp. larva may be transported to and consumed in geographic areas where active transmission does not occur, resulting in human diphyllobothriasis. For example, cases of D. latum infection associated with consumption of imported fish have been reported in Brazil.

Above information from : http://www.dpd.cdc.gov

Clinical Manifestations : Infection with Diphyllobothrium latum is usually asymptomatic, although occasional diarrhea, abdominal pain, fatigue, vomiting, dizziness, or numbness of fingers and toes may be present. Eosinophilia develops during the early stages of worm growth.

Structure: Diphyllobothrium latum is the largest parasite of humans, reaching lengths up to 10 m and consisting of a chain of 3,000 to 4,000 segments, each up to 2 cm wide. The adult worm, a member of the order Pseudophyllidea, is characterized by a scolex with a pair of linear sucking grooves instead of suckers and hooks, and by having a rosette-shaped uterus connected to the outside by a uterine pore through which the eggs are passed. Hence, mature segments produce eggs until they die and are shed, rather than by breaking off as intact egg-filled segments, as in Taenia. Up to a million eggs can be produced daily. The developmental stages are (1) the ciliated, swimming coracidium that hatches from the egg, (2) the procercoid that develops in the copepod primary host, and (3) the plerocercoid (or sparganum), a nonencysted, nonsegmented larval worm, 20 mm or more in length, found in the fish secondary hosts. The plerocercoid develops into the adult tapeworm in the small intestine of a fish-eating final host, such as human, cat, dog, or bear.

Multiplication and Life Cycle: Diphyllobothrium latum is the only adult cestode of humans that has an aquatic life cycle. Eggs are passed in feces of an infected human (or bear, dog, cat, wolf, raccoon, or other freshwater fish-eating reservoir host). If passed into lake or pond water, the eggs develop in 2 or more weeks (varying with the temperature) and hatch, releasing the spherical ciliated coracidium that contains the oncosphere. When ingested by an appropriate water flea (copepods such as Cyclops or Diaptomus), the coracidium sheds the ciliated coat, penetrates into the hemocoel, and changes in 2 to 3 weeks into the 0.5 mm, tailed second-stage embryo, the procercoid. If the infected copepod is then ingested by a minnow or other fish, the procercoid penetrates the fish gut in a few hours and later develops into a third-stage larva, the plerocercoid or sparganum. Usually, these small infected fish are eaten by larger ones; in each new fish host, the plerocercoid penetrates into the fascia or muscles. Eventually, a large game fish, such as a perch or pike, is infected; after being eaten by a human, the fish releases its tapeworm passenger, which attaches and begins adult life. In a few months, the worm is 5 to 10 m long.

From : http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.section.4713