Protozoa : The Single Celled Organisms

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Protozoa : The Single Celled Organisms

 

Protozoa* form part of the Kingdom Protista. This kingdom is made up of all

the single celled organisms (including the dinoflagellates, and the

single-celled algae). The Protozoans are generally (although not always

correctly) defined as those protists which exhibit more " animal-like "

characteristics than the other groups (ie. they do not generally photosynthesise

and are commonly motile). Some protozoans are so small that they parasitise the

cells of the host they inhabit. Protozoans may live in all parts of the body,

including the gut, the mouth, the skin, the blood, the liver, the spleen, the

genital tract, the lungs and the heart.

 

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 Intestinal 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 Intestinal 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 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 Sporozoa (more correctly known as the Apicomplexans) are a

diverse range of parasites found in all parts of the body and in a wide variety

of animal hosts. They do not have a set body plan like the other parasitic

protozoans we have dealt with, although they are characterised by having complex

life-cycles with an alternation of sexual and asexual generations. Many species

are so small that they are parasites within the cells of their host.

Malaria

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)

 

 

 

This page Copyright 1997 Dr Peter Darben.

Last Updated 27.7.2003 O Million Murdering Death . . .

The role of the mosquito in transmitting malaria was first recognised by Ronald

Ross on 20 August, 1895. In honour of his discovery, he wrote a poem for his

wife :

 

This day relenting God

Hath placed within my hand

A wondrous thing; and God

Be praised. At His command

Seeking His secret deeds

With tears and toiling breath

I find thy cunning seeds,

O million murdering Death

I know this little thing

A myriad men will save . . .

Because the malarial parasite reproduces sexually in mosquitos, the mosquito is

said to be the definitive host for the parasite, and we humans, the intermediate

host. Although we commonly think of the mosquito transmitting the infection to

us, we actually transmit the infection to the mosquitos.

 

G & T anyone ?

In the days of the British occupation of India, malaria was a big problem to

the colonists. One of the more effective treatments to keep malaria at bay was a

quinone tonic which became simply known as " Tonic Water " . To make the medicine

more palatable, people used to mix it with Gin, and so the Gin and Tonic was

born. Unfortunately, most strains of malaria are now resistant to this

" medicine " .

 

 

Immunosuppression

Most people have a fully functioning immune system which protects them from

the great majority of parasitic organisms they may come into contact with.

However, if the immune system is depressed for some reason, organisms which are

not normally harmful, may became dangerous. Depression of the immune system can

result from a number of causes, including anti- rejection therapy following

transplant surgery, steroid therapy and the human immunodeficiency virus (HIV).

In fact one of the hints to researchers that HIV might be around was an unusual

number of people coming down with normally harmless parasitic infections (eg.

Pneumocystis carinii). As more and more is understood about immunodeficiency,

parasites of immunocompromised hosts will become a more important area.

 

*Protozoa : A subgroup of the Kingdom Protista, or the single-celled

organisms. The name Protozoa is a carry-over from an old system of

classification and is generally used to described those single-celled organisms

which show more animal than plant characteristics. Naturally, such a distinction

is meaningless, as animals and plants belong to completely different kingdoms,

but in general, Protozoa refers to those organisms which do not carry out

photosynthesis. Parasitic protozoa comprise a number of subgroups : The

Sarcomastigophora (amoebae and flagellates), The Ciliates (ciliated organisms),

the Sporozoa (malaria, Toxoplasma, Cryptosporidium and allies), and the

Microsporidia.

http://home.austarnet.com.au/wormman/wlprot.htm#amoeba

 

 

" Respect means listening until everyone has been heard and understood, only

then is there a possibility of " Balance and Harmony " the goal of Indian

Spirituality. " Dave Chief, Grandfather of Red Dog

 

 

 

 

 

 

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