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Diphyllobothrium latum (Pathogen – Intestinal Cestode)

Organism:
Diphyllobothrium latum belongs to the pseudophyllidean tapeworm group, which is characterized by having a scolex with two bothria (sucking organs) rather than the typical four suckers seen in the Taenia tapeworms. The distribution of this worm is worldwide, with various increased outbreaks reported from time to time.

D. latum strobila of proglottids D. latum scolex D. latum egg

Life Cycle:
Infection with the adult worm is acquired by the ingestion of raw, poorly cooked, or pickled freshwater fish (pike, perch, lawyer, salmon, trout, white fish, grayling, ruff, turbot, etc.) containing the encysted plerocercoid larvae. After ingestion, the worm matures, with egg production beginning in about the fifth or sixth week. The adult worm reaches a length of 10 m or more and may contain up to 3,000 proglottids.  After developing for 2 weeks in fresh water, the eggs hatch and the ciliated, coracidium larvae are ingested by the first intermediate host, the copepod. The copepods, containing the second larval stage (procercoid), are then ingested by fish, which may be ingested by larger fish. In this situation, the final fish intermediate host may contain many plerocercoid larvae, which initiate the infection with the adult worm when ingested by humans.

Acquired:
Infection in humans is acquired through ingestion of infective larvae from infected freshwater fish (raw or poorly cooked fish).

Epidemiology:
Worldwide, primarily human to human transmission

Clinical Features:
Symptoms in the patient depend on the number of worms present, the amounts and types of by‑products produced by the worm, the patient's reaction to these by‑products, and the absorption of various metabolites by the worms. There may be occasional intestinal obstruction, diarrhea, abdominal pain, or anemia. If the worm is attached at the jejunal level, there may be a vitamin B12 deficiency, which resembles pernicious anemia and develops in a very small percentage of persons harboring the tapeworm. Heavy or long-term infections with D. latum may cause megaloblastic anemia due to parasite-mediated dissociation of the vitamin B12-intrinsic factor complex within the gut lumen, making B12 unavailable to the host. This clinical picture is more commonly seen in Finland, where some individuals have a genetic predisposition to pernicious anemia. In patients without this genetic predisposition, symptoms of a D. latum infection may be absent or minimal, consisting of a slight leukocytosis with eosinophilia.

Clinical Specimen:
Stool:  The standard O&P examination is the recommended procedure for recovery and identification of D. latum eggs in stool specimens, primarily from the wet preparation examination of the concentration sediment.
Adult worms:  Gravid proglottids may be recovered in stool; often they can be seen lying on the top or bottom of the stool specimen submitted as a fresh specimen.  Often a partial chain of proglottids may be passed (a few inches to several feet).

Laboratory Diagnosis:
Stool:  The standard O&P examination is the recommended procedure for recovery and identification of Taenia spp.eggs in stool specimens, primarily from the wet preparation examination of the concentration sediment. The eggs are most easily seen on a direct wet smear or a wet preparation of the concentration sediment.  If the egg operculum is difficult to see, the coverslip of the wet preparation can be tapped and the pressure may cause the operculum to pop open, thus making it more visible. The eggs are unembryonated at the time they are passed in the stool.
Adult worms The mature and gravid proglottids are wider than long, with the main reproductive structures (mainly the uterus) located in the center of the gravid proglottid. This configuration of the uterine structure has been called a rosette.

Organism Description:
Egg:  The eggs are broadly oval and operculated). They are yellow-brown and measure 58-75 µm by 40-50 µm in feces.  The egg usually has a small knob at the abopercular end.  The eggs are unembryonated when passed in the feces.  They can sometimes be confused with eggs of Paragonimus westermani; however, the eggs of D. latum have no opercular shoulders into which the operculum fits.
Adult worm:  The scolex of D. latum is elongate and spoon shaped and has two long sucking grooves, one on the dorsal surface and the other on the ventral surface. The mature and gravid proglottids are wider than long, with the main reproductive structures (mainly the uterus) located in the center of the gravid proglottid. This configuration of the uterine structure has been called a rosette. Identification to the species level is usually based on this typical morphology of the gravid proglottids. Both eggs and proglottids may be found in the stool.

Laboratory Report:
Diphyllobothrium latum eggs present
Diphyllobothrium latum proglottids.

Treatment:
Garcia, L.S. 2007.  Diagnostic Medical Parasitology, 5th ed., ASM Press, Washington, D.C.

Control:
Dogs may serve as carriers of the adult worms and infected dogs should be periodically treated.  In areas where human infection is rare, D. latum infections have been recorded in other mammals.  However, the natural transmission cycle from mammals other than humans does not seem to be sustained. It is quite possible to acquire the infection from the ingestion of infected raw freshwater fish that has been shipped under refrigeration to areas where the infection is not endemic. Preventive measures include thorough cooking of all freshwater fish or freezing for 24 to 48 hours at 18°C. This infection has been called the Jewish housewives' disease, since the individual preparing the food may sample the dish (e.g. gefilte fish) prior to cooking and acquire the infection. Other groups who tend to eat raw or insufficiently cooked fish include the Russians, Finns, and Scandinavians. Raw fish marinated in lime juice (ceviche) is also a source of infection (D. pacificum) in Latin America. Since the domestic dog can serve as a reservoir host, infected dogs should be periodically treated.  Other factors contributing to the parasitic cycle include the continued dumping of wastewater into lakes and the possibility of animal reservoirs.