Species belonging to the genus Dibothriocephalus/Diphyllobothrium represent some of the largest cestodes (tapeworms) capable of infecting humans. Historically, Dibothriocephalus latus (previously Diphyllobothrium latus) is considered the more prevalent species (in Europe and Patagonia). However, Dibothriocephalus nihonkaiensis (formerly Diphyllobothrium nihonkaiensis) is endemic to East Asian countries, notably Japan, where raw salmon consumption is common [1, 2]. The global exchange of dietary practices, including the consumption of raw or poorly cooked salmon, driven by increasing globalisation, suggests that D. nihonkaiensis infections are no longer confined to endemic regions and may be more prevalent than currently appreciated.

A 35-year-old Asian ethnic Chinese female, with a past medical history of gallstones, presented with abdominal distension for 6 months and right sided abdominal pain for 3 months. This was associated with 1 month of loose stools, fatigue and significant (7 kg) unintentional weight loss over the past 3 months. There was no per rectal bleeding, malena or tenesmus. She denied having fever, headache, red eyes, skin lumps, rashes or pruritus ani. 3 months prior to her symptom onset, she had travelled extensively throughout Asia (Thailand, Vietnam, Laos, Malaysia and China) and Mexico and had consumed raw salmon throughout her travels.

Physical examination did not reveal any significant findings. Examination of the cardiorespiratory and abdominal systems were unremarkable.

Full blood count showed 6.81 white blood cells (normal 4.0-9.6 × 10³ /µL), 10.7% eosinophilia (0.73 × 10³ /µL) (normal 0–7%; 0-0.60 × 10³ /µL), haemoglobin level of 15.6 g/dL (normal 11.8–14.6 g/dL) with a mean corpuscular volume 90.7 fL (normal 83.0–98.0 fL), mean corpuscular haemoglobin 32.2 pg (normal 28.0–34.0 pg) and mean corpuscular haemoglobin concentration 35.5 g/dL (normal 33.0–35.0 g/dL). Liver and renal function tests were normal. Microscopic evaluation of 3 stool samples for ova, protozoan cysts and parasites was negative, as were stool polymerase chain reaction for Entamoeba histolytica, Giardia lamblia, Cryptosporidium sp. and Cyclospora cayetanensis.

She was referred for colonoscopy. A worm was passed out during bowel preparation for colonoscopy (Fig. 1A). Colonoscopy did not reveal any worms or abnormalities. A pan-Cestoda PCR targeting a fragment of the mitochondrial ribosomal RNA (16 S rRNA – 12 S rRNA) genes based on a protocol developed by Le and colleagues was attempted [3]. The gene target used for this identification spanned 813 base pairs across the 16 S lsu-rRNA and 12 S ssu-rRNA genes. Basic Local Alignment Search Tool (BLAST) analysis revealed a 99.8% similarity to the reference sequence NC_009463 in the GenBank database. The obtained sequence has been deposited in GenBank under accession number PP102438. The worm was eventually identified as D. nihonkaiensis. Microphotographs of the proglottids and egg are presented in Fig. 1B and C, respectively.

She received a single dose of praziquantel at 10 mg/kg, which led to a prompt resolution of her symptoms. Subsequent stool samples, collected one-month post-treatment, confirmed the absence of eggs.

Dibothriocephalus nihonkaiensis remains frequently under-recognized in clinical settings due to its significant morphological similarities with D. latus. This diagnostic challenge carries substantial epidemiological implications. Recent genetic studies conducted across Asia, encompassing Korea, Japan, and China, have indeed demonstrated that numerous cases previously diagnosed as D. latus were, upon re-evaluation, identified as D. nihonkaiensis [4–7].

Beyond this current case, Singapore has documented two additional D. nihonkaiensis infection in 2019 [8]. These occurrences highlight the increasing susceptibility of cosmopolitan societies, characterized by diverse populations and global culinary influences, to the emergence of previously uncommon parasitic infections. In the presented case, precisely pinpointing whether the patient acquired the infection locally in Singapore from imported Pacific salmon may be challenging, given her history of consistent raw salmon consumption during her various frequent international travels. Even so, for accurate diagnosis and epidemiological understanding of tapeworm infections, a detailed dietary history is paramount. By considering the patient’s consumption of raw or undercooked foods and the timing of such ingestion, clinicians may often postulate the likely source and acquisition period of the infection, not only for Dibothriocephalus/Diphyllobothrium species but for cestode infections in general.

Patients infected with Dibothriocephalus/Diphyllobothrium species may present asymptomatically, or manifest with a range of gastrointestinal symptoms including abdominal pain, distension, nausea/vomiting, and diarrhea. Constitutional symptoms such as fatigue, loss of appetite, and weight loss can also occur. Infrequent but serious complications like cholecystitis, cholangitis, pancreatitis, or intestinal obstruction have been reported. Peripheral eosinophilia is occasionally observed [9]. Diagnostic modalities such as colonoscopy and capsule endoscopy can be valuable in detecting these cestodes.

Humans become infected with D. nihonkaiensis through the consumption of raw or undercooked salmonids, which serve as intermediate hosts harbouring the larval plerocercoids [10]. Specifically, Pacific salmon species such as Oncorhynchus masou (cherry salmon) and Oncorhynchus keta (chum salmon) are known to be the second intermediate hosts for D. nihonkaiensis [10–12]. In contrast, D. latus infection is typically acquired from freshwater fish such as Perca fluviatilis (Perch) and Esox lucius (Pike) [10].

Historically, D. latus was recognized as the longest human tapeworm. However, adult Dibothriocephalus species, including D. nihonkaiensis, can achieve lengths exceeding 10 m within human hosts [1, 13]. Solely relying on adult worm length for species differentiation is unreliable due to significant overlap and variability. The incubation period for plerocercoids of both D. latus and D. nihonkaiensis typically ranges from 2 to 6 weeks until their development into adult diphyllobothriids in the small intestine [14]. These adult worms can persist in the human host for approximately 10 years or longer.

While D. nihonkaiensis eggs have generally been reported as smaller than those of D. latus, this morphological distinction is not reliable for definitive species diagnosis. A comprehensive study by Choi et al. (2015) demonstrated wide and overlapping variations in both the length and width of eggs between the two species [15]. Consequently, the highly variable egg size of D. nihonkaiensis precludes its use as a specific diagnostic criterion for differentiating diphyllobothriid tapeworm infections in human patients. Other proposed morphological distinctions between Dibothriocephalus subspecies include variations in eggshell shape, the configuration of the genital atrium opening, and the angle between the long axis of the cirrus sac and the seminal vesicles.

However, given the considerable morphological overlap in adult diphyllobothriids and their eggs, molecular identification proves indispensable for definitive species determination. This typically involves sequencing mitochondrial genes such as NADH dehydrogenase subunit 1 (nad1) and cytochrome c oxidase subunit 1 (cox1), as well as 5.8 S ribosomal RNA gene sequences [16, 17]. In this case, DNA sequence analysis of the region (16 S and 12 S RNA genes) encoded by the mitochondrial genome was useful for identifying the diphyllobothriid species.

Praziquantel is the highly effective anthelmintic agent of choice for treating diphyllobothriasis, irrespective of the infecting Dibothriocephalus/Diphyllobothrium species. Niclosamide serves as an alternative therapeutic option. Praziquantel specifically targets and eliminates adult worms; however, it does not exert ovicidal activity. Therefore, stool samples should be re-examined for the presence of eggs approximately one-month post-treatment to confirm parasite clearance. Furthermore, endoscopic removal of the scolex has also been reported as a successful method for achieving disinfestation in diphyllobothriasis cases [18].

In conclusion, this case highlights that D. nihonkaiensis infections can occur in non-endemic areas, likely driven by evolving food consumption patterns, particularly the increased intake of raw salmon. Therefore, there is a critical need to enhance awareness regarding Dibothriocephalus/Diphyllobothrium species infections. We emphasize the importance of reporting all diagnosed cases and rigorously identifying expelled worms, preferably through molecular methods due to morphological similarities. Such comprehensive data collection will be invaluable for pinpointing infection sources, guiding effective food safety surveillance, and ultimately bolstering public health measures against these emerging parasitic pathogens.