Consumption of infected second intermediate hosts by larger predatory fish, which serve as paratenic hosts; the parasites thereupon migrate into the musculature awaiting consumption by definitive hosts in which adult tapeworms will then finally develop in the small intestine to release up to a million immature eggs per day per parasite. Hosts begin to release eggs 5–6 weeks after infection. Pathophysiology
Tapeworms develop in the small intestine. Adults attach to the intestinal mucosa. Adult tapeworms may grow to over 10m in length and may constitute of over 3,000 proglottids which contain sets of male and female reproductive organs, allowing for high fecundity. Eggs appear in the faeces 5–6 weeks after infection.D. latum tapeworms are the longest and typically reach a length of 4-15m, but may grow up to 25m in length within the human intestine. Growth rate may exceed 22 cm/day. D. latum tapeworms feature an anterior end (scolex) equipped with attachment grooves on the dorsal and ventral surfaces. Host-parasite interactions
Diphyllobothriasis is postulated to cause changes in neuromodulator concentrations in host tissue and serum. D. latum infection has been shown to cause local changes in the host, leading to altered GI function (including motility) via neuroendocrine modulation.Diphyllobothriasis causes mast cell and eosinophil degranulation, leading to pro-inflammatory cytokine release. Diagnosis
Infection may be suspected based upon a patients occupation, hobbies, eating habits, and travel history. During diagnostic procedures, standard safety precautions apply. Eggs are not directly infectious to humans (in contrast to some other tapeworm species).
latum competes with the host for vitamin B12 absorption, absorbing some 80% of dietary intake and causing deficiency and megaloblastic anaemia in some 40% of cases. Vitamin B12 uptake is enabled by the position of the parasite which usually lodges in the jejunum. Vitamin B12 deficiency is - according to research - conversely seldom seen in D. pacificum infection. Gastrointestinal obstructions
Rarely, massive infections may result in intestinal obstruction. Migration of proglottids can cause cholecystitis or cholangitis. Cause
Diphyllobothriasis is caused by infection by several species of the Diphyllobothrium genus. Transmission
Humans are one of the definitive hosts of Diphyllobothrium tapeworms, along with other carnivores; fish-eating mammals (including canids, felids, and bears), marine mammals (dolphins, porpoises, and whales), and (a few) birds (e.g. gulls) may also serve as definitive hosts.Definitive hosts release eggs in faeces; the eggs then mature in ~18–20 days if under favourable conditions. Crustaceans serve as the first intermediate hosts, and Diphyllobothrium larvae develop. The larvae are released when crustaceans are consumed by predators, which serve as second intermediate hosts (these are mostly small fish). Larvae move into deeper tissues in the second intermediate host. Second intermediate hosts do not serve as an important source of infection of humans as these are not regularly consumed raw.