Parasites

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.  

The good Parasites

Are there any good parasites - any that actually have a benefit?  The blood-sucking leech is a parasite, it attaches to its host (animal or human) and feeds on the blood, but it has been and still is very important in medicine! 

Blood-sucking leeches attach to their host and feed until they become full.  They then fall off to digest the blood they have taken.  Leeches are very clever – they have an anaesthetic that stop the host from feeling the leech and they have an anti-clotting agent in their saliva that lets the blood flow more freely.  Leeches from the wild were used to remove poison from the blood but now they grow and live in special laboratories.  These modern leeches eat unwanted blood, numb pain or reduce swelling and keep blood flowing in wounded limbs.  This keeps the limbs alive and allows doctors to fix or reattach them!

Another parasite that features very highly on the ‘yuck’ scale but that actually does good is the maggot (a baby fly).  Maggots, like leeches, grow and live in special laboratories and are used to clean up wounds that are infected or that are not healing well.  Maggots only eat dead skin and tissue so they get rid of the diseased part, leaving the healthy part behind.  Many people are very grateful to these wriggly guys and to leeches as they often save a limb or body part from being amputated!

Another kind of good parasite are fungi!  Fungi cannot make their own food and grow on plants, animals, humans, dead and decaying organic matter, anywhere, in fact, where it is warm and damp!  They get their food by making enzymes that digest food from the surface they are growing on and absorbing the digested nutrients through their cell walls.  Fungi mainly absorb water and digest sugars and starches which they use to grow. 

Some fungi can be very dangerous but the best known fungi, the mushroom, are really good to eat!  Other fungi, like the yeast, Saccharomyces cerevisiae is used in the process of fermentation to make bread.  Yeasts are also involved in the fermentation of wines and beers and in brewing soy sauce which is used in Chinese cooking.  Blue chesses like Cashel blue or Roquefort are injected with certain kinds of fungal spores.  The mold that grows from the fungal spores gives the blue cheeses their colour and yummy taste.  Don’t worry - these fungal spores and molds are not toxic to humans, so they won’t make you sick.

There are some fungi that are used in medicine.  They kill bacteria that invade your body and produce antibiotics like penicillin.  Antibiotics made from fungi are used to treat nasty diseases like tuberculosis (TB) and leprosy.  Not only that, but they can be used to fight off insects, pests and even other fungi that grow on plants!

Fungi, along with bacteria, are one of the best decomposers of organic material.  Without them, dead plants and animals would just hang around and the nutrients from the dead material would not return to the ground.  Other plants, animals and micro-organisms that rely on that food would also die and the delicate balance of the ecosystem would be lost.  So next time you are out for a walk and see mushrooms, do your best to help to spread these ‘fun guys’ around and help them to help you!

So, not all parasites are bad, but it can be very difficult to find a good one.  Make sure you use an expert to figure out which ones are your friends!

The bad parasites 

In humans, parasites usually cause discomfort or disease.  Lice for example are a parasite that can be found in your hair or on your body.  There are over 3,000 species of lice (which are just wingless insects) and the feed on your skin, the oils that your skin and hair secrete and your blood.  You can tell if lice have fed on your blood because their colour changes from the normal beige or grey to a much darker shade! 

Lice have special adaptations that allow them to infest your body, but these depend very much on the type of louse (a single ‘lice’) and where they live.  They have strong short legs with claws that cling on very tightly to your hair or skin and they are small and flat which make them very hard to get rid of.  They lay eggs (called nits) and attaching them to the host using very special saliva that glues the eggs to hair.  These nits are extremely difficult to get rid of unless you use special chemical products.  

Parasites cause lots of different diseases in humans.  Diseases like malaria are caused by protozoa, a single-celled organism that, in the case of malaria, is spread by a certain type of infected mosquito in particular areas of the world.  When the mosquito bites a human, the malaria protozoa parasite gets into their bloodstream and travels to their liver where it makes lots of copies.  These new parasites return to the bloodstream, where they grow even more and multiply so quickly that they burst blood vessels, spreading the parasite and causing huge problems.  Malaria can be treated with drugs but it is very unpleasant even so.  It can stay in the liver for a very long time and symptoms can reappear at any time. 

Protozoa can cause other diseases like amoebic dysentery or problems like liver abscesses.  None of these are fun so your best bet is to avoid getting infected in the first place! 

Some bacteria are parasites, they can invade your body and cause diseases like tuberculosis (TB) and typhoid fever.  Viruses are also common parasites, viruses need a host to grow and multiply but it can survive for a long time without a host, hibernating until one arrives!

Another major type of parasite are the helminths – the worms, like tapeworms or hookworms.  You can get many types of worms from your pets (so make sure your pets are properly wormed!) and these cause lots of problems.  Hookworms have hooks on their bodies that allow them to attach themselves to the inside of your small intestine.  These can cause malnutrition because they eat all of your food!

There are lots of other parasites that can make your life quite unpleasant.  Lice, ticks, fleas and bedbugs all eat your skin, hair, blood and other body fluids!  These parasites are usually very small, but you will still be able to see most of them without a microscope.  You can get these parasites in your home, on your pets, on yourself – anywhere in fact.  You will know they have eaten when you have been bitten – the bite gets very itchy and can be sore.  Some parasites, like fleas, can pass on serious diseases while others, like ticks, burrow deep into your skin and lay their eggs, which can be very difficult to get rid of.  Your best protection from them is to make sure that you pay attention to your personal hygiene and that you make sure your home and environment is clean and tidy!

The ugly parasites 

Parasites like ticks and fleas can make life very uncomfortable.  Others like the parasites that cause malaria or typhus can make you very sick.  Then there are parasites that can look very ugly!  There is a parasite, a family of fungi, called dermatophytes (derma- means anything to do with your ‘skin’) that cause skin infections.  These parasites prefer the keratin in your skin, hair or nails and make a scaly patch wherever they grow.  The patch can sometimes look like a circle like you see on people who have ringworm.  This isn’t a worm at all - it is a fungus that grows in a circle.  It can be itchy and uncomfortable and looks pretty ugly. 

The same dermatophyte parasites, actually a fungus called trichophyton, causes athletes foot.  This fungus can live on skin and socks and floors so it spreads very easily!  It needs a warm and moist environment so remember to dry very well between your toes before you put on your socks!  Athlete’s foot can cause your foot to be very itchy and it looks very ugly because your skin will crack and sometimes even bleed, which can be painful.

Humans aren’t the only ones to suffer from parasites.  Lichens are a partnership between the parasites fungi and algae or bacteria.  They live on rocks, stones, walls and the outside of houses.  They dig deeper and deeper and can make large holes in the stone or wall.  Sometimes, when they are growing, they can look and smell pretty ugly too!  Another type of ugly parasite is mold, a type of fungus that grows on bread, fruit, paper, paint, leather or fabrics.  Mold destroys the thing it grows on – and it looks pretty ugly in the process!

Ugly parasites are everywhere.  Parasites bite, chew, suck your blood and have body parts that are specially made to make this easy for them to do!  Avoid them if you can – make sure your personal hygiene is good and that your environment is clean and tidy.  Parasites don’t look pretty, unless of course you think they look pretty ugly!

Did you know…?

The study of parasites of all kinds, their hosts and the relationship between them is called 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’! 

You might like to kiss under the mistletoe at Christmas, but did you know that mistletoe is a parasite?  It grows on other tress and can kill them.  You can do your bit by making sure you decorate your home with lots of mistletoe at Christmas (and you might even get some kisses!).