Diagnosis of the fish tapeworm requires a thorough history with particular attention to the patient’s occupation, hobbies, eating habits, and travel history. Diphyllobothriasis is known to occur in both endemic and non-endemic areas as a result of globalization. As mentioned previously, it is associated with the consumption of raw or poorly cooked fish. Additionally, due to its extremely high fecundity, it is easily spread in regions with poor hygiene and sanitary practices. Human infection with D. latum can range from an asymptomatic state to mild gastrointestinal symptoms to severe cases of anemia as well as luminal obstruction. Studies have shown that in patients infected with D. latum, twenty-five percent will manifest symptoms of abdominal pain, diarrhea, fatigue, headaches, or pernicious anemia.[9]

A Korean study looking at D. latum infections identified a total of 49 cases between 1971 and 2012. When looking at the composite data in these cases, the patients were most commonly men between the ages of 30 and 49.[10] The significant signs and symptoms encountered were abdominal pain, vomiting, diarrhea, in addition to dizziness, myalgia, anemia, fatigue, and dyspepsia.[9]  In another Korean study looking at D. latum infections, all identified patients were males between the ages of 17 and 35 with primarily abdominal pain and distension as presenting complaints.[11] A case series in 2012 looked at 20 cases of confirmed diphyllobothriasis and found that the most frequently reported symptoms were fatigue and mild abdominal discomfort, identified in 66.6% of patients.[12] A larger Japanese study from 2012 to 2015 with 139 patients with confirmed D. latum infection revealed mild diarrhea as the most common symptom following by abdominal pain.[13]

Diphyllobothriasis can affect different organ systems with varied manifestations and are listed below based on published case reports and series:

• Central nervous system manifestations include paresthesia, demyelinating symptoms secondary to prolonged anemia and vitamin B12 deficiency, headaches, and encephalopathy.[4] 
• Ocular manifestations include optic neuritis secondary to long-standing vitamin B12 deficiency.[4][14]
• Gastrointestinal manifestations include acute abdominal pain, diarrhea, constipation, intestinal obstruction, subacute appendicitis, cholecystitis, as well as cholangitis.[4][9]
• Hematological manifestations include megaloblastic anemia, vitamin B12 deficiency, pancytopenia, eosinophilia, and pernicious anemia.[4][12]
• Respiratory manifestations include dyspnea in the setting of severe vitamin B12 deficiency.[4]
• Dermatological manifestations include glossitis and allergic symptoms, as well as pallor.[4]

In the appropriate clinical picture with signs and symptoms consistent with diphyllobothriasis, the clinician should consider initial laboratory workup. Laboratory workup may reveal peripheral eosinophilia and megaloblastic anemia along with vitamin B12 deficiency.[4] In general, the diagnosis of D. latum is from the presence of eggs or proglottids of the representative shape and structural characteristics in the patient’s stool sample.[4] This allows for the diagnosis at the genus level. Molecular methods to identify D. latum are also available and allow for a more reliable tool to identify the infection down to the species level, which is clinically more relevant for epidemiological studies and includes the use of PCR and restriction fragment length polymorphisms. Additional tests found in the literature include endoscopy, abdominal magnetic resonance imaging, and colonoscopy, but these are both costly and invasive.[15] Humans infected with D. latum will begin to pass eggs in their stools on average 15 to 45 days after the ingestion of the plerocercoid stage of larvae.[4]

Treatment for this parasitic infection, as well as most tapeworm infections, is praziquantel in those without contraindications. A 25 mg/kg dose is noted to be highly effective against D. latum infections.[4][16][17] Lower doses at 10 mg/kg have been noted to be effective against other species of Diphyllobothrium but have shown poor efficacy against D. latum in experimental animal models.[4] A single dose was found to have high cure rates. Side effects of praziquantel include weakness, headache, dizziness, abdominal pain, fever, and possibly urticaria.[4][17]

An alternative anthelminthic drug used for D. latum infections is a single dose of niclosamide, which is given either in a single 2-gram dose for adults or a 1-gram dose for children older than six years of age.

Praziquantel can be given in pregnancy.

The differential diagnosis includes influenza, as well as other helminthic diseases, and can be very broad due to the non-specific symptoms encountered, especially if the clinician neglects to obtain information regarding the consumption of raw or undercooked fish in the initial history.

The prognosis of D. latum infections will depend on the patient’s disease severity, comorbid conditions, as well as worm burden within the intestinal tract. Despite this, the prognosis is excellent, and treatment of this condition is effective.

Patients infected with D. latum are usually asymptomatic or present with a mild disease but may have complications if the worm burden is high this usually manifests secondary to severe vitamin B12 deficiency and its complications, including megaloblastic anemia and development of subacute combined degeneration of the spinal cord as well as cognitive decline.[18]

The best way to prevent infection with D. latum is to avoid the consumption of raw fish. Food safety practices include cooking the fish well or freezing fish at -18 degrees Celsius for 24 to 48 hours to prevent infection.[4]

• Diphyllobothriasis is caused by the fish tapeworm D. latum and is a zoonotic infection mostly attributed to the consumption of raw or undercooked fish.
• Diagnosis relies on thorough patient history, asking about occupational history, hobbies, travel history, eating habits, as well as maintaining a high index of suspicion.
• Most infected individuals present with a mild illness characterized by abdominal pain or discomfort, diarrhea, and malaise.
• Laboratory testing is largely non-specific but may reveal eosinophilia as well as vitamin B12 deficiency, megaloblastic anemia, and/or pernicious anemia.
• Diagnosis is clinical, but confirmation via stool testing is an option.
• Treatment is with a single dose of praziquantel, which is safe in pregnancy.

Given the effect of globalization and the prevalent habit of consuming raw or undercooked fish, clinicians, nurse practitioners, nurses with specialty training in infectious diseases, pharmacists, and public health officials should be aware of the reemergence of diphyllobothriasis, as part of an interprofessional healthcare team approach. The pharmacist should educate the patient on the importance of medication compliance.

Efforts to prevent the spread and acquisition of this disease will rely on a team-based approach to addressing water contamination around the world, anticipating the effect of climate change on parasites, and raising awareness for travelers and locals in endemic areas. [Level 5]