Diphyllobothrium latum infection
Essentials of Diagnosis
- Stool examination reveals ovoid, yellow-brown eggs (60-75 µm by 40-50 µm).
- Chains of proglottids (up to 50 cm long) may be passed in stool.
- Proglottids are wider than long (3 by 11 mm).
- The scolex has no hooklets and has two grooves (bothria).
- The gravid proglottid contains a rosette-shaped central uterus.
General Considerations
D latum is found worldwide, and infection is acquired by ingesting contaminated raw or inadequately cooked freshwater fish. Because of the popularity of raw or undercooked fish, Siberia, Europe, Canada, Alaska, and Japan are endemic regions for D latum infection. After the D latum cyst is ingested, the worm matures within the human intestine and begins producing eggs after 5 weeks. A mature D latum may reach lengths of several metres and contain about 30,000 proglottids.
Eggs and proglottids passed in stool hatch after 14 days in fresh water into ciliated coracidium larvae, which are ingested by the intermediate host, the aquatic copepod. Inside the copepod, the larvae develop into a second larval form, the procercoid. After the copepod is ingested by a freshwater fish, the procercoid larva matures into the plerocercoid larva, which may encyst within fish tissues. Human ingestion of inadequately prepared fish initiates infection via the plerocercoid larval cyst. Bears, seals, cats, mink, foxes, and wolves are alternate definitive hosts for D latum.
Clinical Findings
Signs and Symptoms
Infection with D latum is most often asymptomatic, but symptoms such as bloating, abdominal pain, or diarrhoea may occur. More rarely, intestinal obstruction may occur. A rare complication of chronic small-intestinal involvement with D latum is vitamin B12 deficiency, characterised by anaemia with or without neurological sequelae. This syndrome occurs most often in patients with a genetic predisposition to pernicious anaemia, commonly people of Scandinavia. Patients with unexplained anaemia or neurological symptoms and a history of raw freshwater fish consumption should be evaluated carefully.
Laboratory Findings
Often, the only abnormal finding in a patient infected with D latum is the presence of eggs or proglottids on stool examination for ova and parasites. Blood examination may show a mild leukocytosis with eosinophilia and, occasionally, a megaloblastic anaemia associated with vitamin B12 deficiency.
Imaging
Contrast studies of the gastrointestinal tract may reveal ribbon-like filling defects corresponding to the adult worm.
Differential Diagnosis
The most common manifestation of D latum infection is asymptomatic carriage, which is discovered incidentally. If patients present with abdominal pain and diarrhoea, the differential diagnosis includes a range of infectious and non-infectious causes. Diarrhoea due to D latum infection is not associated with stool leukocytes; this helps in formulating a differential diagnosis. Non-infectious aetiologies to consider include osmotic (eg, lactose intolerance) and secretory (eg, villous adenoma) causes, malabsorption syndromes (eg, coeliac sprue), and motility disorders (eg, irritable bowel syndrome). Infectious aetiologies that cause diarrhoea without stool leukocytes include rotavirus, Norwalk virus, Giardia lamblia, Entamoeba histolytica, Cryptosporidium spp., and toxigenic diarrhoea caused by Staphylococcus aureus, Bacillus cereus, Clostridium perfringens, and enterotoxigenic Escherichia coli.
| Aspect | Findings and notes | Examples |
|---|---|---|
| Transmission and exposure | Ingestion of raw or undercooked freshwater fish in endemic regions | Siberia, Europe, Canada, Alaska, Japan; consumption of contaminated fish |
| Intestinal manifestations | Often asymptomatic; may cause mild to moderate gastrointestinal complaints | Bloating, abdominal pain, diarrhoea; rare intestinal obstruction |
| Haematological manifestations | Interference with the vitamin B12-intrinsic factor complex, leading to deficiency in some patients | Megaloblastic anaemia, especially in individuals predisposed to pernicious anaemia |
| Neurological manifestations | Related to long-standing vitamin B12 deficiency | Peripheral neuropathy, cognitive changes, possible posterior column involvement |
| Laboratory findings | Eggs or proglottids on stool examination; mild blood count abnormalities | Ovoid yellow-brown eggs; mild leukocytosis with eosinophilia; anaemia in deficient patients |
| Imaging findings | Non-invasive visualisation of the adult worm in the intestine | Ribbon-like filling defects on contrast gastrointestinal studies |
| Prevention and follow-up | Safe fish preparation and post-treatment monitoring | Thorough cooking or appropriate freezing of freshwater fish; follow-up stool examinations; assessment of vitamin B12 status when indicated |
Complications
Complications vary with the clinical syndrome associated with infection. Chronic diarrhoea may lead to malnutrition. Megaloblastic anaemia secondary to vitamin B12 deficiency results when the parasite disrupts the vitamin B12-intrinsic factor complex, which makes vitamin B12 unavailable for absorption by the host. Vitamin B12 deficiency may lead to neurological sequelae, including peripheral neuropathy, dementia, and possible severe combined degeneration of the posterior columns. Infection with D latum may also rarely result in intestinal obstruction caused by a mass of entangled worms.
Early recognition of anaemia and neurological changes is important, because timely treatment of the infection and correction of vitamin B12 deficiency can prevent or limit permanent neurological damage.
Treatment
Treatment for infection with D latum consists of either praziquantel or niclosamide. Follow-up stool examinations should be performed 1 and 3 months after treatment to document clearance of the parasite. Drug selection and dosing are individualised and should follow established clinical guidance.
Prognosis
As the disease is not commonly associated with severe symptoms, the prognosis for infected individuals is excellent. One exception is patients who develop vitamin B12 deficiency; in these patients, neurological complications are reversible only if they are recognised and treated early.
Prevention & Control
Prevention of D latum infection is achieved through adequate cooking of all freshwater fish or by freezing fish for 24-48 hours at about -18°C (0°F). Isolation of infected people is not required. Public health measures that promote safe handling and preparation of freshwater fish in endemic areas can further reduce transmission.