Fish tapeworm and sushi

Symptoms and investigations

Diphyllobothrium infections are often asymptomatic. When symptoms occur, they are often mild and vague, including fatigue, constipation, and poorly defined abdominal discomfort. Laboratory investigations tend to present normal results, but might show low vitamin B12 levels or frank pernicious anemia (if the worm has attached in the proximal small intestine, it can compete for vitamin B12 absorption).⁴ Often the first awareness of infection might be passing segments of the tapeworm in the stool,^5–7 as the patient in this report did, whose diarrhea might have been due to an unrelated illness. Diagnosis is made by identification of ova or sometimes worm segments in the stool (Figures 1 and 2).

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Figure 1.

Diphyllobothrium latum ova wet mount

Photo credit: D.M. Raymondo MLT CLS(M)

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Figure 2.

Diphyllobothrium latum proglottids

Photo credit: D.M. Raymondo MLT CLS(M)

Treatment

Treatment is usually 10 to 25 mg/kg of praziquantel given as a single dose.⁴ Stool culture for ova should be negative a week after treatment; occasionally a second dose might be needed.⁶ While treatment is generally completed in a single dose, segments might continue to be evacuated over a prolonged period, which can be distressing to patients and their families. If vitamin B12 levels are low, they generally return to normal ranges within a period of several months.⁴ While praziquantel is not officially indicated for this use in Canada, it is the accepted treatment in the literature. Niclosamide, 2 g as a single dose, is another accepted treatment, but it is not available in Canada.

Prevention

Prevention can be accomplished on a population level by good sewage treatment plants interrupting the cycle at the point where eggs are discharged back into the water, when humans are the definitive host. This is the likely reason for the decline of diphyllobothriasis in previously endemic areas such as Scandinavia⁴ and coastal areas of Japan.⁷

On an individual level, consumers should be aware of the risks of diphyllobothriasis with consuming uncooked fish. Sushi and sashimi are now available not just in restaurants, but also in the deli sections of many grocery stores. Marketers should consider affixing labels to the packaging, assuring consumers that proper preparations have been completed to minimize risks of fish tapeworm. Those who prepare fish should be aware that cooking at a temperature of only 55°C for 5 minutes will kill the larvae; freezing to − 20°C for 7 days, or flash freezing to − 35°C for 15 hours, as long as the flesh is less than 15 cm thick, effectively kills the larvae also.^4,5,7,8 Sushi and sashimi chefs should use only fish frozen in this way. While these freezing protocols are the standards required by public health regulations, they are not easy for public health inspectors to verify.

Relevance

With the increasing popularity and availability of sushi in restaurants and grocery stores, one might expect to see an increase in the number of cases of diphyllobothriasis such as this one. Ching described a substantial increase in the number of cases in British Columbia between the 1970s and the early 1980s, attributed to increasing consumption of uncooked salmon.¹¹ A MEDLINE search using the terms diphyllobothriasis and sushi revealed a number of case reports from a variety of places around the world, only 3 of which had North American connections. In all cases, raw or undercooked fish were consumed, often salmon species, but sometimes perch, char, or pike.⁵ Sushi and sashimi restaurants were specifically implicated in some reports.^7–9 Additional cases are likely occurring but are not being reported in the literature. Primary care physicians should keep the possibility in mind.