Definition. Diphyllobothriasis is a fish-borne zoonotic infection with the cestode Diphyllobothrium latum or other similar species (eg, Diphyllobothrium nihonkaiense, Diphyllobothrium dendriticum, Adenocephalus pacificus, Diplogonoporus balanopterae).
Epidemiology. The adult tapeworm lives in the gut of humans as well as fish-eating carnivores. In humans the adult worm can reach a length of 10 m. The fish tapeworm occurs and endemic in para-polar areas where humans consume raw or pickled fish. The fish is eaten by man or a fish-eating mammal and the plerocercoid matures in the small intestine into an adult worm. Should the fish be eaten by another predator fish the plerocercoid remains in the muscle of the predator fish which simply acts as a carrier host. As the tapeworm, it grows approximately 5 cm/day, releases up to 1 million eggs per day, and can survive for 10 years, the production of eggs is high, so that water bodies can become contaminated very quickly. Besides humans, dogs and cats can also become infected.
Morphology. The Broad Fish tapeworm is 15 – 20 mm wide and has approximately 4000 proglottids. The scolex has two attachment organs called bothria and no hooks. The scolex is finger-like and has a simple longitudinal groove each on the ventral and dorsal sides. The small follicles of testes and vitellaria are difficult to distinguish from one another. They are arranged in two broad lateral bands, leaving free only a narrow part in the middle of the proglottid with the bilobed ovary situated at the posterior margin. In front of it, the uterus leads in tight coils to the anterior part of the proglottid. Here, the seminal vesicle, the cirrus pouch, and the male genital opening are located together with the vaginal opening and the opening of the uterus, which is called a ‘tocotrem’ (Greek: tókos = parturition, tréma = opening). Through this, the operculated (yellow-brown eggs) 60–66×40–49μm in size, are shed. The emptied proglottids then fall off the strobila.
Symptoms. In most cases, humans are infected with one specimen only and usually do not show symptoms of infection, possibly for several years. Diphyllobothriasis can manifest as symptoms of intestinal obstruction, vitamin B-12 deficiency, and/or, most commonly, the passage of proglottids in the stool. The scope of infection has become more cosmopolitan with the increasing popularity of ethnic raw fish containing food and the rapid global distribution of fresh foodstuff.
Diagnosis. Laboratory studies that may be helpful for diphyllobothriasis include the following: (1) Microscopic stool examination for ova and parasites (the usual basis for the diagnosis); (2) Complete blood count (CBC) demonstrating anemia; and (3) Vitamin B-12 level of less than 150 pmol/L and total serum homocysteine level of more than 13 µmol/L or methylmalonic acid level of more than 0.4 µmol/L in the absence of renal failure and folate and vitamin B-6 deficiencies.
Treatment. Treatment of the worm infection is pharmacologic, involving one of the following agents Praziquantel (drug of choice) 5-10 mg/kg orally in a single dose; taken with liquids during a meal and Niclosamide (an acceptable alternative).
Chernin, J. Parasitology – Lifeline (Modules in life sciences). CRC Press, London: 2000.
David R Haburchak, D.R. and Chandrasekar, P.H. Diphyllobothriasis [Updated: Jun 13, 2019]. Available from: https://emedicine.medscape.com/article/216089-overview
Lucius R., Brigitte Loos-Frank, Richard P.L., Robert P., Craig W.R., and Richard K.G. The Biology of Parasites. Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim, German: 2017.