What is a parasite?
A parasite is a living thing that lives on or in another living thing and takes its food from it. The living thing it feeds on is the host. The host loses food and energy, so it gets weaker. A parasite does not usually want to kill its host, because then it would lose its home.
Parasites that live inside the body (in blood, gut or liver) are endoparasites. Parasites that live outside, on skin or hair, are ectoparasites.
Parasites come from many groups
- Bacteria: some live only inside cells of a host and cause disease, for example the germs of tuberculosis and typhus.
- Protozoa (one-cell animals): Plasmodium causes malaria, Entamoeba causes amoebic dysentery, Giardia gives stomach upset, Trypanosoma causes sleeping sickness.
- Fungi: ringworm and athlete's foot grow on skin, hair and nails. Yeast like Candida can grow in the mouth.
- Worms (invertebrates): tapeworm, roundworm and hookworm live in the gut. Filarial worms live in lymph vessels.
- Arthropods (animals with jointed legs): lice, fleas, ticks and itch mites live on the skin.
Most worm parasites enter through food, water or soil. Eggs are passed out in faeces, so a toilet and clean hands are important.
Vectors: the carriers
A vector is an animal that carries a germ or parasite from one host to another. The vector is usually not harmed.
- Arthropod vectors: Anopheles mosquito (malaria), Aedes mosquito (dengue, chikungunya), tsetse fly (sleeping sickness), ticks (Lyme disease), fleas on rats (plague). A housefly can also carry germs on its feet from dirt to food.
- Mammal vectors: rats and mice carry fleas and germs; dogs, bats and foxes can pass on the rabies virus through bites.
- Bird vectors: wild and farm birds can carry bird flu virus and bring ticks and mites to new places.
Remember: the parasite lives in the host; the vector only gives it a ride (sometimes the parasite also grows in the vector, as Plasmodium does in the mosquito).
How to break the chain
The chain is: parasite, then vector or food or water, then a new host. Cut any link:
- Remove stagnant water so mosquitoes cannot breed. Use nets, screens and repellents.
- Drink boiled or filtered water, wash vegetables, cook meat and fish well.
- Use toilets, wash hands with soap, keep nails short, wear shoes.
- Check your skin and pets for ticks and lice. Control rats and flies.
- Treat sick people early with medicine given by a doctor, so they do not pass the parasite on.
Try it: hunt for mosquito homes
Walk round your home and school with an adult. Count every place where water can stand for more than a week: tyre, bucket, plant tray, cooler, roof gutter. Predict how many you will find, then count. Empty or cover each one, and check again after a week. In the 3D above, press Vector then Net on and compare the second host.
Key formulas and definitions
- Parasite: lives on or in a host and feeds on it
- Endoparasite = inside the body; ectoparasite = outside on skin or hair
- Vector: carries a parasite or germ from one host to another
- Chain: parasite โ vector or water or food โ new host
Worked examples
1. A tapeworm lives in a person's gut. Name the parasite type, the host and say endo or ecto.
Parasite: tapeworm (an invertebrate worm). Host: the person. It lives inside the gut, so it is an endoparasite.
2. Malaria is caused by Plasmodium and spread by Anopheles. Which one is the parasite and which is the vector?
Plasmodium (a protozoan) is the parasite. The Anopheles mosquito is the vector that carries it to people.
3. Lice live on the scalp and drink blood. Are they endoparasites or ectoparasites? Which animal group are they?
They live outside on the skin, so they are ectoparasites. They are arthropods (insects with jointed legs).
Common mistakes
- Mixing up parasite and vector. The parasite harms the host; the vector just carries it.
- Thinking all mosquitoes spread malaria. Only some Anopheles females carry Plasmodium.
- Thinking parasites are only worms. Bacteria, protozoa, fungi and ticks can be parasites too.
- Thinking a parasite always kills the host. Many stay for years and slowly weaken it.