1st year DDS - Practical class 2

General characteristics of Platyhelminthes (flatworms)

• The common name of the phyllum – the flatworms (they are flat);
• Of medical importance are two classes: Trematoda (flukes) and Cestoda (tapeworms);
• Protonephridial excretory system (flame cells);
• Tegument and parenchyma – characteristic kinds of tissue;
• Hooks and suckers for attachment;
• Hermaphroditic organisms (with exception of flukes Schistosoma spp.);
• Heteroxenous parasites (with exception of a tapeworm Hymenolepis nana);

General characteristics of Trematoda (flukes)

additional characters:
• Two suckers – oral and ventral;
• The digestive system is incomplete (without the anus);
• The general life cycle involves following stages: eggs → miracidium → sporocyst → redia → cercaria → (metacercaria, adolescaria, furcocercaria) → adult stage.
Species specific snails act as primary intermediate hosts of the flukes of man, but some species of flukes have a second intermediate hosts or have not any additional host. The life cycle is modified in relation to the species;
• Human is always a definitive host (typical or accidental);
• Hermaphroditic and heteroxenous parasites (with the exception of Schistosoma);
• The morphology of the eggs is generally within a specific range and can be used for species identification.

Fasciolopsis buski
(the giant intestinal fluke)

Kind of parasite: heteroxenous and polyxenous.

Definitive host(s): some mammals (i.e. pigs and human; rarely dogs and rabbits).

Intermediate host(s): snails.

Infective stage: metacercaria (adolescaria) which encysts on freshwater plants (i.e. water chestnut, Chinese water chestnut).

Transmission by ingestion of raw water chestnuts which are contaminated with adolescaria stages.

Site of infection: the small intestine.

Diagnostic method(s):

• microscopic identification of the eggs in stool;

• rarely, identification of the adult stages of the parasite in stool, or in vomit (sometimes, very rare, during gastroscopy).

Geographical distribution: endemic (Far East and Indian subcontinent).

Remarks:
• During diagnosis the multiply stool examination using concentration techniques are recommended.
• Sometimes geographical distribution areas of Fasciola hepatica and Fasciolopsis buski may be the same and, moreover, the eggs of F. hepatica and F. buski are very similar. 

Clonorchis sinensis
(the Chinese liver fluke)

Kind of parasite: heteroxenous and polyxenous.

Definitive host(s): all the fish-eating mammals (including human).

Intermediate host(s): 1st – snails, and 2nd – freshwater fish.

Infective stage: metacercaria which encysts in the muscle of freshwater fish.

Transmission by ingestion of raw fish which are infected with metacercariae of the parasite.

Site of infection: the biliary ducts of the liver.

Diagnostic method(s):

• microscopic identification of the eggs in stool;

• rarely, examination of duodenal contents.

Geographical distribution: endemic (Japan, Korea, China, Taiwan and Vietnam).

Remarks:
• The adult stages and eggs of C. sinensis are very similar to Opisthorchis felineus.
• Human infections caused by C. sinensis due to eating of sushi prepared from infected fish can be diagnosed also in non-endemic areas (i.e. USA, western Europe).

Paragonimus westermani
(the lung fluke)

Kind of parasite: heteroxenous and polyxenous.

Definitive host(s): numerous mammals (including human).

Intermediate host(s): 1st – snails, and 2nd – crabs and crayfish.

Infective stage: metacercaria which encysts in the muscle of crabs and crayfish.

Transmission by ingestion of raw crabs or crayfish which are infected with metacercariae of the parasite.

Site of infection: the lungs or rarely the other organs (i.e. brain) of definitive host.

Diagnostic method(s):

• microscopic identification of the eggs in stool or in sputum;

• laboratory diagnosis should be repeated (2-3 times in a week) because of irregularity in the excretion of the eggs of parasite;

• serological methods.

Geographical distribution: endemic (Far East and Africa).

Schistosoma spp.
(the blood fluke)

There are three Schistosoma species of medical importance: S. haematobium, S. mansoni and S. japonicum.

Kind of parasite:

• S. haematobium - heteroxenous and monoxenous;

• S. mansoni and S. japonicum - heteroxenous and polyxenous.

Definitive host(s): numerous mammals (including human).

• S. haematobium - human;

• S. mansoni – human and non-human primates;

• S. japonicum – human and numerous mammals.

Intermediate host(s): fresh-water snails.

Infective stage: furcocercariae which swim free in the water.

Transmission by actively penetration of furcocercarial stages through the skin of the definitive host.

Site of infection: the lungs or rarely the other organs (i.e. brain) of definitive host.

• S. haematobium – the veins of the urinary bladder plexus;

• S. mansoni – the portal veins of the large intestine;

• S. japonicum – the veins of the small intestine (sometimes the veins of the liver and lungs).

Diagnostic method(s):

• microscopic identification of the eggs in urine (S. haematobium) or in stool specimens (S. mansoni and S. japonicum);

• examination of biopsied tissue for the presence of the eggs;

• immunological tests.

Geographical distribution:

endemic

• S. haematobium – Africa and Asia Minor;

• S. mansoni – Africa and Southern America;

• S. japonicum – the Far East.

Remarks:

• the most important epidemiological factor in water-borne schistosomosis is water polluted by waste which forms the living site of specific intermediate hosts of Schistosoma. The transmission of human schistosomosis occurs by contact with contaminated water used for domestic, recreational or religious practices (e. g. ablution of Moslems);

• over 200 million people living in rural, agricultural and suburban areas are estimated to be infected, and over 400 million people are at the risk of infection.