Other Cestodes
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Direct answer
Other cestodes of exam interest include Taenia saginata, Hymenolepis nana, Diphyllobothrium latum and Dipylidium caninum, each with a distinct intermediate host or egg feature. Taenia saginata is the beef tapeworm acquired from undercooked beef, while Diphyllobothrium latum causes megaloblastic anaemia by absorbing vitamin B12. Praziquantel is the usual drug for most of these infections, but exam questions often test the specific host or egg.
What you must remember
- Taenia saginata: The beef tapeworm has four suckers and no rostellar hooks, and cattle are its intermediate host.
- Hymenolepis nana: The dwarf tapeworm completes its cycle without an intermediate host and can cause autoinfection.
- Hymenolepis diminuta: The rat tapeworm uses arthropods as intermediate hosts and rarely infects humans.
- Diphyllobothrium latum: The broad fish tapeworm has operculated eggs and causes vitamin B12 deficiency with megaloblastic anaemia.
- Dipylidium caninum: The dog and cat tapeworm is transmitted by fleas, and its gravid proglottids look like cucumber seeds and contain egg packets.
- Egg features: Hymenolepis nana eggs have polar filaments, Diphyllobothrium eggs are operculated, and Taenia eggs are radially striated.
- Drug of choice: Praziquantel treats most of these cestodes, including Hymenolepis, Diphyllobothrium and Dipylidium.
- Clinical clue: Taenia saginata proglottids are motile and may be seen passed actively in stool.
How the life cycle decides the diagnosis
These cestodes share an adult stage in the intestine but differ in how larvae reach humans. Each step of the life cycle links to a typical exam question, so tracing the route is the fastest way to recall the answer.
- Egg or proglottid in stool: Gravid proglottids or eggs appear in stool, and each cestode has a characteristic egg morphology that helps identify the species.
- Intermediate host: Taenia saginata uses cattle, Diphyllobothrium uses copepods followed by fish, and Dipylidium uses fleas. Hymenolepis nana needs no intermediate host at all.
- Human infection: Humans acquire Taenia saginata from raw or undercooked beef, Diphyllobothrium latum from raw freshwater fish, and Dipylidium caninum by accidentally ingesting an infected flea.
- Hymenolepis route: Hymenolepis nana eggs can hatch in the intestine and reinfect the same host, leading to autoinfection and heavy worm burdens in children.
- Clinical outcome: Most infections are mild or asymptomatic, but Diphyllobothrium latum absorbs vitamin B12 and so produces megaloblastic anaemia and neurological symptoms in some patients.
- Treatment: Praziquantel is the usual single-dose drug for most cestodes, while niclosamide is an older alternative used in some settings.
Where the exam tries to catch you
Most mistakes come from assigning the wrong intermediate host or the wrong anaemia.
- Trap: Choosing pig as the intermediate host for Taenia saginata Correct: cattle are the intermediate host, while pigs serve Taenia solium.
- Trap: Assuming Hymenolepis nana needs an intermediate host Correct: H. nana completes its cycle in the same host, which makes it the exception among cestodes.
- Trap: Linking Diphyllobothrium to iron-deficiency anaemia Correct: this worm causes vitamin B12 deficiency with megaloblastic anaemia, not microcytic anaemia.
- Trap: Calling Dipylidium an infection from dog faeces alone Correct: infection requires ingesting an infected flea, and the egg packets are released from proglottids.
Frequently asked questions
Which cestode lacks an intermediate host?
Hymenolepis nana is the cestode that completes its life cycle without an intermediate host, so eggs passed in faeces can reinfect the same person. This also allows autoinfection in heavy infections.
Which tapeworm causes megaloblastic anaemia?
Diphyllobothrium latum, the broad fish tapeworm, causes vitamin B12 deficiency and megaloblastic anaemia. It is acquired by eating raw or undercooked freshwater fish that carry the plerocercoid larvae.
What egg feature identifies Diphyllobothrium latum?
Diphyllobothrium latum eggs are operculated, meaning they have a small lid at one end, and they are passed unembryonated in stool. This feature separates them from the radially striated eggs of Taenia species.
Which drug is used for most intestinal cestode infections?
Praziquantel is the drug of choice for most intestinal cestode infections, including Taenia saginata, Hymenolepis nana, Diphyllobothrium latum and Dipylidium caninum. A single oral dose is usually effective.
How is Dipylidium caninum acquired by humans?
Humans acquire Dipylidium caninum by accidentally ingesting an infected flea that carries the cysticercoid larval stage. Children are often affected after close contact with dogs or cats that harbour fleas.