Platyhelminthes/Trematodes Flashcards
Describe the platyhelminthes.
- A phylum commonly known as flatworms since they are dorso-ventrally flattened and worm like in appearance.
- Invertebrates that are parasites of other organisms.
- They are bilaterally symmetrical with a mouth end where waste is egested since they have no anal opening.
- No circulatory system –> they depend on diffusion through their highly flattened body.
- The platyhelminth body wall (tegument) in endoparasitic forms can absorb nutrients – especially in cestodes which lack a mouth and gut.
- Most are hermaphrodites, meaning they have both male and female organs.
What the four main classes of the phylum platyhelminthes?
- Trematoda (flukes) – endoparasitic mostly, there are a few ectoparasitic.
- Cestoda (tapeworms) – endoparasitic.
- Turbellaria – free-living.
- Monogenea – mostly ectoparasitic.
(Ectoparasitic exist outside host, endoparasitic exist inside host.)
Describe the class trematoda.
– 10,000 known species, all parasitic.
– Commonly known as flukes – those living in blood as ‘blood flukes’, in the gut as ‘intestinal flukes’ and those in liver as ‘liver flukes’.
– Complicated life cycles - they have one or two intermediate hosts:
o The first host is nearly always a ‘gastropod mollusc’ – snail. This is where asexual reproduction takes place.
o There is passive (e.g. eating) and active (penetration of the parasite) transmission between the different hosts.
– They possess an alimentary canal, also uptake some nutrients via their tegument.
– They are nearly all hermaphrodites, except for schistosomes.
What is polyembryony?
Form of asexual reproduction where the presence of several larvae stages will allow one miracidium to produce a lot of cercariae.
- The snail is infected by either eating the parasite egg or by being actively invaded by larvae stage. The parasite will penetrate the digestive system of the snail, where they undergo a cycle of different larvae stages.
- They first form a sporocyst which can either form a daughter sporocyst or redia by asexual reproduction.
- Eventually the daughter redia/sporocyst will form an active swimming stage called the cercaria. The cercaria will leave the snail and then infect the 2nd host.
- This mechanism will increase the chance of transmission significantly as one sporocyst produces masses of cercaria by different pathways – ‘multiplication stage’.
Describe transmission to the second host.
–> Host seeking by cercariae is not a random process, and is stimulated by light, gravity, turbulence, and rarely by chemicals released by the second intermediate host.
–> The 2nd host can be a fish, amphibian or some species do not have a second intermediate host so they will infect a human (e.g. schistosomes).
What are the two types of trematode infections?
- Passive infection - food-borne, liver and intestinal flukes.
- Active infection - blood flukes - schistosomiasis.
(they are called ‘flukes’ due to their leaf like shape)
Describe the trematode food-borne infections - their prevalence.
- Fasciola, Clonorchis are liver flukes which are major public health problem, especially in south-east Asia.
- Its prevalence has reduced in the past decades due to increased urbanisation and socio-enconomic development, along with the use of drugs.
- However there endemic areas are expanding due to increased production of aquatic foods (snails crave aquatic areas, also irrigation schemes which have created more ‘wet’ areas) as well as increased migration.
Describe fasciola hepatica - transmission, hosts.
Sheep liver fluke.
- Include some of the largest flukes – the adult worm is 20-50mm long x 10-13mm wide.
- They are found in liver/bile duct in mammals.
- Important parasitic infection of livestock.
- Zoonotic infection, ‘accidental host of humans’.
- Widespread disease distribution and its most important cattle helminth infection – causes high economic loss.
— Most transmission occurs through eating contaminated aquatic food, salad or by eating raw infected liver in Middle East which generates an alternative form of the infection - Halzoun.
- It occurs mainly in sheep/cattle areas especially those that eat aquatic plants such as raw watercress.
- A large number of hosts act as a reservoir for the parasitic infection, which makes control more difficult.
- Only have one intermediate host (an aquatic snail).
Describe the life cycle of fasciola hepatica.
– Adult flukes live in bile ducts of the host and release eggs that pass with bile and are excreted with faeces. The eggs will hatch, releasing the free-living miracidium larva.
In the snail:-
- The miracidium will search and actively bore its way into an aquatic/semi-aquatic snail.
- The miracidium will then develop into a sporocyst which will undergo asexual reproduction to generate first generation redia.
- The redia are motile and can move through the snail’s tissues once released.
- The redia will then undergo another round of asexual reproduction to form second and sometimes third gen rediae.
- The redia will give rise to several cercariae (polyembryony) which will burrow out of the snail –> in optimal conditions – over 10 degrees
- The cercariae consist of two body regions – the ‘body’ with a mouth with oral and vetral suckers and a tail of striated muscle which will allow it to swim actively/
- The cercariae will settle on grass/foliage to form metacercial cysts. They come out around midnight.
In the definitive host:-
– The definitive host will become passively infected when it eats plants infected with metacercariae – the larvae are released from the cyst and will penetrate the intestinal wall of the vertebrate to reach the bile duct where they will mature into an adult fluke worm.
What are the stages of fascioliasis?
Acute –>
- The migration of large numbers of larvae to liver will cause acute fascioliasis. (Rare in man)
- it requires more than 10k metacercial cysts to be ingested.
- can cause sudden death as a large number of larvae infect the liver and cause liver failure or peritonitis (inflammation of the peritoneum, the thin tissue that lines the inner wall of the abdomen).
Chronic –>
- The adult fluke can survive for up to 10 years in biliary ducts of its host – depending on how long the host lives. — The fluke can produce 20k-50k eggs a day.
- Often asymptomatic, but with the maturation of the parasites there may be an obstructive phase causing – hepatitis, infection of biliary tract, inflamed pancreas.
Describe clonorchis sinensis
- Trematode infection mainly of humans.
- It can also infect dogs, cats, pigs – large number of reservoir hosts makes control difficult.
- It is endemic is East Asian countries – China, Korea, Taiwan and Vietnam.
- Transmission is mainly from eating undercooked or pickled fish that contain metacercariae.
- Control is also difficult as these metacercariae are resistant to harsh environmental conditions.
How is clonorchis sinensis different from fasciola hepatica
- 10 times more common than fasciola but not as pathogenic.
- Three hosts – passive transmission of egg to snail and then active transmission to fish. the human is then passively infected by eating infected fish.
- Wider range of snail host – over 100 species.
Describe the life cycle of clonorchis sinensis.
NB: Fish eating carnivores can act as reservoir hosts.
Life cycle consists of two intermediate hosts (snail and fish) and definitive (usually human) host.
- The snail will eat the egg (passive transmission) from human faeces and the larvae will reproduce inside the snail into cercaeriae which will come out and actively seek out and force their way into the skin/ flesh of fresh water fish – using their suckers.
- In the fish, the cercariae will encyst as metacercarie within the muscle.
- When the fish is ingested by human host, the metacercariae will then travel to human’s bile ducts where they will develop into the adult form.
Describe symptoms and pathology of clonorchis sinensis.
- Most infections are asymptomatic in humans.
- Acute symptoms include abdominal pain, nausea, and diarrhoea.
- Most pathology is due to obstruction and inflammation of biliary duct.
- Chronic infections include cholangitis, gallstones, and pancreatitis.
– Recently, some cases of Clonorchis have been linked to cholangiocarcinoma – cancer of the bile duct.
There is yet no effective chemotherapy for this cancer so we have to prevent infection with this parasite. It’s thought that infection with Clonorchis increases the susceptibility of DNA to damage by carcinogens.
What is Opisthorchis Viverrini ?
- another liver fluke that is very similar to clonorchis sinensis in terms of its biology and pathology
- also acquired when eating raw or undercooked fish.
- It’s endemic throughout Thailand, Laos and Cambodia with a high prevalence in humans.
- Has three host life cycle – snail (single species only), fish and final host being a fish-eating mammal.
- Thought to be responsible for high levels of cholangiocarcinoma – WHO estimates up to 1/3 of infected will contract cancer.
