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3. Techniques for parasite assays and identification in faecal samples


3.1 Introduction
3.2 Collection of faecal samples
3.3 Qualitative techniques for separating and concentrating eggs/larvae
3.4 Quantitative techniques for separating and concentrating eggs/larvae
3.5 Preparation of faecal cultures
3.6 Isolation and identification of lungworm larvae and infective larvae harvested from faecal cultures (the Baermann technique)
3.7 Diagnostic techniques for filarial nematodes
3.8 Identification and examination of snails


3.1 Introduction

To diagnose gastro-intestinal parasites of ruminants, the parasites or their eggs/larvae must be recovered from the digestive tract of the animal or from faecal material. These are subsequently identified and quantified. This chapter presents diagnostic techniques within the reach of most laboratories to identify and quantify parasite infections from the examination of faecal material. The following are the main tasks involved in this process:

· Collection of faecal samples
· Separation of eggs/larvae from faecal material, and their concentration
· Microscopical examination of prepared specimens
· Preparation of faecal cultures
· Isolation and identification of larvae from cultures

It is important to understand the following basic limitations of faecal examination in the diagnosis of gastro-intestinal parasitism.

(a) The demonstration of parasite eggs or larvae in the faeces provides positive evidence that an animal is infected but does not indicate the degree of an infection.

(b) The failure to demonstrate eggs or larvae does not necessarily mean that no parasites are present; they may be present in an immature stage or the test used may not be sufficiently sensitive.

(c) There is generally no correlation between the numbers of eggs/larvae per gram of faeces and the number of adult nematodes present in cattle. An exception to this may occur in a primary infection in young grazing animals during their first exposure. There are some indications that the correlation is stronger in sheep and goats with mixed infections.

Various factors can limit the accuracy and significance of a faecal egg count.

(a) There is a fairly regular fluctuation in faecal egg output.

(b) Eggs are not evenly distributed throughout the faeces.

(c) The quantity of faeces passed will affect the number of eggs per unit weight.

(d) The egg output is influenced by the season of the year (large infections may be acquired during rainy seasons).

(e) The resistance of the host can depress or entirely inhibit the egg production of parasites.

(f) Immature worms do not indicate their presence by producing eggs.

(g) Immunity may result in a marked extension of the prepatent period and a lower egg output by female parasites.

(h) An egg count often refers to the total number of eggs of a mixture of species, which differ widely both in their biotic potential and their pathogenicity.

(i) Eggs may not be detected due to low numbers of them or to a low test sensitivity.

3.2 Collection of faecal samples

Faecal samples for parasitological examination should be collected from the rectum of the animal

If rectal samples cannot be obtained, fresh faecal samples may be collected from the pasture.

Several samples should be collected. Samples should be dispatched as soon as possible to a laboratory in suitable containers such as:

· screw cap bottles
· plastic containers with lids
· disposable plastic sleeves/gloves used for collecting the samples
· plastic bags

Each sample should be clearly labelled with animal identification, date and place of collection.

Samples should be packed and dispatched in a cool box to avoid the eggs developing and hatching. If prolonged transport time to a laboratory is expected, the following may help to prevent the eggs developing and hatching.

(a) Filling the container to capacity or tightening the sleeve/glove as close to the faeces as possible. This is to exclude air from the container.

(b) Adding 3% formal in to the faeces (5-20 ml, depending on the volume of faeces). This is to preserve parasite eggs. (N.B Formalin-fixed faeces cannot be used for faecal cultures.) When samples are received in the laboratory they should immediately be stored in the refrigerator (4 °C) until they are processed. Samples can be kept in the refrigerator for up to 3 weeks without significant changes in the egg counts and the morphology of eggs. SAMPLES SHOULD NEVER BE KEPT IN THE FREEZER.

3.3 Qualitative techniques for separating and concentrating eggs/larvae


3.3.1 Simple test tube flotation
3.3.2 Simple flotation method
3.3.3 Sedimentation technique (for trematode eggs)
3.3.4 Microscopical examination of prepared samples


A number of different methods are available for separating, concentrating and demonstrating eggs, oocysts and larvae in faecal samples. Three methods are described:

· Simple test tube flotation
· Simple flotation
· Sedimentation technique (for trematode eggs)

3.3.1 Simple test tube flotation


3.3.1.1 Principle
3.3.1.2 Application
3.3.1.3 Equipment
3.3.1.4 Procedure


3.3.1.1 Principle

The simple test tube flotation method is a qualitative test for the detection of nematode and cestode eggs and coccidia oocysts in the faeces. It is based on the separating of eggs from faecal material and concentrating them by means of a flotation fluid with an appropriate specific gravity.

3.3.1.2 Application

This is a good technique to use in initial surveys to establish which groups of parasites are present.

3.3.1.3 Equipment

· Beakers or plastic containers
· A tea strainer (preferably nylon) or double layer cheesecloth
· Measuring cylinder or other container graded by volume
· Fork, tongue blades or other type of stirring rod
· Test tube
· Test tube rack or a stand
· Microscope
· Microslides, coverslips
· Balance or teaspoon
· Flotation fluid (see the Appendix to this handbook for formulation)

3.3.1.4 Procedure

(a) Put approximately 3 g of faeces (weigh or measure with a precalibrated teaspoon) into Container 1.

(b) Pour 50 ml flotation fluid into Container 1.

(c) Mix (stir) faeces and flotation fluid thoroughly with a stirring device (tongue blade, fork).

(d) Pour the resulting faecal suspension through a tea strainer or a double-layer of cheesecloth into Container 2.

(e) Pour the faecal suspension into a test tube from Container 2.

(f) Place the test tube in a test tube rack or stand.

(g) Gently top up the test tube with the suspension, leaving a convex meniscus at the top of the tube and carefully place a coverslip on top of the test tube.

(h) Let the test tube stand for 20 minutes.

(i) Carefully lift off the coverslip from the tube, together with the drop of fluid adhering to it, and immediately place the coverslip on a microscope slide.

3.3.2 Simple flotation method


3.3.2.1 Principle
3.3.2.2 Application
3.3.2.3 Equipment

3.3.2.1 Principle

The principle for the simple flotation method is the same as for the simple test tube flotation method.

3.3.2.2 Application

This is another good technique for use in initial surveys. In addition, it can be used in conjunction with the McMaster technique (see section 3.4.1) to detect low numbers of eggs (when present below the McMaster sensitivity of 50 eggs per gram of faeces).

3.3.2.3 Equipment

· Two beakers or plastic containers
· A tea strainer or cheesecloth
· Measuring cylinder or other container graded by volume
· Fork, tongue blades or other type of stirring rod
· Test tube (dry)
· Microscope
· Microslides, coverslips
· Balance or teaspoon
· Flotation fluid (see the Appendix to this handbook for formulation)

(a) Put approximately 3 g of faeces (weigh or measure the faeces with a precalibrated teaspoon) into Container 1.

(b) Pour 50 ml of flotation fluid into Container 1.

(c) Mix (stir) the contents thoroughly with a stirring device (tongue blade, fork).

(d) Pour the resultant faecal suspension through a tea strainer or a double-layer of cheesecloth into Container 2.

(e) Leave the container to stand for 10 minutes.

(f) Press a test tube to the bottom of the filtrate, lift it quickly and transfer a few drops adhering to the surface to a microslide.

(g) The test tube ought to touch the microslide for at least 2-4 seconds for the drops to run off.

(h) Mount the coverslip on the microslide for microscopical examination.

3.3.3 Sedimentation technique (for trematode eggs)


3.3.3.1 Principle
3.3.3.2 Application
3.3.3.3 Equipment
3.3.3.4 Procedure


3.3.3.1 Principle

The sedimentation technique is a qualitative method for detecting trematode eggs (Paramphistomum) in the faeces. Most trematode eggs are relatively large and heavy compared to nematode eggs. This technique concentrates them in a sediment.

3.3.3.2 Application

This is a procedure to assess the presence of trematode infections. It is generally run only when such infections are suspected (from previous postmortem findings on other animals in the herd/flock area), and is not run routinely. The procedure can be used to detect liver fluke (Fasciola) and Paramphistomum eggs.

3.3.3.3 Equipment

· Beakers or plastic containers
· A tea strainer or cheesecloth
· Measuring cylinder
· Stirring device (fork, tongue blade)
· Test tubes
· Test tube rack
· Methylene blue
· Microslide, coverslips
· Balance or teaspoon
· Microscope

3.3.3.4 Procedure

(a) Weigh or measure approximately 3 g of faeces into Container 1.

(b) Pour 40-50 ml of tap water into Container 1.

(c) Mix (stir) thoroughly with a stirring device (fork, tongue blade).

(d) Filter the faecal suspension through a tea strainer or double-layer of cheesecloth into Container 2.

(e) Pour the filtered material into a test tube.

(f) Allow to sediment for 5 minutes.

(g) Remove (pipette, decant) the supernatant very carefully.

(h) Resuspend the sediment in 5 ml of water.

(i) Allow to sediment for 5 minutes.

(j) Discard (pipette, decant) the supernatant very carefully.

(k) Stain the sediment by adding one drop of methylene blue.

(l) Transfer the sediment to a microslide. Cover with a coverslip.

3.3.4 Microscopical examination of prepared samples

The prepared samples on microslides from the simple test tube flotation method, the simple flotation method and the sedimentation method are examined under a microscope at the magnifications listed in Table 3.1.

Table 3.1 MAGNIFICATION LEVELS FOR EXAMINING PREPARED SAMPLES

Magnification

Parasites

10 x 10

Nematode and cestode eggs

10 x 40

Coccidia oocysts

10 x 4

Trematode eggs

WARNING: In case of a time delay between processing the sample and reading the count, egg numbers may decline dramatically. Also, eggs may change their appearance, becoming crenated and "ghost-like". It is therefore advisable to prepare only a few samples at a time. These changes can be prevented by keeping prepared samples in the refrigerator after mixing. Using the salt-sugar solution as flotation fluid also reduces the morphological changes.

3.4 Quantitative techniques for separating and concentrating eggs/larvae

The simplest and most effective method for determining the number of eggs or oocysts per gram of faeces is the McMaster counting technique described below.


3.4.1 McMaster counting technique


3.4.1 McMaster counting technique


3.4.1.1 Principle
3.4.1.2 Application
3.4.1.3 Equipment
3.4.1.4 Procedure
3.4.1.5 Guideline to the interpretation of faecal egg counts in young animals


3.4.1.1 Principle

The McMaster counting technique is a quantitative technique to determine the number of eggs present per gram of faeces (e.p.g.). A flotation fluid is used to separate eggs from faecal material in a counting chamber (McMaster) with two compartments. The technique described below will detect 50 or more e.p.g. of faeces.

3.4.1.2 Application

This technique can be used to provide a quantitative estimate of egg output for nematodes, cestodes and coccidia. Its use to quantify levels of infection is limited by the factors governing egg excretion.

3.4.1.3 Equipment

· Beakers or plastic containers
· Balance
· A tea strainer or cheesecloth
· Measuring cylinder
· Stirring device (fork, tongue depressor)
· Pasteur pipettes and (rubber) teats
· Flotation fluid (see the Appendix to this handbook for formulation)
· McMaster counting chamber*
· Microscope

* Suppliers are:

Philip Harris

Fisons Scientific Equipment

618 Western Avenue

Bishop Meadow Road

Park Royal

Loughborough

London W3 0TE

Leicestershire LE 11 ORS

England

England

Tel: 081-992-5555

Tel: 0509-231166

3.4.1.4 Procedure

(a) Weigh 4 g of faeces and place into Container 1.

(b) Add 56 ml of flotation fluid.

(c) Mix (stir) the contents thoroughly with a stirring device (fork, tongue blade).

(d) Filter the faecal suspension through a tea strainer or a double-layer of cheesecloth into Container 2.

(e) While stirring the filtrate in Container 2, take a sub-sample with a Pasteur pipette.

(f) Fill both sides of the McMaster counting chamber with the sub-sample.

(g) Allow the counting chamber to stand for 5 minutes (this is important)

(h) Examine the sub-sample of the filtrate under a microscope at 10 x 10 magnification.

(i) Count all eggs and coccidia oocytes within the engraved area of both chambers.

(j) The number of eggs per gram of faeces can be calculated as follows: Add the egg counts of the two chambers together.

Multiply the total by 50. This gives the e.p.g. of faeces. (Example: 12 eggs seen in chamber 1 and 15 eggs seen in chamber 2 = (12 + 15) x 50 = 1350 e.p.g.)

(k) In the event that the McMaster is negative (no eggs seen), the filtrate in Container 2 can be used for the simple flotation method (section 3.2.2), steps f, g and h.

WARNING: In case of a time delay between processing the sample and reading the count, egg numbers may decline dramatically. Also, eggs may change their appearance, becoming crenated and "ghost-like". It is therefore advisable to prepare only a few samples at a time. These changes can be prevented by keeping prepared samples in the refrigerator after mixing. Using the salt-sugar solution as flotation fluid also reduces the morphological changes.

3.4.1.5 Guideline to the interpretation of faecal egg counts in young animals

Table 3.2 below provides guidelines to aid in interpreting faecal egg counts in young animals.

Table 3.2 FAECAL EGG COUNTS IN YOUNG ANIMALS

Parasite

Degree of infection (eggs per gram of faeces)

Light

Moderate

Heavy

CATTLE


Mixed infection

50-200

200-800

800+


Pure Haemonchus infection

200

200-600

600+


Pure Trichostrongylus infection

50-100

100-400

400+


Pure Cooperia infection

200-300

300-2500

2500+

SHEEP


Mixed infection

50-800

800-1200

1200+


Mixed infection with Haemonchus absent

300-800

800-1000

1000+


Pure Haemonchus

100-2000

2000-7000

7000+


Pure Trichostrongylus

100-500

500-2000

2000+


Pure Nematodirus

50-100

100-600

600+


Pure Oesophagostomum

100-800

800-1600

1600+

If possible guidelines for the interpretation of faecal egg counts should be established for each area/country/region according to different climatic zones, as the composition and pathogenicity of parasite populations may differ from area to area.

3.5 Preparation of faecal cultures


3.5.1 Principle
3.5.2 Application
3.5.3 Equipment
3.5.4 Procedure


3.5.1 Principle

Many nematode eggs are alike and species such as Haemonchus, Mecistocirrus, Ostertagia, Trichstrongylus, Cooperia, Bunostomum, and Oesophagostomum cannot be clearly differentiated from the eggs in faecal samples. For these parasites, differentiation can be achieved by the use of faecal cultures. They provide a suitable environment for the hatching and development of helminth eggs into the infective stage (L3).

3.5.2 Application

The identification of parasite species present is an important component of initial surveys and of the investigation of clinical disease caused by gastrointestinal nematodes.

3.5.3 Equipment

· Fork, spoon, tongue depressor, spatula

· Water

· Jars, containers

· Charcoal (dried, sterile bovine faeces may be used if charcoal is not available. This is prepared as follows. Faeces should be sterilized to remove any helminth eggs present, completely dried by heating to 70 °C and ground to a fine powder.)

3.5.4 Procedure

(a) Break up collected faeces finely using a stirring device.

(b) Faeces should be moist and crumbly.

If faeces are too dry, add water.

If faeces are too wet, add charcoal (or sterile bovine faeces) until the correct consistency is obtained.

(c) Transfer the mixture to jars or other containers.

(d) Leave the culture at room temperature for 14-21 days, by which time all larvae should have reached the infective stage.

(e) If an incubator is available, the culture can be placed at 27 °C and left for 7 to 10 days.

(f) Add water to cultures regularly (every 1-2 days).

(g) Larvae are recovered using the Baermann technique (see section 3.6).

3.6 Isolation and identification of lungworm larvae and infective larvae harvested from faecal cultures (the Baermann technique)


3.6.1 Principle
3.6.2 Application
3.6.3 Equipment
3.6.4 Procedure
3.6.5 Identification of infective larvae


3.6.1 Principle

The Baermann technique is used to isolate lungworm larvae from faecal samples and infective larvae from faecal cultures. It is based on the active migration of larvae from faeces suspended in water and their subsequent collection and identification.

3.6.2 Application

This is a procedure for harvesting infective larvae for identification purposes.

3.6.3 Equipment

· Funnel (size according to need)
· Funnel stand
· Rubber or plastic tubing
· Rubber bands
· Clamp or spring clip
· Cheesecloth or screen
· Simple thin stick (about 15 cm long)
· Strainer
· Microscope
· Test tube
· Pasteur pipette
· Small petri dish(es)

Equipment

3.6.4 Procedure

(a) Fit a short piece of tubing which is closed at one end with a clamp or spring clip, to the stem of a funnel of appropriate size.

(b) Support the funnel by a stand.

(c) Weigh or measure about 5-10 g of faecal culture/faeces and place it on a piece of double-layer cheesecloth.

(d) Form the cheesecloth around the faeces as a "pouch".

(e) Close the pouch with a rubber band.

(f) Fix a supporting stick under the rubber band - Step 1

Fix a supporting stick under the rubber band - Step 2

(g) Place the pouch containing faecal culture material or faeces in the funnel. Trim the surplus cheesecloth off.

(h) Fill the funnel with lukewarm water, covering the faecal material.

(i) Leave the apparatus in place for 24 hours, during which time larvae actively move out of faeces and ultimately collect by gravitation in the stem of the funnel.

Examination for longhorns

(j) Draw a few ml of fluid from the stem of the funnel into a small petri dish.

(k) Examine under dissecting microscope for live lungworm larvae (L1).

(l) For positive samples a transfer of larvae to a microslide for identification at 10 x 10 magnification may be required. It is important to differentiate between Muellerius capillaris and other species as the treatment is different.

Examination for infective larvae from faecal cultures

(m) Draw 10-15 ml of fluid from the stem of the funnel into a test tube or other container.

(n) Leave the tube to stand for 30 minutes. Remove the supernatant with a Pasteur pipette.

(o) Transfer a small aliquot of the remaining fluid using a Pasteur pipette to a microslide, add a drop of iodine and cover with a coverslip.

(p) Examine under 10 x 10 magnification. (See Tables 3.3 and 3.4 below for larval identification).

(q) Repeat steps m and n until 100 larvae have been identified.

(r) The counts for each species provide an estimate of the composition (%) of the parasite population of the host.

3.6.5 Identification of infective larvae

Table 3.3 provides a key to the infective larvae of some common nematodes of cattle.

Table 3.3 KEY TO THE INFECTIVE LARVAE OF SOME COMMON NEMATODES OF CATTLE (after Keith [1953])

1.


Sheath absent, oesophagus more than 1/3 the length of the body.

Strongyloides

Sheath present, oesophagus short.

2

2.


Length, including sheath, less than 600 m.

Bunostomum

Length, including sheath, more than 600 m.

3

3.


Tail of sheath less than 200 m.

4

Tail of sheath more than 200 m.

5

4.


Two conspicuous oval bodies at anterior end of oesophagus.

6

No such structures at anterior end of oesophagus.

7

5.


Length including sheath more than 1000 m; tail of sheath dorsal and ventral lobes with a rod-like process between.

Nematodirus

Length including sheath less than 1000 m, tail of larva ending in a simple point.

Oesophagostomum radiatum

6.


Length, including sheath, usually more than 850 m; tail of sheath usually more than 150 m long, gradually to end bluntly.

Cooperia oncophera

Length, including sheath, usually less than 850 m; tail of sheath tapering rapidly to a point or short fine filament less than 150 m long.

Cooperia punctata
C. pectinata

7.


Tail of sheath short and conical, less than 110 m long.

Trichostrongylus axei

Tail of sheath at least 126 m long.

8

8.


Tail of sheath ending bluntly.

Ostertagia ostertagii

Tail of sheath ending in a fine whip-like filament.

Haemonchus contortus

Table 3.4 Provides a key to the infective nematode larvae of sheep and goats

Table 3.4 KEY TO INFECTIVE NEMATODE LARVAE OF SHEEP AND GOATS (Adapted from Dikmans and Andrews [1933])

Total length of larva (m)

Length, end of larva to end of sheath (m)

Species, with range of total length (m)

Other differential features

Short
500-700

No sheath
85-115

Strongyloides
570-700

Slender body with oesophagus, 1/3 to ½ total length of larvae.

Short
500-700

Long
85-115

Bunostomum
510-670

Wide body with sudden tapering to long thin tail. "Band" constriction on oesophagus.

Medium
650-900

Short
20-40

Trichostrongylus
620-910

Short straight larva, conical tail sheath. Tubercles on tail of larva. Intestinal cells usually prominent.

Medium
650-900

Short
20 40

Ostertagia
790-910

Long, conical, "finger like" tail sheath.

Medium
650-900

Short
20 40

Cooperia curtice
710-850

Oval bodies at anterior end of larva. Tail of larva rounded.

Medium
650-900

Medium
30-60

Haemonchus
650-750

Tail sheath is usually "kinked". Pointed tail of larva.

Medium
650-900

Medium
30-60

Cooperia oncophera
800-920

Oval bodies anterior end of larva. Tail of larva rounded.

Long
900-1200

Long
60-80

Chabertia
710-790

Stout body with 24 to 32 rectangular intestinal cells.

Long
900-1200

Long
60-80

Oesophagostomum
770-920

Usually longer than Chabertia. Has 16 to 24 triangular intestinal cells.

Long
900-1200

Extremely long
250-290

Nematodirus
922-1180

Tail of larva is forked.

3.7 Diagnostic techniques for filarial nematodes


3.7.1 Stephanofilaria
3.7.2 Onchocerca
3.7.3 Parafilaria
3.7.4 Setaria


Diagnosis of filarial infections in living animals is done by isolating and identifying the microfilaria of these parasites.

As several of them have distinct predilection sites that have evolved in accordance with the feeding patterns of the insects which serve as their intermediate hosts, the majority of these infections can usually be diagnosed on the basis of clinical signs, site of lesions and type of tissue from which the microfilaria are isolated.

3.7.1 Stephanofilaria

The microfilaria (L1 larvae) of this parasite are present in the skin lesions caused by the adult parasites. Skin scraping taken after the removal of the crust and exudate from the lesion may contain the microfilaria.

3.7.2 Onchocerca

Microfilaria of Onchocerca species are located in the subcutaneous tissue with the highest concentration found in the predilection sites. These sites vary according to the feeding habit of the intermediate hosts. For example, in Sudan the microfilaria are generally concentrated in the midline area of the hump and back of cattle; in Tanzania, microfilaria are found in the tissues of the neck and ears.

Infections can be diagnosed by demonstrating the microfilaria in a skin biopsy taken from the predilection site. The tissue sample is teased and placed in warm physiological saline solution for at least six hours. If present the larvae migrate out and can be found in the saline. If the sample is negative after six hours it should be left overnight to detect low levels of microfilaria in the skin.

3.7.3 Parafilaria

The microfilaria of Parafilaria are present in blood oozing from the lesions caused by the adult female penetrating the skin from the subcutaneous tissue. Blood from the "bleeding points" can be examined on a microslide. A drop of blood placed on the microslide should be covered by a cover slip and if present, microfilaria can be seen moving in the smear. Thin or thick blood smears taken from the bleeding points are also suitable for diagnosing Parafilaria.

3.7.4 Setaria

The microfilaria circulate in the blood and they can be detected by examining a blood sample as described for Parafilaria.

3.8 Identification and examination of snails

Aquatic lymnaid snails which act as intermediate hosts for Fasciola hepatica and Fasciola gigantic can be identified fairly easily as compared to other freshwater snails. The opening of the lymnaid snails is on the right when the snail is held with the spires pointing away from the viewer. When the apex of the spire is facing the viewer the spires turn clockwise.

For closer identification of the species and examination of the snails for larval stages of the flukes, samples should be sent to the nearest appropriate laboratory.


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