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How parasites work?

A parasite is an organism that lives on or inside another organism, its host.  It is a type of symbosis, which simply means ‘living together’ but the relationship isn’t equal.  The parasite takes all of its nourishment (food) from its host and doesn’t give anything back so quite often the parasite makes the host quite ill or causes discomfort.

A parasite will grow, feed and be sheltered by its host but you cannot always see it.  Parasites range in size from the large, like the ticks or lice you can see in hair or on pets, to tiny, microscopic parasites like single-celled protozoa.  These protozoa are one of the earliest forms of life but you can’t see them without using a microscope.  There are many different types of parasites and each has adapted to the different hosts within or on which they live. 

Parasites that live inside the host are called endoparasites (endo– means ‘inside’) while parasites that live outside, on the host are called ectoparasites (ecto– means ‘outside’).  Endoparasites get inside their hosts either by burrowing into their tissues or when the host eats contaminated food or drinks contaminated water.  Parasites that get into the gut when contaminated food or water is consumed produce lots of eggs very quickly.  These grow inside the gut, eating the food the host eats!  When they are mature, they travel through the gut and are excreted into toilets, sewage systems and water sources.  The parasites are then free to find more hosts.  If humans have bad hygiene, the parasites can infect lots of people very quickly.

Ectoparasites have to work a little harder to find their hosts!  Some, like the leech, can sense a host when it moves.  They check out if the host is good for them by measuring lots of different things like temperature and nutrient levels and then attach.  Others, like fleas and ticks, either move around until they find a host or they wait for a host to pass by and jump on!  A lot of ectoparasites eat their host’s skin, hair or drink their blood!  They have lots of special body parts that help them to attach and feed – like strong legs, claws and very sharp teeth and jaws to feed!


Most parasites cannot survive without their host, mainly because they have adapted to life on their host and they have lost some of the organs necessary for to live on their own.  Some parasites even have different hosts at different stages of their life cycles – the liver fluke (a type of flatworm) for example, has two hosts.  Young fluke first infest the freshwater snail.  They grow, using the snail for food (which kills the snail), and when they run out of food, they attach to grass at the waters edge.  These fluke are eaten by grazing animals like cows and sheep, where they grow to adults and lay eggs, completing their life cycle.  Eggs are excreted in the cow pats and washed into the water so the cycle begins again.  Humans can be infected by drinking contaminated water from rivers and streams.  Live fluke are not very nice so it is best not to drink unpurified water, even if it looks clean!

It is not possible to avoid parasites, they have survived because they are so clever!  The best thing to do is to make sure that you take care of your personal hygiene every day and that you keep your environment clean.  


Did you know…?

The study of parasites of all kinds is call parasitology.

The word parasite comes from the Greek word ‘parasitos’ which means ‘eating at the side of or at the same table as’ (‘para’ means – alongside of; ‘sitos’ means food).  It originally meant someone who came to live with another person but who didn’t pay their way or work for their keep.  It was much later (in the 18th century) that ‘parasite’ was used in biology.

Protozoa are one of the simplest forms of life, with just one cell.  Their name comes from the Greek ‘proto’ meaning first and ‘zoa’ meaning animals!

Cuckoos are a type of parasite!  Cuckoos put their eggs into the nests of other birds where they are fed and raised by the ‘host’ parent.  When the cuckoo is ready to leave the nest, they just fly away – without even saying ‘thank you’! 



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