The protozoa which parasitise humans may be divided into a number of different groups :

The Amoebae
The Flagellates
The Ciliates
The Sporozoa
The Microsporidia

Amoebae

The Amoebae form part of the Phylum Sarcomastigophora, along with the Flagellates. These two groups were originally classified separately (and may still be presented that way in older texts), although taxonomists have recently found it difficult to draw a dividing line between the two - Flagellates frequently pass through amoebic stages in their life-cycles, and amoebae may also have flagellated stages.

When picturing amoebae, try to imagine a tiny version of "The Blob". Amoebae creep about by extending parts of their bodies to form pseudopods (SUE-dough-pods). They also use these pseudopods to capture food, surrounding what they are about to eat and eventually engulfing it.

Amoebae can live in a number of places around the human body, but most are found in the intestine.

Entamoeba histolytica (ENT-ah-MEE-bah HISS-tow-LIT-ik-ah) is an amoeba which feeds on the lining of the gut. The irritation this creates can lead to the condition known as amoebic dysentery. This amoeba forms cysts to protect it from harsh conditions. People contract the condition if they eat or drink faecally contaminated food or water. In extreme conditions this amoeba may create abscesses in the liver.

The are two other species of Entamoeba which may be encountered in the human gastrointestinal tract : Entamoeba coli (COE-lie) and Entamoeba hartmanni (HART-man-eye). Both of these species may be found in the body without causing any ill-effects - they are said to be commensal. Both are significant, however, as diagnostic parasitologists must be able to differentiate the pathogenic E. histolytica from the non-pathogenic species.

The majority of the other intestinal amoebae also live inside us without causing significant problems.

Other Amoebae

Two rare types of amoebae may cause significant problems. Acanthamoeba (ah-CAN-thah-ME-bah) and Naegleria (nay-GLARE-ee-ah) are amoebae which normally in still bodies of freshwater. If they get into our bodies (eg. jumping into a pond or dam can force the water containing these organisms up into the nose), they can cause a severe and often fatal infection of the membranes of the brain (eosinophilic meningitis). These infections are so nasty, that sufferers may die before the infection can be diagnosed. In Australia, the only reported cases have been isolated around Rockhampton and Adelaide.

Images of the following amoebae are found in WormLearn :

Pathogens :
Entamoeba histolytica

Commensals :
Entamoeba coli
Entamoeba hartmanni
Endolimax nana (en-DOE-lim-axe NAH-nah)
Iodomoeba butschlii (eye-OWE-doe-ME-bah BOOT-shlee-eye)
Blastocystis hominis (BLASS-tow-SISS-tiss HOM-in-iss)

Flagellates

Although the Flagellates are closely related to the amoebae, their bodies tend to keep the one shape (with some minor bends and twists). Flagellates move about by moving one or more whip-shaped bodies known as flagella (fla-JELL-ah). Beating these whips through the water moves them around, just like a snake uses its body to swim.

Like the amoebae, parasitic flagellates can live in a number of different parts of the body. There are pathogenic and commensal species.

One species that lives in the gut is Giardia intestinalis (jee-ARE-dee- ah in-TESS-tin-ARE-liss). Giardia is a tiny (only 15µm long) teardrop shaped organism which feeds off the lining of the upper part of the small intestine. People catch Giardia by eating food or drinking water which has been contaminated by the organism - usually from faeces. When there are a lot of Giardia present, this generates inflammation, which causes nausea, stomach ache and diarrhoea. When people have Giardia for a long time, they may become malnourished, because they can't absorb food across their intestinal wall. Giardia is found in Australia, but it is only a problem where there is no adequate disposal of sewerage, or where the water supply may be contaminated.

The human intestine also plays host to a range of commensal flagellates, including Trichomonas hominis (TRY-ko-MO-nass HOM-in-iss), Dientamoeba fragilis (DIE-ent-ah-ME-bah FRAJ-ih-liss) and Chilomastix mesnili (KY-low-MASS-tix MEZ-nih-lie).

Flagellates From Other Body Sites

Another flagellate which was, until recent times, not uncommon in Australia is Trichomonas vaginalis (TRY-ko-MO-nass VAJ-in-ARE-liss). As the name suggests, this organism lives in the reproductive tract. It causes irritation to the vaginal membranes and the production of a foul-smelling white discharge. It is transmitted mainly through sexual contact, as unlike Giardia it is too delicate to survive outside. Men may harbour the infection (and infect their partners) without knowing they have it. The incidence of this parasite has dropped in recent years, due to the increase in the use of barrier methods of contraception such as condoms.

The Haemoflagellates (HEEM-oh-FLAJ-eh-ets), as the name suggests, are flagellates which live principally in the blood of their hosts. They require the presence of an arthropod vector (normally a fly or other insect) to complete their lifecycle. There are normally three stages observed : the Promastigote (PRO-mass-tih-goat) found in the arthropod vector, the Tryptomastigote (TRIP-oh-mass-tih-goat) found in the blood and other fluids, and the Amastigote (AY-mass-tih-goat) found in the tissues.

In the golden age of colonial expansion, explorers in darkest Africa had more to fear than lions, elephants or other terrible animals. In fact, many were laid low by the bite of a fly. The Tsetse Fly (Glossina sp.) transmitted another form of flagellate in its saliva. These flagellates, known as the Trypanosomes (TRIP-an-oh-soams or trip-AN-oh-soams) live in the blood and spinal fluid, and may cause African Sleeping Sickness. This disease manifests as a steady decline in consciousness, with sufferers eventually lapsing into a coma, some never to wake again. The two African species of Trypanosomes are Trypanosoma rhodesiense (TRIP-an-oh-soam-ah roe-DEEZ-ee-zee-EN-see) from East Africa, which generally causes an acute (or fast acting) illness, and Trypanosoma gambiense (GAM-bee-EN-see), found in West Africa and which generally results in a chronic illness. Only Trypomastigotes are generally seen in the human host.

There is a South American species of trypanosome, Trypanosoma cruzi (TRIP-an-oh-soam-ah CREW-zee) which is spread by the bite of a blood-sucking bug (the "kissing" bugs), and which causes cardiac problems. This species may be found as far north as the southern regions of the United States. The Trypomastigotes may be found in the blood, while the Amastigotes are found in the heart muscle.

Leishmania is a diverse genus of haemoflagellates causing a range of clinical conditions. This group is characterised by having no known Trypomastigote stage - diagnosis is usually achieved through detection of Amastigotes in affected tissues. Leishmania donovani causes visceral leishmaniasis, or Kala Azar, and the amastigotes infect certain cells in the spleen and liver. Cutaneous Leishmaniasis is caused by a range of species, and the amastigotes may be found in cells in the skin. This conditions may result in disfiguring skin lesions. All species of Leishmania are transmitted by the bite of certain species of sandfly (eg. Phlebotomus spp.)

Images of the following flagellates are contained in WormLearn :

Pathogens :
Giardia intestinalis
Trichomonas vaginalis
Trypanosoma cruzi
Leishmania donovani

Commensals
Dientamoeba fragilis
Chilomastix mesnili

Ciliates

Ciliates (silly-ates) are larger protozoans, growing up to greater than 100µm. They tend to have a rigid body shape and move about by means of thousands of beating hairs called cilia (silly-ah). These hairs are frequently too small to see, even at high magnification, and so the ciliates seem to glide effortlessly through the water (think of the oars on a Roman galley).

There is only one species of pathogenic ciliate known to parasitise humans : Balantidium coli (BAH-lan-TID-ee-um CO-lie). This organism lives in the large intestine and causes diarrhoea.

Ciliates are widely spread as pathogens and commensals in other animals. One species lives in the gut of termites and aids these animals in digesting wood. Other species live in the stomach (or rumen) of cattle and helps them digest the grass they eat. There is even a species that lives in the gut of the cockroach.

Images of Balantidium coli are contained in WormLearn

Sporozoa

The most well known of all the sporozoans are the organisms which cause the disease malaria - Plasmodium falciparum (plaz-MOW-dee-um fal-SIP-are-um), Plasmodium vivax (VY-vax), Plasmodium malariae (mah-LAIR-ee-ay) and Plasmodium ovale (oh-VAR-lee). This disease was once found all around the world, but is now mostly confined to the tropics. It is spread by the bite of the Anopheles (ah-NOFF-el-ees) mosquito. The name "malaria" comes from the French for "bad air", as the disease was most common around the swamps where the mosquitos breed (people used to think the disease came from breathing the foetid vapours from the marshes).

In the human body, malarial parasites mostly infect the red blood cells, although upon initial infection the parasites live in the cells of the liver. In the blood cells, the parasites look like tiny signet rings. They multiply until the host cell bursts, whereupon the parasites find the next cell to infect. In falciparum malaria, the presence of the parasites makes the red cells "sticky", causing them to clump together. These miniature clots may shut off the blood supply to important parts of the body. If this happens in the brain, a potentially fatal condition known as cerebral malaria may develop. The other symptoms associated with malaria are periodic fevers, which coincide with the time the red cells burst to release the parasites (they all tend to do this at once).

All reproduction carried out by malarial parasites in the human body is asexual in nature. Sometimes, instead of the parasites reproducing and bursting the host cell, they form specialised cells called gametocytes (gam-EE-toe-sites). If these gametocytes are drawn up by a mosquito, the conditions inside the mosquito's stomach make them form gametes (similar to sperm and eggs in humans). These unite to form an organism capable of infecting the mosquito. This stage forms cysts on the edge of the mosquito's stomach which generate the sporozoites (SPOH-roe-ZO-ites), the stage infective to humans. When mature, the sporozoites migrate to the mosquito's mouthparts, ready to be injected into the next human host.

Malaria is an important disease worldwide and is one of the top infectious killers. The World Health Organisation estimated that

" . . . in the time taken to read this sentence out loud, one child will have died from falciparum malaria in sub-Saharan Africa."

Think of the limiting statements to that sentence: not contracted the infection but died, not adults but just children, not all four forms of malaria but just falciparum, not in the whole world, but just in one part of Africa. And all in under 30 seconds.

Malaria continues to be a problem because of its tendency to develop resistance to new drugs. In fact these resistances may develop so fast that drug-resistant strains may already be present by the time a drug is ready to go on the market. Control methods now include a staggering array of drugs, plus methods to keep the mosquitos under control (fogging of breeding grounds, netting, etc). Recent work at the Queensland Institute of Medical Research has identified a minute crustacean which preys on the larvae of mosquitos, and hopes are held to use this as a form of biological control for the vector. Researchers are also trying to develop a vaccine, although the parasite seems to be just as capable of building up resistance to the vaccine as it is to new drugs. One drug which is showing promise is an ancient Chinese herbal cure for fevers, known as Artemisia.

Australia is free of malaria for the time being, even though we have the correct mosquitos and our climate is just right. What we do lack is large numbers of people living in places where the disease is likely to spread. There were some temporary outbreaks around north Queensland during World War 2 with the return of soldiers fighting in the Pacific theatre.

Images of all four species of malaria may be found at the Worm Pages.
Plasmodium falciparum
Plasmodium vivax
Plasmodium malariae
Plasmodium ovale

Other Sporozoans

Relatives of malaria may be found in other parts of the body. These parasites have similar life-cycles (ie. alternation of sexual and asexual generations), but they do not need another host in order to reproduce. One species, Cryptosporidium (CRIP-tow- spor-ID-ee-um), lives in the cells of the intestine, causing irritation and diarrhoea. Cryptosporidium is a not uncommon cause of chronic diarrhoea infants. It is spread through faecal contact under low hygienic conditions. It is also a reasonably common cause of illness in immunosuppressed patients.

Another important sporozoan parasite is Toxoplasma gondii (TOX-oh-PLAZ-mah GON-dee-eye). This parasite has a similar life-cycle as Cryptosporidium, although its normal host is the cat, and it may be found in a wide variety of other hosts (eg. cattle, sheep, pigs, etc). Normally, it has little effect on humans. However, if someone has an immune system which is not working properly, it may cause serious problems. Even worse, if a pregnant woman comes into contact with the parasite, there is a chance that it will cause birth deformities in her child (eg. brain damage, blindness, etc). It is therefore advisable that pregnant women limit their contact with cat faeces or try to avoid eating rare meat in order to limit the chances of contracting the infection. Most women now have a specialised blood test known as the TORCH assay (TOxoplasma, Rubella, Cytomegalovirus, Herpes) when they find out they are pregnant.

Images of the following Sporozoa are contained in WormLearn :
Cryptosporidium
Sarcocystis lindemanni(SAR-co-SISS-tiss LIN-deh-man-eye)

Microsporidia

Microsporidia are tiny protozoan parasites (< 5µm long) about which very little is known. Because they are so difficult to diagnose, very little work has been done into their importance in human disease, although they are known to be a major cause of productivity loss in aquaculture facilities such as prawn farms. Microsporidia have also been implicated in causing disease in immunocompromised hosts.

The following images of Microsporidia are contained in WormLearn :
Pleistophora spp.(ply-STOF-or-ah)
Enterocytozoan bineuzi (EN-ter-row-sy-tow-ZO-an bin-YOU-zee)