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Digenea Flukes

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Digenea Flukes

GENERAL: Flukes are flatworms adapted to extreme parasitism on both vertebrate and invertebrate animals. The adult is found usually in the former and is internal. The image above is of two adults. By contrast the larvae are varied during the development and at one or two stages are free living in water, in search of a host. This is the miracidium or cercaria stages. Some cercaria are found encysted on grass, left by one host and waiting to be picked up by the second. These larvae are microscopic and easily overlooked. The Liver Fluke (Fasciola hepatica), above, is the species of most concern to humans.
Flukes like most parasites are specific to a particular host and therefore their presence and distribution will be closely linked to that host. They are, however, very widely dispersed across Europe.


Cercaria larva under a light microscope
Cercaria larva under a light microscope
Cercaria larva under a scanning electron microscope
Cercaria larva under a scanning electron microscope
Miricidium larva under light microscope
 Miricidium larva under light microscope
A fluke as seen under a scanning electron microscope
A fluke as seen under a scanning electron microscope
An egg of a fluke (scanning EM)
An egg of a fluke (scanning EM)

ECOLOGY: As with most internal parasites there are basic problems for the species to overcome. Not least, how to enter a new host. This invariably needs huge numbers of offspring as well as a way to meet the new host. In the latter case internal parasites utilised two different hosts. The faeces of the host is the best way of taking eggs out of the body but for reinfection it needs to get those offspring back into contact with the host. The Liver Fluke (Fasciola hepatica) has an adult which lives in the liver of a mammal such as sheep or even humans. There the egg and sperm meet to produce fertilised eggs which pass out with the faeces. The hatching miracidium larvae require water to swim in and some will make it to a stream. Here they need to find a snail, Lymnaea, to infest. It penetrates through the muscular foot and soon settles down to a period of further reproduction. Only a few miracidia may have arrived so it needs to boost the numbers again. This it does by asexual means to give rise to huge numbers of redia larvae. After sometime these emerge from the snail to form cercaria larvae. Escape from the snail is via its breathing apparatus. They may form cysts or swim around in the water. Either way, they are in search of the main mammal host once more, to be swallowed and taken into the gut. This may be by drinking or by the larva resting on vegetation. Water Cress is a notorious carrier in this respect and it is important to wash this before humans ingest it. Once in the gut of the animal the young fluke moves to the liver where it begins the cycle once more. The number of different parasite species and hosts is enormous. Birds show a very high incidence of being infected, paraticularly water birds such as gulls, waders and ducks. The sea water species use periwinkles as a secondary host instead of Limnaea, which is in freshwater.

 Classification:

 

 Kingdom

Animal

 Phylum

Platyhelminthes

 Class

Trematoda

 Order

Digenea



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