The Ciliates, Coccidia and Microsporidia

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The Ciliates, Coccidia and Microsporidia

 

2.1 Infections acquired through the Gastrointestinal Tract - Part 2

 

In vertebrates, by far the most favourable sites for intestinal

parasites are the duodenum, ileum, caecum and large intestine. To

survive to reproduce in the gastrointestinal tract the parasites have

to adapt to continuous physiological changes relative to the feeding

habits of the host, the battery of protein, fat and

carbohydrate-splitting enzymes, pH changes and the almost oxygen-free

environment. Despite these features many parasites, both protozoa and

helminths are capable of colonising the gastrointestinal tract

successfully.

 

Protozoa are single-celled animals which resemble a single cell of a

higher organism. However, the protozoan cell is capable of carrying

out vital functions such as reproduction, feeding and locomotion.

Intestinal protozoa include species which can live in the lumen of the

intestine and others which additionally live and reproduce in the

cells of the intestinal walls.

 

The protozoa make up a wide spectrum of organisms which have different

life cycles and variable characteristics.

 

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2.2 The parasites

 

The Ciliates

 

The ciliates belong to the family Ciliophora. They possess simple

cilia or compound ciliary organelles, 2 types of nuclei and a large

contractile vacuole. The only member of the ciliate family to cause

human disease is Balantidium coli.

 

Balantidium coli

 

Introduction

 

Balantidium coli is widely distributed in warmer climates, which is

where human infections most commonly occur. The organisms inhabit the

large intestine, caecum and terminal ileum where they feed on

bacteria. The most common hosts being humans, pigs and rodents. Human

infection is usually from pigs and is rare. Diagram 1 illustrates the

life cycle of Balantidium coli.

 

Diag 1.gif (11291 bytes)

 

Diagram 1. Life cycle of Balantidium coli. (Professor JH. Cross)

 

Morphology of cyst

 

The cyst is spherical or ellipsoid and measures from 30-200m m by

20-120m m. It contains 1 macro and 1 micronucleus. The cilia are

present in young cysts and may be seen slowly rotating, but after

prolonged encystment, the cilia disappear. (Diagram 2 & Fig 1) Cysts

form when diarrhoea subsides and the rectal contents become formed.

The cyst, ingested by a fresh host, excysts to liberate the

trophozoite. Diagram 2 illustrates a B. coli cyst.

 

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Diagram 2. Systematic diagram of a Balantidium coli cyst. 40 x 60m m

(Dr. AJ. Frisby)

 

 

 

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Fig 1. Balantidium coli cyst

 

 

 

Morphology of trophozoite

 

Trophozoites of B. coli measure approximately 30-150m m in length x

25-120m m in width but have been known to attain lengths of up to 200m

m. They are oval in shape and covered in short cilia. A funnel shaped

cytosome can be seen near the anterior end. Multiplication is by

binary fission in the trophozoite stage. In an unstained preparation,

the organisms are easily recognised because of their size and rapid

revolving rotation. In a stained preparation, the characteristic macro

and micronuclei may be observed. (Diagram 3 & Fig 2)

 

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Diag. 3 Balantidium coli

 

 

 

Clinical Disease

 

Severe B.coli infections may resemble amoebiasis. Symptoms include

diarrhoea, nausea, vomiting and anorexia. The diarrhoea may persist

for long periods of time resulting in acute fluid loss. Balantidium

coli also has the potential to penetrate the mucosa resulting in

ulceration just as those of Entamoeba histolytica, but perforation is

more common. Metastatic lesions do not occur. Extra-intestinal disease

has also been reported, but is rare.

 

Laboratory Diagnosis

 

Wet preparations of fresh and concentrated stool samples reveal the

characteristic cysts and motile trophozoites. (Fig 2) They are easier

to identify in direct-smear saline preparations than permanently

stained faecal smears.

 

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Fig. 2 wet

preparation of B.coli trophozoite

 

 

 

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The Coccidia

 

The Coccidia are a group of organisms which parasitise the epithelial

cells of the intestinal tract. This group includes Cryptosporidium

parvum, Cyclospora cayetanensis and Isospora belli. Most of the

coccidian infections in man are zoonoses (having the potential to

infect animals or arise from animals). In immunocompetent individuals,

they usually produce mild, self-limiting infections.

 

Cryptosporidium parvum

 

Introduction

 

Cryptosporidium species, are coccidian protozoa, which are

cosmopolitan in distribution, occurring in both developed and

underdeveloped countries and causing infection in both humans and

their live stock. Cryptosporidium parvum is the species responsible

for human infection.

 

Life cycle and Morphology

 

Diag 4 vol 2.gif (20485 bytes)Development of Cryptosporidia occurs in

a parasitophorous vacuole located on the microvillous surface of the

epithelial cells. Diagram 4 illustrates the life cycle of Cryptosporidia.

 

Diagram 4. Life cycle of Cryptosporidium sp. (www.dpd.cdc.gov)

 

Sporulated oocysts, containing 4 sporozoites, are excreted by the

infected host through faeces (and possibly other routes such as

respiratory secretions). Following ingestion (and possibly inhalation)

by a suitable host, excystation (a) occurs. The sporozoites are

released and parasitize epithelial cells (b, c) of the

gastrointestinal tract (or other tissues such as the respiratory

tract). In these cells the parasites undergo asexual multiplication

(schizogony or merogony)(d, e, f) and then sexual multiplication

(gametogony)(g). Upon fertilization of the macrogamonts (female) by

the microgametes (male)(i), oocysts (j, k) develop that sporulate in

the infected host, and are excreted. Because the oocysts sporulate

inside the infected host, autoinfection can occur. Oocysts (measuring

4-5m m in diameter and containing 4 sporozoites) are infective upon

excretion, thus permitting direct and immediate faecal-oral

transmission. (Fig 3 & 4)

 

Clinical Disease

 

C. parvum is now widely recognised as a cause of acute

gastro-enteritis, particularly in children. The infection produces a

persistent, watery, offensive diarrhoea often accompanied by abdominal

pain, nausea, vomiting and anorexia.

 

In immunocompetent persons, symptoms are usually short lived (1 to 2

weeks). The small intestine is the site most commonly affected,

symptomatic Cryptosporidium infections have also been found in other

organs including other digestive tract organs, the lungs, and possibly

conjunctiva.

 

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Fig 4. Cryptosporidium parvum oocysts (Acid-fast stain) The oocysts

are variable in size (4-5µm) and in colour ranging from pale pink to

vermillion red. The oocyst are easily identifiable. The slide

background is blue-green even though the counter colour uses was

malachite green. This is not unusual if the stools have been preserved

in 10% formalin soultion (x825)

 

Cryptosporidiosis in immunocompromised individuals, especially in HIV

patients, can be life threatening, as many as 10% may pass oocysts of

C. parvum. Infections are characterised by the production of frequent,

large volume watery stools and sometimes there is invasion of the

pancreas, biliary or respiratory tract.

 

Oocyst excretion and symptoms may fluctuate during the course of

infection. Asymptomatic infections are commonly found in developing

countries with poor hygiene, where there is close contact with livestock.

 

Laboratory Diagnosis

 

Definitive diagnosis of cryptosporidiosis is by finding the

characteristic spherical oocysts in faecal samples. They do not

concentrate well using standard concentration techniques and are

identified using various staining techniques.

 

Using the modified Ziehl-Neelsen staining method (fuschin followed by

methylene blue), the oocysts are acid fast. (Fig 3) However, staining

results within a smear and between specimens are diverse, varying from

unstained to partial red staining and complete staining. Fully

sporulated forms can be seen in which the red staining sporozoites are

within an unstained oocyst wall. When staining the faecal smear with

phenol-auramine/carbol-fuchsin, the oocysts appear as bright yellow

discs with an " erythrocyte " pattern of staining against a dark red

background.

 

Detection of the oocysts can also be achieved by using specific

polyclonal or monoclonal antibodies conjugated to fluorescein. (Fig 5)

These tests are now commercially available and offer a high degree of

sensitivity. However, caution must be exercised when they are used to

detect oocysts in the faecal smears distributed by NEQAS parasitology.

Such specimens are preserved in formalin, which interferes with the

fluorescent staining of the parasites, and are thus difficult to detect.

 

Oocysts in stool specimens (fresh or in storage media) remain

infective for extended periods. Thus stool specimens should be

preserved in 10% buffered formalin or sodium acetate-acetic

acid-formalin (SAF) to render oocysts non-viable. (Contact time with

formalin necessary to kill oocysts is not clear; we suggest at least

18 to 24 hours).

 

Image11.gif (22893 bytes)

 

Fig 5. Staining of Cryptosporidium parvum oocysts in a stool smear

with monoclonal antibodies conjugated to fluorescein. The

Cryptosporidium oocysts appear with a peripheral green fluorescence.

This technique could be of interest in epidemiological inquiries. (x 670)

 

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Isospora belli

 

Introduction

 

Isospora belli is a coccidian protozoan of cosmopolitan distribution,

occurring especially in warm regions of the world infecting both

humans and animals.

 

Life cycle and morphology

 

The life cycle of I. belli involves an asexual (schizogonic stage) and

a sexual (sporogonic stage). (Diagram 5)

 

Infection with I. belli occurs in both immunocompetent and

immunocompromised patients and begins when the mature oocyst is

ingested in water or food.

 

 

 

 

 

Morphology of oocysts The mature oocyst contains 2 sporocysts, each

containing 4 sporozoites measure on average 35 x 9m m.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Diagram 5. Diagrammatic illustration of Isospora belli life cycle.

(Adapted and redrawn from NCDC.)

 

Diagram 6 illustrates diagrammatically an immature and a mature oocyst

with each sporocysts containing 4 sporozoites. Fig 6 and 7 demonstrate

faecal smears of oocysts. The sporulated oocyst is the infective stage

of the parasite and they excyst in the small intestine releasing

sporozoites which penetrate the epithelial cells, thus initiating the

asexual stage of the life cycle. The sporozoite develops in the

epithelial cell to form a schizont, which ruptures the epithelial cell

containing it, liberating merozoites into the lumen. These merozoites

will then infect new epithelial cells and the process of asexual

reproduction in the intestine proceeds. Some of the merozoites form

macrogametes and microgametes (sexual stages) which fuse to form a

zygote maturing finally to form an oocyst.

 

 

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Diagram 6. Isospora belli immature oocysts showing a central

individual mass of protoplasm. The mature oocysts illustrates the 2

sporocysts each containing 4 sporozoites. (10 x 16m m)

 

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Fig 6. Isospora belli oocyst with 2 sporoblasts. (Imature oocyst)

(Direct wet preparation without staining). The sporoblast divides

itself into 2 identical round masses x990.

 

 

 

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Fig 7. Isospora belli oocyst. with its elongated rugby ball shape.

(28µm).The wall is smooth and refractive. One end is tapered and the

other often shows a slight striction. The rounded granular sporoblast

ocupies the central area, this is the most prominent feature. x660

 

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Fig 8. Isospora belli mature oocyst. The oocyst keeps its elongated

shape but the striction at one end is very pronounced. The sporoblasts

are transformed into 2 sporocysts each containing 4 sporozoites and a

residual body. (Direct wet preparation with no staining). (x660)

 

Clinical Disease

 

In the immunocompetent, infection is generally asymptomatic or a

self-limiting gastro-enteritis. However, in chronic infections, severe

non-bloody diarrhoea with cramp-like abdominal pain can last for weeks

and result in fat malabsorption and weight loss. Eosinophilia may be

present (atypical of other protozoal infections).

 

In immunocompromised individuals, infants and children, infection

ranges from self-limiting enteritis to severe diarrhoeal illness

resembling that of cryptosporidiosis.

 

Laboratory Diagnosis

 

Oocysts are thin walled, transparent and ovoid in shape. They can be

demonstrated in faeces after a formal ether concentration where they

appear as translucent, oval structures.

 

Alternatively, oocysts can be seen in a faecal smear stained by a

modified Ziehl-Neelsen method, where they stain a granular red colour

against a green background, or by phenol-auramine.

 

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Cyclospora cayetanensis

 

Introduction

 

Cycloclospora cayetanensis, a coccidian protozoan, has been described

in association with diarrhoeal illness in various countries, in

particular Nepal, Pakistan and India. Infection results in a disease

with non-specific symptoms. Quite often the disease is the cause of

unexplained summer diarrhoeal illness and similar illness following

travel to tropical areas.

 

Life Cycle

 

The life cycle of this organism is unknown, however environmental data

suggest that Cyclospora, like Cryptosporidium species, is a

water-borne parasite. The oocysts of C. cayetanensis are spherical,

measuring 8-10m m in diameter and the mature oocyst contains 2

sporocysts. Oocysts of Cyclospora cayetanensis, are twice as large in

comparison with C. parvum and are not sporulated (do not contain

sporocysts - upon excretion). Fig 9, 10 and 11 illustrate immature and

mature oocysts of Cyclospora species

 

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Fig 9 and 10. Cyclospora sp. immature oocysts. In the centre of each

picture is a rounded body, 9µm in diameter. The outer wall is distinct

and thick. Each body contains several refractive granules. (Direct wet

preparation with no staining) (x660, x1650)

 

Clinical Disease

 

Patients from whose stools the organism has been isolated have

reported nausea, vomiting, weight loss and explosive watery diarrhoea.

Flatulence and bloatedness, nausea and vomiting, myalgia, low-grade

fever, and fatigue are associated symptoms. The site of infection is

the small bowel. The disease is usually self-limiting to three to four

days but untreated infections can last from several days to a month or

longer, and may follow a relapsing course. Some infections are

asymptomatic.

 

Fig 11 vol2.gif (68416 bytes)

 

Fig. 11. Cyclospora sp. oocyst. If the examination is not done

immediately after stool emission some oocsts can start sporulation.

You then observe globular bodies which vary in size and are very

refractive. Thediagnosis is therefore very diffcult. (Direct wet

preparation with no staining) (x1650).

 

Laboratory Diagnosis

 

The oocysts of C. cayetanensis are spherical as can be seen in

formol-ether concentrated stool samples by light microscopy. They are

refractile spheres which exhibit blue autofluorescence under

ultraviolet light. It is important to note that UV microscopes set up

for FITC and auramine microscopy only (450-500nm) will fail to detect

the autofluorescence of the oocyst. Iodine-quartz microscopes do not

produce UV wavelength below 400nm, while both mercury vapour and xenon

vapour microscopes must be fitted with a 340-380nm excitation filter

to demonstrate autofluorescence.

 

The oocysts are variably acid-fast when stained by the modified

Ziehl-Neelsen method. Some cysts are acid-fast whereas others appear

as round holes against a green background. They do not stain well with

phenol-auramine.

 

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Microsporidia species

 

Introduction

 

The term microsporidia is also used as a general nomenclature for the

obligate intracellular protozoan parasites belonging to the phylum

Microsporidia. To date, more than 100 genera and 1,000 species have

been described as parasites infecting a wide range of vertebrate and

invertebrate hosts. There are at least 7 microsporidian species that

are well characterised as human pathogens. (Table 1)

 

Microsporidia are characterised by the production of resistant spores

and the polar tubule (or polar filament) which is coiled inside the

spore as demonstrated by its ultrastructure.

 

They have recently come to medical attention as opportunistic

pathogens in humans with Acquired Immune Deficiency Syndrome (AIDS)

and have been implicated in conditions ranging from enteritis to

keratoconjunctivitis.

 

Morphology and Life cycle

 

Microsporidia are primitive organisms. They possess no mitochondria

and have prokaryotic like ribosomes. Classification is based on the

ultrastructural features, which include the number of coils in the

polar tubes, the configuration of nuclei and the spore size 1-4m m,

depending on the species.

 

Microsporidium.JPG (48927 bytes)

 

Diagram 7. The general intracellular cycle of Microsporidia.

 

The spores are the infective stage of the organism (1), spores can

survive for a long time in the environment. The spore hatches by

extruding its polar tubule and infects the host cell (2). Infection

occurs when the infective sporoplasm within the organism is injected

into the host cell through the polar tube (3). Microsporidia multiply

rapidly within the cytoplasm of the cell to produce sporoblasts

(merogony) (4), followed by sporogony (5). This development can occur

either in direct contact with the host cell cytoplasm (A) or inside a

vacoule termed a parasitophorous vacoule (B). This results in the

production of infective, in the case of Enterocytozoon and

Encephalitozoon species thick walled spores which are released into

the intestinal lumen. (6)

 

 

 

Microsporidia

 

 

Size

 

 

Associated disease

 

Enterocytozoon bieneusi

 

 

1m m x 1.5m m

 

 

Gastrointestinal and biliary tract infections

 

Encephalitozoon intestinalis

1.5m m x 2.5m m

 

Gastrointestinal tract and systemic infections

 

Encephalitozoon hellem

 

 

1.5m m x 1m m

 

 

Keratopathy, respiratory tract infection

 

Encephalitozoon cuniculi

 

 

1.5m m x 1m m

 

 

Central nervous system disease

 

Nosema connori

 

 

2m m x 4m m

 

 

Systemic infections

 

Nosema corneum

 

 

2m m x 4m m

 

 

Keratopathy

 

Pleistophora species

2.8m m x 3.4m m

 

Myositis

 

Table 1. Microsporidia found in humans and their associated disease.

 

In addition to the species in Table 1 above there are other, not

well-characterised microsporidian human pathogens. These are

designated as Microsporidum, a collective taxon, that includes

Microsporidium africanum and Microsporidium ceylonensis.

 

Clinical disease

 

The most common microsporeans found in patients with AIDS are

Enterocytozoon bieneusi, Encephalitozoon intestinalis and

Encephalitozoon hellem. Patients with these infections tend to be

severely immuno-deficient with a CD4 count less than 100 x 106/L.

Additionally, cases of microsporidiosis have been reported in

immunocompromised persons not infected with HIV and in immunocompetent

individuals. The clinical manifestations of microsporidiosis are very

diverse, varying according to the causal species, with diarrhoea being

the most common. (Table 1)

 

Enterocytozoon bieneusi

 

Infections with E. bieneusi are restricted to the enterocytes of the

small intestine, resulting in villous atrophy and malabsorption.

Clinical symptoms include chronic watery, non-bloody diarrhoea,

malaise and weight loss.

 

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Encephalitozoon intestinalis

 

Infection with Encephalitozoon intestinalis occurs in the enterocytes

of the small intestine but is more widely disseminated than E.

bieneusi and has been found in the colon, liver and kidney.

 

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Encephalitozoon hellem and Encephalitozoon cuniculim

 

These organisms have also been found in disseminated microsporidiosis.

Clinical symptoms may include sinusitis, nephritis, hepatitis,

keratoconjunctivitis and peritonitis.

 

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Nosema corneum

 

This organism has been detected in AIDS patients with

keratoconjunctivitis.

 

Microspora spore.gif (95565 bytes)

 

Fig 12. Microsporidia spore. (Electron microscopy). In this tiny,

oval-shaped spore can be seen the polar filament rolled up in a number

of coils, the exact number of which will vary according to the genus

and species of the microsporidia under study.

 

fig 13 vol 2.gif (78384 bytes)

 

Fig. 13. Microsporidia spores in stool smear. (Modified Weber staining

technique). The spores appear as minute bodies (1.5-2.0µm), oval

shaped and coloured bright pink. An uncoloured vacuole can be seen in

several of them. (x1650)

 

Laboratory Diagnosis

 

Initially, the diagnosis of intestinal microsporidiosis depended on

tissue biopsies which were stained with Gram's stain and examined by

light microscopy. However, in order that ill patients were not

subjected to unnecessary invasive procedures, non-invasive diagnostic

procedures were developed. The modified Trichrome stain and the

Fungiqual fluorescent stain are the stains of choice.

Immunofluorescence assays (IFA) using monoclonal and / or polyclonal

antibodies are being developed for the identification of microsporidia

in clinical samples.

 

2.6 Examination of faecal specimens for Parasites

 

Introduction

 

Many intestinal disorders are due to intestinal parasites which cannot

be diagnosed symptomatically. Laboratory investigation is therefore

required and the staff responsible should have adequate expertise in

examining faecal specimens for parasitic organisms.

 

2.7 Relevant information required

 

The request form should always state the patients clinical symptoms

and signs and whether the patient is resident in the UK, or had recent

overseas travel. If the patient has had no recent history of overseas

travel, examination for Cryptosporidium, Giardia and Microsporidia, if

immunocompromised should be considered. If overseas travel has been

undertaken, it is important to note is the patient ill or whether a

routine post-tropical screen is requested. The Geographical location

is also important as it may indicate these parasites which could be

present.

 

2.8 Collection of samples

 

If a faecal sample is not properly collected and taken care of before

examination, it will be of little or no value for accurate diagnosis.

This is especially true if protozoa are present. Amoebic trophozoites

begin to degenerate 1-2 hours after passage, as do flagellate

trophozoites. Cysts will deteriorate if the faecal specimen is left

standing for many hours or overnight, especially at high temperatures.

 

Helminth eggs and larvae are less affected by the age of the specimen

than are protozoa. Nevertheless, changes may occur that could affect

their identification e.g. hookworm larvae may become embryonated and

larvae may hatch from the eggs risking confusion with Strongyloides

larvae. Larvae themselves may disintegrate thus making their

identification difficult.

 

To ensure that good specimens are provided for examination, it is

important to note the following points.

 

1. A clean dry container must be used for the collection of faecal

samples. Urine and water will destroy trophozoites, if present, and

the presence of dirt also causes identification problems.

1. Ideally the specimen should be brought to the lab as soon as it

is passed, to avoid deterioration of protozoa and alterations of the

morphology of protozoa and helminths.

2. The specimen container should be clearly labelled with the

patient's name, date, and time of passage of the specimen.

3. An amount of stool adequate for parasite examination should be

collected and a repeat sample requested if too little is supplied. The

smallest quantity that should be accepted is about the size of a

pigeon's egg.

4. Diarrhoeal specimens, or those containing blood and mucus,

should be examined promptly on arrival in the laboratory. The

specimens may contain motile amoebic or flagellate trophozoites and

may round up and thus be missed if examination is delayed. Where

amoebic dysentry is suggested, the laboratory should be informed that

a " hot stool " is being supplied so that it can be examined within

twenty minutes of being passed.

5. With the exception of " hot stools " if specimens cannot be

examined as soon as they arrive, they should be put in the refrigerator.

 

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2.9 Visual observation of the faecal sample

 

It is important to observe the macroscopic appearance of the stool as

this can give a clue to the type of organisms present. Therefore the

consistency; formed, unformed or liquid; the colour and the presence

or absence of the exudate are reported. The presence of adult worms

can also be seen in a freshly passed stool e.g. adult stage of Ascaris

lumbricoides and Enterobius vermicularis. Proglottids of Taenia

species can also be seen.

 

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2.10 Routine procedure for the microscopic examination of faecal

samples for parasites

 

1. Direct microscopy should be done on all unformed and liquid

samples by mixing a small amount of the specimen in 0.9% sodium

chloride solution. This permits detection of trophozoites of Entamoeba

histolytica and Giardia lamblia. It can also provide information on

the content of the stool i.e. the presence of leucocytes and red blood

cells.

2. A formol-ether concentrate should be done on all faecal samples

examined for parasites. This reveals the presence of most protozoan

cysts, eggs of nematodes, cestodes and trematodes and also the larval

stages of some nematodes.

3. A permanently stained direct faecal smear should be used for all

bloody, liquid or semi-formed stools. The smear can reveal the

presence of intestinal parasites that can be either destroyed or

missed by the formol-ether concentration method e.g. Dientamoeba fragilis.

4. Specimens from patients with HIV should be left in 10% formalin

for one hour before proceeding with parasite examination.

 

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2.11 Principals of diagnostic methods for the identification of parasites

 

The principal of the successful identification of faecal parasites is

based upon,

 

1. Measurement - The use of an eyepiece graticule is of the utmost

importance, especially for cyst identification.

2. Morphology - In protozoan cysts, the number of nuclei and the

presence of inclusions e.g. glycogen mass and chromidial bar, aid the

identification of protozoa. In trophozoites, the presence of red cells

in amoebae is diagnostic of Entamoeba histolytica and flagella also

aid identification of some protozoan trophozoites.

3. Appearance - In helminth eggs, the shape of the egg, the

thickness of the shell, the colour of the ovum and the presence or

absence of features such as an operculum, spine or hooklets are

diagnostic pointers to the identity of the parasite.

4. Stains also aid in identification of the parasite.

 

The addition of iodine to formol ether concentrates highlights the

internal

 

structure of cysts and helps distinguish between vegetable matter and

 

cysts. Permanently stained faecal smears are useful in demonstrating the

 

nuclear pattern of cysts.

 

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2.12 Problems of identification

 

Many things in stool specimens look like parasites but are not.

 

Epithelial cells and macrophages can be confused with amoebic

trophozoites, especially macrophages that show slight amoeboid

movement and may contain red blood cells. Pus cells can be confused

with amoebic cysts. The nuclei appear as 3 or 4 rings and usually

stain heavily. The cytoplasm is ragged and the cell membrane is often

not seen. Amoebic cysts have a distinct cell wall.

 

Hair and fibres may be confused with larvae, but they do not have the

same internal structure as larvae.

 

Plant cells can be confused with cysts or eggs. Though plant cells

usually have a thick wall and cysts have a thin wall.

 

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2.13 Reporting of parasites

 

Ideally, the presence of all parasites should be reported, whether

they are pathogens or non-pathogens. This particularly applies to the

presence of cysts. However, if it is laboratory practice to report all

cysts, the report should state whether they are pathogenic or

non-pathogenic.

 

The stage of the parasite should always be reported. For the protozoa,

whether cysts or trophozoites are present; the stage of larvae as in

Strongyloides; and whether adult stages or eggs of helminths are present.

 

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References

 

Murray, PR, Drew, WL, Koyayashi, GS & Thomson, JH: Medical

Microbiology. Mosby Books Inc., New York (1990)

 

Peters, W & Gilles, HM: Tropical Medicine & Parasitology. Wolfe

Medical Publications Ltd.

 

Jeffrey & Leach: Atlas of Medical Helminthology and Protozoology. E &

S Livingstone Ltd.

 

Ash, LR & Orihel, TC: Atlas of Human Parasitology. ASCP Press, Chicago.

 

Garcia, LS & Bruckner, DA: Diagnostic Medical Parasitology. Elsevior

Science Publishing Co. Inc.

 

Muller, R & Baker, JR: Medical Parasitology. Gower Medical Publishing.

 

Snell, JJS, Farrell, ID & Roberts, C: Quality Control, Principles and

Practice in the Microbiology Laboratory. Public Health Laboratory

Service. ISBN 0 901 144 312.

 

World Health Organisation: Basic Laboratory Methods in Medical

Parasitology. ISBN 92 4 154410 4. (1991)

 

Brisdon, E: M, Moody, A, Edwards, H & Chiodini, PL: Evaluation of the

Parasep® (faecal parasite concentrator).

 

Pennell, DR et al: Advances in Giardia Research. p211-213. University

of Calgary Press.

 

Samways, KL et al: Assessments of ParasepÒ , a novel parasite egg

retrieval system; use in faecal and waste water testing. Presented to

Royal. Soc.Trop.Med.

 

Howells, H: A Critical evaluation of methods available for diagnosis

of Cryptospridium parvum and Giardia lamblia in faecal samples. St

Mary's Hospital Portsmouth.

 

Juranek, DD. Cryptosporidiosis. In: Hunter's Tropical Medicine. 8th

edition, Strickland, GT (editor).

 

Brown, VC. A Longitudinal study of the prevalence of intestinal

helminths in baboons (Papio doguera) from Tanzania. (1994) Thesis,

Liverpool.

 

Acknowledgements

 

We would like to thank the authors of the following web sites:

 

www.cdfound.to.it/atlas.htm

 

www.aisr.li.tju.edu,

 

www.medlib.med.utah.eduparasitology

 

www.ferris.edu

 

www.jeffline.tju.edu,

 

www.dpd.cdc.gov

 

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