Describe general features of the cestodes.
- Different from trematodes in that they are typically intestinal and that most do not reproduce inside an intermediate host.
- 5,000 known species.
- All stages lack a mouth and gut but they have a metabolically active tegument with microvilli that can uptake nutrients and remove waste.
- Have a scolex at anterior end with attachment organs (hooks) that will embed into the intestinal wall so that the cestode stays in place. The scolexes can also uptake nutrients as well as the tegument.
- Nearly all segmented (hundreds of segments called proglottids) and are hermaphrodites.
- There are 2 or 3 hosts in life cycle, some can carry out auto-infection.
- Transmission is always passive.
- The main exception of cestodes is the Echinococcus (Hytadid cysts).
Describe the cestode family taeniidae and their general life cycle.
– three species where humans are the only definitive host: Taenia saginata, Taenia solium and Taenia asiatica.
General life cycle
o The adult worm lives in the intestine and reproduces sexually to form eggs, which are passed out in faeces. The intermediate host (usually a herbivore mammal) will consume eggs via faecal oral transmission.
o The egg then hatches inside intermediate’s intestine to release an oncosphere which will penetrate gut and transform into a metacestode.
o The life cycle is completed once the metacestode is ingested by definitive host
Describe the features of taenia saginata - structure, transmission and prevalence.
It is the beef tapeworm.
- Very long, their mean length differs from 5 to 20m with up to 2000 proglottids.
- However their scolex are only 1-2mm long with four suckers.
- Each proglottid (segment) can contain up to 200k eggs. Has around 1,000 segments. This gives it a high reproductive potential.
- One of the commonest tapeworm parasites of humans.
- They have worldwide distribution, common in Africa, east Europe, SE Asia, and South America.
- Humans are infected by eating raw or undercooked beef.
- The adult tapeworm can live for up to 25 years in human intestine.
Describe the life cycle of taenia saginata.
Two-host life cycle.
In the cow:-
- Cattle are infected by eating vegetation contaminated by the tapeworm eggs.
- The eggs will hatch in their small intestine and release oncospheres.
- The oncospheres will penetrate the gut, enter circulation and travel around the body until they penetrate muscle fibres in which they encyst to form cysticerci.
- The cysticerci have pearly white discoloration.
In human:-
- Humans are then infected by eating raw/undercooked beef and the taeniae will then attach to intestinal wall via their scolex.
- Eggs and proglottids will then be excreted in faeces and can cause further transmission.
Describe the features of taenia solium - structure, prevalence and transmission.
It is the pork tapeworm.
- Most important tapeworm parasite of humans and it’s cause of considerable morbidity and mortality.
- Worldwide distribution, about 4 million cases annually.
- Meat hygiene regulations means that it’s rare in developed countries but it’s endemic in poorer countries.
- Adults mean length varies from 2-7m (about 1000 proglottids) with a scolex of four suckers and two rows of hooks (smaller than T. saginata).
- Each mature proglottid contains about 50,000 eggs
- Infection of humans is by eating raw or undercooked pork (e.g. BBQ pork sausages!)
- The usual life cycle has the pig as intermediate host and humans as its definitive host.
- Pathology of adult worm is highly variable, there is often few symptoms. Similar symptoms to T. saginata.
Describe the life cycle and T.solium and explain how cysticercosis is caused.
In the pig:-
- Pigs become infected through faecal contamination of their vegetation and the eggs will hatch, releasing oncospheres which will travel to skeletal or cardiac muscle where they will develop into cysticerci.
- Because T.solium’s proglottids are not motile, the pig can be simultaneously be infected by hundreds of eggs as they do not disperse.
In the human:-
- Humans are then infected when they consume pork containing mature cysticerci.
- In the human, the scolex attaches to the intestinal wall and the proglottid chain grows.
- Eggs will be released with the faeces which can then be taken up by the pigs.
- Dogs and cats can also be intermediate hosts and where these are consumed by humans (e.g. parts of Asia) they can also be infected.
–> Auto-infection can also occur when a gravid proglottid is transported back to the stomach by reverse peristalsis. — This results in the eggs receiving stimuli to hatch even though they haven’t left the host’s body. This can result in sudden massive infection.
– This leads to retro infection – cysticercosis. The parasite larva will encyst in various parts of body.
– If it encysts in the brain, it causes neurocysticercosis – which often presents as hydrocephalus, epileptic seizures and paralysis.
(auto-infection always occurs in hytadid cysts)
What are the treatment options for cysticercosis?
- Skeletal Muscle/Subcutaneous tissues - Main problem aesthetic, usually untreated unless painful; remove by surgery
- Extraocular Muscle - Treated with praziquantel or albendazole (both with steroids); or remove by surgery
- Intraocular Cysts - Surgery preferable to drugs; latter would cause inflammation in the eye
- Neurocysticercosis - Depending on location, removal by surgery or treatment with high doses of albendazole (with steroids). If many cysts albendazole may not be used as it could cause massive swelling as it destroyed the cysts
Describe the features of Diphyllobothrium latum - structure, transmission and prevalence.
It is the broad fish tapeworm.
- It is the largest human parasite. It is up to 10 m long and very broad (the proglottids nearest to scolex are broader than they are long), hence the name.
- Its scolex has slit-like grooves for attachment. 3-4k proglottids, each up to 2cm wide.
- Up to a million eggs are produced per day by each worm.
- It consists of 3 host life cycle –> crustacean – fish – fish-eating mammal.
- Infected fish may sometimes be eaten by a larger fish (can occur several times).
- Wide range of reservoir hosts.
- Common in temperate/subarctic regions where raw fish is eaten (e.g. Baltic). It’s now spread to non-endemic areas such as Brazil and in developed countries due to increasing consumption of raw fish.
Describe the life cycle of diphyllobothrium latum.
Definitive host:-
- The parasite attaches to the gut wall and eggs are passed in its faeces.
- The eggs take a few weeks before they hatch and release the free-living coracidia which consists of the oncosphere surrounded by cilia.
- The coracidium will die unless it’s eaten by a crustacean, which acts as the first intermediate host.
Intermediate host:-
–The coracidium will penetrate its gut and develop into the procercoid larvae in the body cavity.
In fish:-
- The crustacean will then be ingested by a small freshwater fish (usually salmon), which acts as the second intermediate host.
- The parasite will penetrate the gut of the fish and make its way to muscle where it will develop into plerocercoid larva.
- The life cycle is complete once the fish is ingested by its definitive host.
