Microbes: parasites

Public summary: 

Parasites are a massive problem for global health, particularly in the developing world. Find out more about these creatures, who get a bit too close for comfort!

Get up close an personal to some parasites (well, preserved slides), and find out more about them...
Useful information
Kit List: 

• 6 x cuddly parasites (4x protozoa, 2x insects)
• 2 x cuddly mosquitos (parasite "vectors")
• laminated fact sheets
• laminated parasite matching game
• microbiology text book

There are also some slides and a microscope in the experiment "Microscopes & Cells" which can be used with this experiment (Risk Assessment is in "Microscopes & Cells" - you would need to sign for this RA separately). The slides are: head louse, bed bug, malaria parasite, liver fluke, hookworm and guinea worm.

Packing Away: 

Please make sure that we have all the cuddly microbes, and that they go back in the right box.

Frequency of use: 


In a nutshell...

- Cuddly parasite toys and laminated sheets showing parasites and their effects.
- It might be easier to get a couple of toys out at a time and get people to ask questions about them - it might help to control the flow of questions!
- If you're reading this in advance, more information can be found on the CDC website (homepage http://www.cdc.gov/)

Talking about parasites:

What is a parasite?

- Being a parasite is essentially free-loading off the host that it feeds on.
- Some parasites live on the surface of their hosts, these are ectoparasites (e.g. some mites/lice, ticks).
- Some types parasites live inside their "prey", these are endoparasites. Some of these live in gaps between the host tissues (eg trypansomes, tapeworms) while others live inside the host cells (eg malaria parasites)
- Most parasites live in more than one species at some point in their lifecycle (see individual descriptions).
- The parasites that you're most likely to hear about are single-celled Protozoa (including the parasites that cause malaria and trypanosomiasis), and parasitic worms (such as tapeworms).
- There is constant conflict between parasite and host: the host’s immune system is trying to kill the parasite, and the parasite is trying to hide from the immune system. Co-evolution of the parasite with the host has led to them becoming well adapted to each other – a good parasite is one that manipulates the host so it can reproduce maximally but doesn’t kill the host because it needs somewhere to live!
- Parasitic diseases are a massive problem for global health. Malaria kills nearly 1 million people per year, most African children under 5 years old. Parasitic worms might reduce your quality of life so much that you can’t care for your family, leading to a wider impact (eg elephantiasis). In many regions where parasitic diseases are a problem there is very basic healthcare, making treatment more difficult to access.
- Also a big problem in animals - cost to pet owners of preventative treatment (to avoid illness), most farm animals have a significant parasite burden (gives economic costs of reduced production, greater food intake required, medications etc.

Some examples of parasites

PROTOZOA: Single-celled eukaryotes

1) Malaria (Plasmodium falciparum):
4 parasites cause malaria: Plasmodium falciparum (the most common and most deadly), Plasmodium vivax, Plasmodium malariae, Plasmodium ovale. These are carried by Anopheles mosquitoes, the vector, which are active and hence bite people at night. When they bite an infected person they take up the parasite into their digestive system (parasite stage = sporozoite), and then secrete it into the next person they bite in saliva (used to inject anticoagulants to keep the person bleeding so the mosquito can get its blood meal). The parasite then travels in the bloodstream to the liver where cells are infected (parasite stage = merozoite). Parasites are released and enter red blood cells in the circulation, where they multiply and burst the red blood cell, causing the symptoms of malaria. Inside some red blood cells the parasites become gametocytes which are taken up by another mosquito if it bites the person.
Symptoms at first include fever, chills, headache and vomiting. In some cases this can rapidly progress to severe anaemia, respiratory distress or cerebral malaria - and can be fatal. However if you live in a malarial area (and get infected often) you may develop partial resistance to malaria and be infected without symptoms (asymptomatic infection). When you travel to a malarial area (Asia, South America, Africa - but the range is spreading) you use chemoprophylaxis (drugs to stop an infection becoming established), in combination with avoiding being bitten (mosquito nets, DEET mosquito repellent) - and there are also drugs available to treat malarial infections.

2) African Sleeping Sickness (Trypanosoma brucei):
Parasites are injected into you by the bite of a tsetse fly in Africa (tsetse fly = the vector), or they can cross the placenta to infect a fetus. Trypanosomes don’t live inside cells like malaria but they live in the blood stream – this means they have to do clever things to hide from the immune system, such as constantly changing their protein coat so that the immune system can't recognise them as a parasite. 2 different trypanosome subspecies cause sleeping sickness: T. brucei rhodesiense gives a rapidly-progressing disease (death in weeks to months) whereas causes a slowly-progressing disease (you might not have any symptoms for years!). Sleeping sickness occurs when the trypanosomes enter the brain and start to destroy the areas that control sleep and wakefulness, leading to blurred sleep/wake transitions and falling asleep at inappropriate times, progressing to coma and death! Trypanosomiasis (disease caused by trypanosomes) is treatable - but it's much easier at the early stage (drugs: pentamidine/suramine) than once the parasites enter the brain (drugs: melarsoprol - which comes from arsenic and has nasty side effects that can be fatal themselves!).
African trypanosomes also infect cattle, making agriculture in areas with tsetse flies really unproductive - cattle become really thin and so can't work (pulling ploughs etc) and aren't useful as food for people either!

3) Chagas disease/ American trypanosomiasis (Trypanosoma cruzi)
Chagas disease, also known as American trypanosomiasis, is caused by the protozoan parasite Trypanosoma cruzi (T. cruzi). 10 million people in the world are infected, mainly in South America. The vector transmitting the trypanosomes to humans is the triatomine bug ('kissing bugs'). These live on the walls of houses and are nocturnal, biting people on their exposed faces when they are asleep at night. Unlike the African trypanosomes, South American trypanosomes aren't injected with the bug's saliva but the bug defecates next to the bite and it's the faeces that contain the parasites. They get into the person's bloodstream when they rub their face and smear the faeces into the bite!
The first sign of infection is often a large swelling of the eyelid on the side of the face that was bitten. Parasites can then enter the heart (30% cases) causing problems with the heartbeat, and maybe sudden death. Digestive problems can also occur (10% cases) when there is enlargement of the oesophagus as food can't enter the stomach - the food that gets trapped can start rotting which is pretty disgusting! Chagas disease is highly treatable though, with nearly 100% of people cured if treated early (benznidazole or nifurtimox).

4) Toxoplasmosis (Toxoplasma gondii)
Toxoplasma gondii is a species of parasitic protozoa.The definitive host (where the sexual stage of the life cycle occurs) is the cat - gametes are formed in the digestive tract and exit in the faeces. Then they enter another host (intermediate host) when they eat or drink contaminated food or water. So far every warm-blooded animal tested can act as a host - including humans. Here the parasite can enter any nucleated cell and replicates to form a tissue cyst. This can be passed on to another intermediate host or a cat by carnivory.
Usually toxoplasmosis has very mild symptoms (there might be some fever) and in France up to 90% of adults are infected (due to the French love of rare meat)! However serious disease can be caused in immunocompromised people (AIDS sufferers or post-transplant patients on immunosuppressive drugs) and pregnant women (increasing the risk of spontaneous abortion and birth defects). This is why pregnant women are advised not to touch cat litter! (also explains why pregnant women are advised to clean fruit/veg thoroughly as gametes can contaminate them, and women become infected this way)
T.gondii infection of the brain can lead to changes in behaviour by changing the amounts of chemicals in the brain (dopamine). Rats and mice lose their fear response to the scent of cats (tested using cat urine) and are more curious - very important effect, as means rats are more likely to be caught and eaten by cats giving greater rate of infection of cats (completing the parasites life cycle). Studies in humans have linked toxoplasmosis to schizophrenia (including hallucinations and reckless behaviour), slower reaction times and greater chance of causing traffic accidents.


(There's some slides for these in "Microscopes & Cells" which can be borrowed - same for some preserved specimens in "Horrible Housemates".)

Flukes (Trematodes): Adult flukes are leaf-shaped flatworms. Prominent suckers at the mouth and on the stomach help maintain position. Flukes are hermaphroditic (both male and female) except for blood flukes (schistosomes), which are bisexual. The life-cycle includes a snail intermediate host.

Tapeworms (Cestodes): Adult tapeworms are elongated, segmented, hermaphroditic flatworms that inhabit the intestinal lumen. You can eat the cysts in undercooked animal tissues (pork is probably the greatest risk if undercooked), and then they develop in your intestines. They eat your food from your intestine – instead of you getting the nutrients. They attach to the intestinal wall using suckers in the head. Problematic in the developing world where there is already malnutrition. People used to use tapeworms as a slimming aid… They can grow up to 15 metres long and live for 20 years! Larval forms live in extraintestinal tissues.

Roundworms (Nematodes): Adult and larval roundworms are bisexual, cylindrical worms. They inhabit intestinal and extraintestinal sites.

1) Schistosomiasis (caused by a fluke/ trematode)
• Schistosomiasis is a chronic, parasitic disease caused by blood flukes (schistosomes)
• At least 230 million people require treatment every year – praziquantel
• Infection from larval forms released from freshwater snails in contaminated water
• Penetrate the skin and live in circulation where females release eggs
• Cause damage because of the host immune response to the parasites

2) Liver fluke (Fasciola hepatica) (caused by a fluke)
• Freshwater snail intermediate host, where reproduction occurs, then they are released as cercariae and swim through water to be ingested by ruminants normally, or sometimes humans eating uncooked foods like watercress
• Adult lives in the liver where they feed on the lining of bile ducts – makes cheese-like holes in the liver
• Produce eggs – up to 25000 a day per female

3) Dracunculiasis (guinea-worm disease, caused by a nematode)
• Dracunculiasis (caused by Dracunculus medinensis, a long thread-like worm) is a parasitic disease on the verge of eradication
• Exclusively transmitted by drinking water contaminated with parasite-infected fleas such as rural isolated ponds
• Takes 10-14 months for worm to mature in the body

4) Hookworm (Ancylostoma duodenale and Necator americanus, kinds of nematodes)
• Soil-transmitted helminths (parasitic worms)
• Major burden of disease worldwide - estimated 576-740 million people infected with hookworm
• Hookworms live in the small intestine, eggs are passed in faeces of infected person – this is a problem in the developing world where people may defecate outside (near bushes, in a garden, or field) of if the faeces of an infected person are used as fertilizer, eggs are deposited on soil.
• Eggs then mature and hatch, releasing larvae (immature worms).
• The larvae mature into a form that can penetrate the skin of humans, and infection is mainly acquired by walking barefoot on contaminated
• Most people infected with hookworms have no symptoms. Some have gastrointestinal symptoms, especially persons who are infected for the first time. The most serious effects of hookworm infection are blood loss leading to anemia, in addition to protein loss.
• Infection is treatable – anthelminthic medications eg. albenadazole


(There are two cuddly toys for this, so you can show a second uninfected mosquito feeding off a human host)

Mosquito (Culex pipiens):
• Most mosquitoes are harmless but some can transmit disease
• Viral diseases, such as yellow fever, dengue fever and Chikungunya, transmitted mostly by Aedes aegypti
• Parasitic disease malaria, carried by mosquitoes of the genus Anopheles
• Lymphatic filariasis (the main cause of elephantiasis – worms block lymphatics impairing lymph drainage causing swelling of the limbs)
• We have cuddly toys of mosquitoes – in the parasites box


(There's some cuddly toys and slides for these, and hopefully some preserved samples borrowed from the "Horrible Housemates" experiment)
1) Head louse (Pediculus humanus capitis)
• Lice are wingless biting or sucking insects. Lice infestation of any part of the body is called pediculosis!
• Live on your head (an ectoparasite) - has a single strong claw on each leg that can grab onto 6 hairs so they can move rapidly (may be on several heads in one day). But their short stumpy legs mean they can’t jump or walk well on flat surfaces.
• Your head provides a source of food - blood
• Eggs are called nits and the female attaches the eggs close to the scalp with a transparent quick-setting glue
• The time taken to hatch depends upon temperature
• Head lice have no lungs! They take in air by muscle contraction of the abdomen (via spiracles)
• Head lice have been recovered from prehistoric mummies!

2) Bed bug (Cimex lectularius)
• Ectoparasites that feed on human blood (haematophagous)
• Live in houses and especially beds – most active at night so they can feed on the host without being noticed
• Adults can survive more than a year without feeding
• They don’t usually spread disease but they can cause allergic skin reactions
• Light brown flattened oval-shaped body, vestigial front wings, microscopic hairs on abdomen that give a banded appearance

Risk Assessment
Date risk assesment last checked: 
Mon, 08/01/2018
Risk assesment checked by: 
Date risk assesment double checked: 
Fri, 02/02/2018
Risk assesment double-checked by: 
Risk Assessment: 
Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Microbe toys and accessories Trip hazard if dropped on the floor. Likelihood score Severity score Overall Keep all props in contained area. Call first aider in case of injury. Likelihood again Severity again New overall (hopefully better than the first)
Publicity photo: