Updated 19 May 2015


Parasites are organisms which live in or on another organism and harm that organism.



  • Parasites are organisms which live in or on another organism and harm that organism.
  • Three main groups infect humans – protozoa, helminths and arthropods.
  • Parasites are extremely common around the world, particularly in third world countries.
  • Parasites can cause serious diseases such as sleeping sickness and bilharzia.
  • Parasitic infection can be effectively treated or prevented in most cases.

What is a parasite?

A parasite is an organism that lives in or on another organism and in the process harms this organism. The three main groups of parasites that infect humans are:

  • Protozoa
  • Helminths
  • Arthropods

Protozoa are single-celled microscopic parasites, while helminths are larger worm-like parasites that can usually be seen with the naked eye. Arthropods (insects, and other organisms with jointed legs) usually infest the surface of the body, and include ticks, fleas and lice.

Parasites are extremely common throughout the world, and are important causes of disease, especially in developing countries. For example, it is estimated that over one billion people are infected with roundworm, a helminth. Malaria, caused by a protozoan parasite, is one of the commonest causes of death in Africa, responsible for over one million deaths annually.

How do you get a parasitic infection?

This depends on the type of parasite. Some are acquired by ingesting contaminated food or water. Others (such as the parasite that causes malaria) can be spread by insects. Some, such as bilharzia, are able to burrow through your skin into the bloodstream. More detail is available in the sections dealing with the specific parasites.

How are parasitic infections diagnosed?

Diagnosis can sometimes be quite difficult, since many parasites produce only vague, non-specific symptoms – such as nausea, tiredness and abdominal pain. Such common symptoms can have a number of causes. The best way to diagnose a parasitic infection is to send samples of different tissues and fluids to the laboratory, where the parasite will be identified. Some parasitic infections do have slightly more specific symptoms, but it is always best to prove the presence of the parasite.

How are parasitic infections treated?

Most parasites can be treated with specific drugs. If one person in a family is infected, others often become infected as well (particularly with parasites that live in the gut). Therefore, the whole family should be treated, otherwise re-infection is likely. Again, details of different therapies are available in the sections on specific parasites. 


These are single-celled parasites that can infect several different sites in the body. They can cause symptoms and disease by damaging tissues directly, by producing toxins, or as a result of the body’s immune response to the parasite.

Malaria is one of the most important protozoal infections and is dealt with in a separate article.

Common examples of protozoa include amoebae (certain of which cause amoebic dysentery); trypanosoma (which causes sleeping sickness); and toxoplasma.

Amoebiasis or amoebic dystentry

Amoebiasis is caused by the parasite Entamoeba histolytica, a small, single-celled organism that usually causes bowel infections. In the bowels, it multiplies and lives as a free moving organism. Before it can be excreted and survive outside the human, it acquires a protective outer “shell” – this is the cyst form. The cysts are excreted and survive in contaminated soil or sewage until they are ingested by another human. When they get back into the intestine, they lose their protective covering and can move around again and multiply.

Most people who are infected with E. histolytica get dysentery. Dysentery means that you pass blood and mucus in your stools, and is a result of the organism breaking down tissues lining the bowel wall. (There are other causes of dysentery, both infectious and non-infectious. If you notice blood or mucus in your stools, you should see your doctor). The inside of the bowel of someone with amoebic dysentery has small superficial ulcers.

Sometimes people with amoebiasis develop a severe, and sometimes life-threatening, form of bowel infection with extensive ulcers and necrosis (tissue death) of the bowel wall. Fortunately, this is an unusual complication of amoebiasis.

Occasionally the amoebae find their way to sites in the body other than the bowel. They may enter the liver, where they form large abscesses (collections of dead tissue and pus). People who have this form of amoebic infection usually complain of pain in the upper right area of the abdomen, may have a fever, and often loss of appetite and weight. Very rarely, the amoebae may infect other areas such as the brain.

How do you know if you have amoebiasis?

Symptoms are often not specific enough to allow for easy diagnosis. For example, symptoms of abdominal pain, fever and loss of appetite could be caused by many different conditions. In addition, there are many possible causes of dysentery. Certain investigations need to be done. If someone has amoebic dysentery, stool samples will usually show the free-living parasites or the cysts.

In the case of a liver abscess, a scan of the abdomen will show the presence of the abscess, but that doesn’t necessarily mean it is amoebic. If the person comes from an area where the organism is commonly found (such as KwaZulu-Natal), it is likely that a liver abscess is amoebic. Pus may be drained from the abscess, but it is unusual to see the organisms in the pus. A blood test can be performed to show if the patient has antibodies to E. histolytica: this is the commonest way of confirming the diagnosis of amoebic liver abscesses. (Antibodies are proteins that the body makes to help fight off infection. You can only make antibodies if you have been infected with the organism).

How is amoebic dysentery treated?

An antibiotic called metronidazole is used to treat amoebic dysentery. People with severe colitis (inflammation of the large bowel) may need to have surgery as well. Liver abscesses are usually treated with a combination of drainage of the abscess and metronidazole. Ensuring clean water supplies and satisfactory hygiene standards helps prevent amoebiasis. Unfortunately many areas of the world still do not have adequate access to clean water and sewage disposal.

Sleeping Sickness (Trypanosomiasis)

This condition is caused by the organism Trypanosoma brucei. There are two subspecies of the organism – T. brucei subsp rhodesiensae and T. brucei subsp gambiae. T. brucei subsp rhodesiensae causes East African sleeping sickness, which often has a more rapid onset, while T. brucei subsp gambiae is found more often in West Africa, and has a slower onset.

Where does sleeping sickness occur?

The disease occurs in sub-Saharan Africa, with the highest risk areas being Angola, Zaire, Sudan, Uganda and Cameroon.

What is a trypanosome?

A trypanosome is a single-celled parasite with a flagellum - a tail-like apparatus that allows it to move.

The trypanosome is spread by the bite of the tsetse fly. The fly bites an infected person (or other animal such as a cow) and ingests the trypanosome. In the fly, the trypanosome undergoes various changes which take three to four weeks, and eventually moves into in the fly's salivary glands. If the fly then bites a human, the parasites can spread from the fly's saliva into human's tissues and blood.

What happens if you have sleeping sickness?

At first, you will develop a small chancre (ulcer) at the site of the tsetse fly bite. The trypanosomes will stay here for a short time, and then move into the bloodstream.

Once in the bloodstream, they spread throughout the body and multiply. You will experience episodic waves of fever, headache and feel generally unwell. Sometimes the spleen and lymph nodes enlarge.

An interesting feature of trypanosomes is that they regularly change the proteins on their surface. Your body’s immune system recognises these proteins as foreign and attacks them, and every time the trypanosomes change their surface proteins, the immune system reacts as though it’s a new infection, since it doesn’t recognise the proteins. This may account for the waves of fever experienced by the patient.

After spreading through the bloodstream, the organisms move to the brain. This may result in behavioural changes, and eventually drowsiness (hence the name "sleeping sickness"), leading to coma and possibly death.

How is sleeping sickness diagnosed?

Symptoms tend to be vague: fever, headaches and sleepiness could be associated with a number of causes. Anybody who has been to a high-risk area and who has such symptoms should be suspected of having sleeping sickness. The best way to diagnose the condition is to identify the organism from blood smears sent to a laboratory. It may also be necessary to send CSF (cerebrospinal fluid – the fluid that surrounds the brain and spinal cord) for examination. There are some blood tests available that look for antibodies, but they are no better than direct examination of the blood, and are still somewhat experimental.

How is sleeping sickness treated?

Drugs are available to treat the condition. Suramin and pentamidine can be used if the brain is not involved. These drugs do not cross into the brain, and if involvement of the brain is suspected, alternative agents may need to be used. One such drug is melarsoprol, which is very effective, but also quite toxic. There have been reports of cases of sleeping sickness that do not respond to these drugs, which is obviously of grave concern. For this reason, as well as because of the toxic effects of melarsoprol, other less toxic drugs able to treat the cerebral form of trypanosomiasis are being investigated. One of these agents is called eflornithine.

Can sleeping sickness be prevented?

Prevention is difficult. The best way to prevent the disease, as with malaria, is to avoid being bitten by using insect repellents, mosquito nets, etc. Attempts have been made to eradicate the flies, but this is difficult and often not economically viable.


This is an infection caused by the single-celled organism Toxoplasma gondii. It is primarily a parasite that infects cats, but humans can become infected (usually by pet cats). It is extremely common – some reports say that over 80% percent of people in parts of Europe (especially France) have been exposed to the organism.

How do you get toxoplasmosis?

Toxoplasma reproduces sexually in the cat's gut, and the eggs are excreted in the cat's faeces. The eggs undergo further maturation outside the cat, and can then be ingested by a human (or other animals). In humans and other animals, toxoplasma forms cysts in the tissues. The organisms can also penetrate into the tissues of the cat, where they form cysts and can lie dormant for months or years. If these tissues are eaten, these cysts are infectious to the animal that eats the cat.

Another important way the infection is acquired is from mother to child. If a pregnant woman is infected, toxoplasma can move through the placenta and infect the foetus. This is called congenital toxoplasmosis.

What happens if you get toxoplasmosis?
  • Acquired toxoplasmosis (i.e. acquired by ingesting contaminated food)

    When humans ingest toxoplasma, it penetrates through the gut wall and spreads via the bloodstream to various organs in the body.

    Most people have no obvious symptoms of infection. However, in those who have symptoms, the commonest are enlarged lymph nodes (usually in the neck), a general feeling of malaise (feeling generally unwell), and sometimes mild fever. This form of the disease usually resolves spontaneously over a period of weeks to months, and people often don’t realise they’ve been ill.

    In people with impaired immunity (such as people with Aids), toxoplasma cysts may develop in the brain form and cause symptoms such as weakness, seizures, headaches and disorientation. This is a potentially fatal form of the disease.

    In some people, the course of the infection is more long-term. The main problem with this type of infection is involvement of part of the eye. Specifically, the infection results in inflammation of the retina (which lies at the very back of the eye) although other parts of the eye can be affected. Patients usually experience blurry vision, pain, and light sensitivity. This form of the infection can be fairly non-specific and difficult to diagnose.

  • Congenital toxoplasmosis

    Toxoplasma infection of the foetus may manifest in various ways. If the infection was transmitted early in pregnancy, spontaneous abortion may occur. Infection later in pregnancy can result in stillbirth or clinical disease in the child. The disease can take the form of an acute severe infection that is often fatal, or the child may appear normal at birth, and only show signs of infection later on. These signs may include abnormal vision, jaundice, rashes, seizures and hydrocephalus (fluid accumulation in the brain).

    Sometimes children infected before birth show no obvious symptoms at all and live perfectly normal lives.

How is toxoplasmosis diagnosed?

Diagnosis may be difficult. Blood tests are available to check for the presence of toxoplasma antibodies, which will confirm if you have been exposed to the organism. However, these tests are not always accurate – they may give a negative result even if you have been infected.

If biopsies of infected tissues can be done, toxoplasma can sometimes be shown with special stains. Other tests are being developed to assist diagnosis, but sometimes doctors may have to make the diagnosis based on clinical signs alone, and by excluding other possible causes of the symptoms. In certain circumstances this is not too difficult; for example, a person with advanced AIDS who has brain lesions as shown by CT scan probably has toxoplasmosis (although other causes should still be excluded).

How is toxoplasmosis treated?

Drugs such as cotrimoxazole, pyrimethamine and clindamycin can be used to treat toxoplasmosis. Cotrimoxazole is probably the best agent. It is usually only necessary to treat infections in pregnant women, children (i.e. congenital infections), immunosupressed individuals, and people with involvement of organs such as the brain or eye.

Can toxoplasmosis be prevented?

Prevention is difficult as cats are so ubiquitous. No vaccine is available, so the best way of preventing toxoplasmosis is to practise good hygiene with regard to disposal of cat waste, and to ensure food is properly cooked. In people with advanced AIDS, the infection can be prevented by taking cotrimoxazole on a long- term basis.


These wormlike parasites usually (but not always) live in the gastrointestinal tract (the gut). Some helminths can also infect the bloodstream or tissues. They can cause symptoms either by their very presence (they cause a blockage in the gut, the bile duct in the liver, or the lymphatic vessels), or they deprive the body of nutrients. In the latter case, it is only when there is heavy infestation, or if you are malnourished to start with, that you would have obvious symptoms. Some worms also cause bleeding in the gut, and with continued loss of small amounts of blood, you can become anaemic.

Helminth life cycles include a stage in the human host, and sometimes a stage outside the human host – either in another animal, or free-living in soil. Parasites often have different forms at different stages in their life cycles.

Common examples of helminths include roundworm (Ascaris lumbricoides); tapeworm (Taenia solium and saginata); and bilharzia (schistosomiasis, caused by Schistosoma haematobiuma and S. mansoni).

Roundworm (Ascaris lumbricoides)

Over one billion people are thought to be infected with roundworm. The condition is also called ascariasis. You get the worm by ingesting food or water which contains its eggs. The eggs hatch in your gut, and larvae are released. These larvae penetrate the gut wall, and move into the bloodstream. They reach the lungs, are coughed up and swallowed, and thus return to the gut. The larvae then mature into adult worms, which live in the gut and reproduce. Mature females are 30 cm long, with the males a bit smaller. They are about 2 – 6 mm in diameter.

The eggs are passed in the faeces and stay in the soil until another host ingests them. One female worm can produce nearly 250 000 eggs per day!

As with many helminth infections, symptoms are generally mild and vague, and may include abdominal pain, nausea, weight loss and possibly diarrhoea. Some people develop a cough and mild fever from the larvae moving through the lungs. Large numbers of adults in the gut may block the gut itself. Sometimes the worms migrate around the gut, and move into structures such as the bile duct and appendix, causing cholangitis (inflammation/infection of the bile duct) and appendicitis (inflammation/infection of the appendix).

Drugs are available to treat ascariasis: mebendazole is probably the commonest. Hygienic disposal of faeces is very important in controlling this and other worm infections. In some countries, mass treatment campaigns of children are carried out to try to reduce the worm burden in the population.

Tapeworm (Taenia solium and saginata)

As the name suggests, these worms are flat, ribbon-like and segmented. They can reach up to 10 metres in length with over 1000 segments, each segment capable of reproducing. Tapeworms are hermaphroditic – each worm contains both male and female sex organs, and one worm is able to reproduce on its own.

The adult worms reproduce in the gut, and mature segments break off the worm and migrate out through the anus into the soil. These segments are filled with eggs, and are ingested by a herbivorous host (usually cows for T. saginata and pigs for T. solium). In the herbivore’s gut, the eggs hatch to release larvae, which move through the gut wall into the tissues. Once in the tissues (e.g. muscle) of the herbivore, the larvae fill with fluid and form a structure called a cysticercus. If a human eats undercooked meat, these cysticerci can hatch, and a mature tapeworm can develop in the gut after two to three months.

An important variation can occur in Taenia solium infections. If the eggs are eaten by another human (or by the original host), the eggs can hatch in the human host’s gut, and larvae can reach the tissues and form cysticerci there as well. This gives rise to the condition called cysticercosis. The cysticerci for some reason tend to migrate mostly to the brain.

Infections with adult worms normally produce only mild symptoms, such as some abdominal pain and nausea. However, cysticercosis (cysts in human tissues) can cause a variety of problems, such as epilepsy and hydrocephalus (fluid on the brain). However, many people with cysticercosis have no symptoms at all. Eventually the cysticerci die, and become calcified.

Bilharzia (Schistosomiasis, caused by Schistosoma haematobiuma and S. mansoni)

An estimated 200 million people worldwide are infected with Schistosoma, the parasite that causes bilharzia. The worms are approximately 15 – 18 mm long, with the females slightly longer than the males. The female worms actually lie within a fold on the surface of the male.

The life cycle is fairly complex. Eggs are excreted in the human host’s urine or faeces. If the eggs reach water, they hatch and release a miracidium – the larval form. The miracidium is able to infect aquatic snails, in which it undergoes various developmental changes to become a cercaria (a small, free-living, motile form of the parasite), which swims from the snail. Cercariae are able to penetrate human skin, so if you’re swimming in water which has infected snails, you may become infected. The snails live in still or slowly moving water. (There is little risk of bilharzia in rapidly running water, where the snails are seldom found.)

Once the cercariae penetrate the skin, they develop into yet another larval form in the bloodstream. They move through the lungs, and eventually reach the blood vessels around the liver. As they migrate through the bloodstream, the worms mature. Once they reach the vessels in the liver, they mate, and then move to the blood vessels around either the gut or the bladder. The females lay eggs, which pass into the gut or bladder, and are excreted in the faeces or the urine. S. mansoni usually finds its way to blood vessels around the gut, while S. haematobium reaches vessels around the bladder.

Symptoms can be due to the cercariae entering the skin, or to the effects of the adult worms and eggs. Some people develop an itchy rash at the site the cercariae penetrated the skin. This usually lasts about a week.

Katayama fever is a syndrome that starts a few weeks after first exposure to the parasite, and can last a few weeks. It is not very common with either S. mansoni or S. haematobium infections, but is seen more commonly with a form of schistosomiasis that occurs in the Far East (S. japonicum). Symptoms include cough, loss of appetite, abdominal pain, fever, chills, diarrhoea, and enlargement of the spleen, liver and lymph nodes.

In chronic schistosomiasis due to S. mansoni, symptoms are initially uncommon, but may include fatigue, nausea and diarrhoea. However, with time, the presence of the parasites and eggs in the vessels around the gut and liver can cause liver damage and enlargement of the spleen.

In S. haematobium infection, symptoms are initially of a burning sensation on passing urine, and blood in the urine. With time, there can be damage to the urinary tract and the kidneys.

The infection can be diagnosed by looking for the eggs in stool or urine samples. Alternatively, blood tests are available to see if the patient has antibodies to the parasite. This test is not as reliable as finding the eggs, however.

Bilharzia can be treated with the drug praziquantel. Like many infections, prevention can be aided through sanitary waste disposal. Where bilharzia is common, the water can be treated with chemicals to help kill the snail population.


These are larger parasites that usually infest the skin rather than the bowel or tissues. They are also extremely common.

Examples include the mite Sarcoptes scabei, which causes scabies; and lice.


Scabies is caused by a mite called Sarcoptes scabei. It makes small burrows into the skin and lives there, often at the web spaces of your fingers. The main symptom is intense itching, which is usually due to an allergic reaction to the mite. If one person in the house has scabies, many others may get it too. Apart from the intense itch, you may see small bumps (papules) on your arms, which sometimes become secondarily infected. Scabies is usually diagnosed on physical examination, and sometimes by finding the mite in the skin.

Scabies is treated by applying an ointment to kill the mite, washing linen and clothes carefully, and sometimes using mild steroid creams for the itching. It is very important to treat all household members, otherwise reinfection is likely.


There are three different types of lice that infest humans:

Pediculus humanus capitis – head louse

Pediculus humanus humanus – body louse

Phthirus pubis – pubic louse

There is some debate as to whether head and body lice are in fact separate species, although distinct differences between the two do exist.

Lice feed on human blood, and can bite up to five times a day. When they bite, they inject a substance which acts as anaesthetic and anticoagulant (stops blood clotting at the site of the bite). This substance is what causes the itching. Excessive scratching can infect the bites.

The diagnosis is made by finding the lice or their eggs. Lice and their eggs are usually found in clothing that lies close to the skin. The best way to eliminate lice from clothing is to wash clothes in water of at least 50°C. Certain substances can also be used on the clothing to eliminate lice, such as DTT, permethrin dust or malathion. It is also important to wash recently used linen. The best way to prevent lice is regular changing and washing of clothes.

(Written by Dr Andrew Whitelaw, University of Cape Town and Groote Schuur Hospital)

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