Dibothriocephalus nihonkaiensis infection accompanied with vitamin B12 deficiency: A case report.

Diphyllobothriosis is a tapeworm infection and is associated with the consumption of raw or undercooked fish. Among causative pathogens of human tapeworm infection, Dibothriocephalus nihonkaiensis is the second most prevalent species, contributing to over 2000 cases of human infection annually. Although D. nihonkaiensis is primarily distributed in the northern Pacific region, several cases have recently been reported from other parts of the world, including the United States. As there is a concern regarding the further spread of D. nihonkaiensis infection, accurate diagnosis and understanding related complications is essential. Although Dibothriocephalus latus, one of the major species involved in human diphyllobothriasis, is widely recognized to occasionally accompany vitamin B12 deficiency, to the best of our knowledge, a case of D. nihonkaiensis with decreased vitamin B12 level has not been previously reported. However, the issue of vitamin B12 deficiency caused by tapeworms should be discussed carefully because the two species were not separated before 1986. We herein report the first case of molecularly proven D. nihonkaiensis infection in Japan associated with vitamin B12 deficiency. The case was a 19-year-old male without any significant past history. The patient presented with tapeworm excretion at 2 months following raw trout fish consumption. At the initial visit, his laboratory data showed a slightly decreased vitamin B12 level; however, anemia was not noted. Two months later, his vitamin B12 level further decreased; however, it spontaneously resolved following anthelmintic treatment. This case provides valuable insights into the clinical picture of D. nihonkaiensis infection.