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 .  Under appropriate conditions, the eggs mature (approximately 18 to 20 days) and yield oncospheres which develop into a coracidia .  After ingestion by a suitable freshwater crustacean (the copepod first intermediate host) the coracidia develop into procercoid larvae .  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) .  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 .  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 .  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 .  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) 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.

Reference:

Sampaio JLM, Andrade VP, Lucas MC, et al.  Diphyllobothriasis, Brazil.  Emerg Infect Dis 2005;11:1598-1600.

Clinical Features:Diphyllobothriasis can be a long-lasting infection (decades).  Most infections are asymptomatic.  Manifestations may include abdominal discomfort, diarrhea, vomiting, and weight loss.  Vitamin B12 deficiency with pernicious anemia may occur.  Massive infections may result in intestinal obstruction.  Migration of proglottids can cause cholecystitis or cholangitis.

Laboratory Diagnosis:Microscopic identification of eggs in the stool is the basis of specific diagnosis.  Eggs are usually numerous and can be demonstrated without concentration techniques.  Examination of proglottids passed in the stool is also of diagnostic value.

Treatment:Praziquantel* is the drug of choice.  Alternatively, Niclosamide can also be used to treat diphyllobothriasis.  For additional information, see the recommendations in The Medical Letter (Drugs for Parasitic Infections).

* This drug is approved by the FDA, but considered investigational for this purpose.

Microscopy

EggsDiphyllobothrium spp. eggs are oval or ellipsoidal and range in size from 55 to 75 µm by 40 to 50 µm. There is an operculum at one end that can be inconspicuous, and at the opposite (abopercular) end is a small knob that can be barely discernible. The eggs are passed in the stool unembryonated.

A B

A, B: Eggs of D. latum in an iodine-stained wet mount. Note the knob at the abopercular end in Figure B. Images courtesy of the Oregon State Public Health Laboratory.

C D

C, D: Eggs of Diphyllobothrium latum.

AdultsAdults of Diphyllobothrium spp. may be 4-10 meters in length.  The scolex is elongated, 1mm by 3mm, with two shallow, longitudinal grooves. As proglottids mature, they may break off from the stroblia, in lengths containing few to many segments.  Proglottids are broader than they are long and range from 2 to 4 mm long by 10 to 12 mm wide.  The uterus is coiled in rosette appearance and the genital pore is at the center of the proglottid.  There are numerous testes that appear in the lateral fields of each proglottid.

E F

E: Section of an adult D. latum containing many proglottids.  The scolex was not present in this specimen.  Image courtesy of the Florida State Public Health Laboratory.F: Close-up of a few of the proglottids from the specimen in Figure E, showing the rosette-shaped uterus at the center of each proglottid.

G H

G, H: Carmine-stained proglottids of D. latum, showing the rosette-shaped ovaries.

I

I: Scolex of D. latum.

J K

J, K: Eggs within the proglottids of Diphyllobothrium sp., observed in an intestinal biopsy specimen stained with hematoxylin and eosin (H&E).  Images taken at 500x magnification.  Images courtesy of the University of Washington Medical Center, Seattle, WA.