Diphyllobothrium latum
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p678-680
2025-10-29
61
GENERAL CHARACTERISTICS
Diphyllobothrium latum, the freshwater broad fish tape worm, is the largest human tapeworm. Adults have been known to reach up to 10 m in length, with more than 3000 to 4000 proglottids, and reside within a host for 30 years or more. The proglottids are characteristically wider than long with a central rosette-shaped uterine structure (Figure 1). The scolex is spatulate and contains two shallow sucking grooves referred to as bothria (Figure 1, B). D. latum has unembryonated eggs. The eggs are operculated with a terminal knob, similar to trematode eggs (Figure 1, C). The intermediate hosts include crustaceans and freshwater fish.
Fig1. A, Diphyllobothrium latum scolex. B, D. latum scolex, bothria visible. C, D. latum ovum. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)
EPIDEMIOLOGY
Diphyllobothrium is not found in the tropics; it is commonly found worldwide where cool lakes are contaminated by sewage. D. latum can be found wherever freshwater fish are eaten raw or marinated. This includes fish such as trout, pike, and salmon, which have been shipped to nonendemic areas. In the United States, D. latum is generally found in and around the Great Lakes.
PATHOGENESIS AND SPECTRUM OF DISEASE
Diphyllobothrium latum is the only cestode to have an aquatic life cycle (Figure 2). Fish serve as the reservoir host, with humans serving as the definitive host. D. latum eggs are found in the feces of infected humans and other fish-eating mammals. Once passed into a water source, such as a lake, the life cycle requires two intermediate hosts. After incubation in freshwater for approximately 2 weeks, the mature eggs release the first larval stage (coracidium). The coracidia are ingested by copepods. The fish feed upon the small crustaceans ingesting the procercoid larvae. Within the freshwater fish, the larvae develop into the infective larvae. D. latum infection occurs through the ingestion of poorly cooked freshwater fish containing the plerocercoid larval form. D. latum matures to an adult tapeworm within the human small intestine. Infection is usually asymptomatic, but mild gastrointestinal symptoms may occur such as diarrhea, abdominal pain, fatigue, vomiting, or dizziness. Symptoms vary depending on the worm burden and the host’s immune response to the organism. The tapeworm nutritional requirements may decrease the host’s vitamin B12 level, resulting in megaloblastic anemia (Table 1).
Fig2. Life cycle of Diphyllobothrium latum.
Table1. Epidemiology of the Intestinal Cestodes That Cause Disease in Humans
LABORATORY DIAGNOSIS
Both eggs and proglottids may be found in patient’s feces. Visualization of the eggs is enhanced using a wet preparation of the patient’s stool sample. Diagnosis is made by identification of the ovoid, operculated, yellow brown eggs (58 to 75 µm by 40 to 50 µm) passed in abundance in the stool. They are sometimes confused with the eggs of Paragonimus. The mature gravid proglottids are wider than long (3 × 11 mm), often in chains, and contain a rosette-shaped central uterus (Figure 3). Species identification is through assessment of the morphologic characteristics of the proglottids as previously described (Tables 2 and 3).
Fig3. Proglottid demonstrating rosette-shaped uterus in D. latum. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)
Table2. Common Human Parasites, Diagnostic Specimens, Tests, and Positive Findings
Table3. Cestode Parasites of Humans (Intestinal)
ANTIPARASITIC SUSCEPTIBILITY TESTING AND THERAPY
Humans infected with D. latum develop little or no protective immunity. Reinfection is common. Treatment with praziquantel or niclosamide is effective and non toxic. Subsequent stool specimens should be reexamined 6 weeks following treatment. The patient may require a vitamin B12 supplement if anemia develops.
PREVENTION
Prevention simply includes avoiding the consumption of raw fish. The larva stage is destroyed when food is thoroughly cooked or frozen. Treatment of patients infected with adult tapeworms is indicated to prevent accidental autoinfection. Good hygiene and proper sanitation measures will also prevent reinfection. Treatment of sewage before it enters lakes may help reduce the prevalence of infection.
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