Diphyllobothriasis associated with eating raw pacific salmon

Abstract

The incidence of human infection with the broad tapeworm Diphyllobothrium nihonkaiense has been increasing in urban areas of Japan and in European countries. D. nihonkaiense is morphologically similar to but genetically distinct from D. latum and exploits anadromous wild Pacific salmon as its second intermediate host. Clinical signs in humans include diarrhea and discharge of the strobila, which can be as long as 12 m. The natural life history and the geographic range of the tapeworm remain to be elucidated, but recent studies have indicated that the brown bear in the northern territories of the Pacific coast region is its natural final host. A recent surge of clinical cases highlights a change in the epidemiologic trend of this tapeworm disease from one of rural populations to a disease of urban populations worldwide who eat seafood as part of a healthy diet.

Figure 1

Wood print depicting a man passing a strobila of a broad tapeworm. The caption (not shown) said, “The man ate masu salmon. After a time, a strange object emerged from the anus and was pulled out: it turned out to be 2–3 m long.” From Shinsen Yamaino Soushi, by Daizennosuke Koan (1850). Courtesy of the Tohoku University Medical Library.

Figure 2

Diphyllobothriasis cases, Department of Medical Zoology of the Kyoto Prefectural University of Medicine in Kyoto and Department of Infectious Diseases of the Tokyo Metropolitan Bokutoh Hospital in Tokyo, Japan, 1988–2008.

Figure 3

Seasonal occurrence of diphyllobothriasis nihonkaiense, 149 cases, Department of Medical Zoology of the Kyoto Prefectural University of Medicine in Kyoto and Department of Infectious Diseases of the Tokyo Metropolitan Bokutoh Hospital in Tokyo, Japan, 1988–2008.

Figure 4

Age distribution of patients with diphyllobothriasis nihonkaiense, Department of Medical Zoology of the Kyoto Prefectural University of Medicine in Kyoto and Department of Infectious Diseases of the Tokyo Metropolitan Bokutoh Hospital in Tokyo, Japan, 1988–2008.

Figure 5

Possible distribution area of Diphyllobothrium nihonkaiense. Open circle, open square, and open triangle represent brown bears, humans, and Pacific salmon, respectively, from which D. nihonkaiense adult worms or plerocercoids were isolated and identified by DNA sequencing (DNA sequences refer to reference 21). Patients in European countries are suspected to have eaten salmon imported from the Pacific coast of North America.