10. Haemagglutination test

Introduction

All strains of Newcastle disease virus will agglutinate chicken red blood cells. This is the result of the haemagglutinin part of the haemagglutinin/neuraminidase viral protein binding to receptors on the membrane of red blood cells. The linking together of the red blood cells by the viral particles results in clumping. This clumping is known as haemagglutination.

Haemagglutination is visible macroscopically and is the basis of haemagglutination tests to detect the presence of viral particles. The test does not discriminate between viral particles that are infectious and particles that are degraded and no longer able to infect cells. Both can cause the agglutination of red blood cells.

Note that some other viruses and some bacteria will also agglutinate chicken red blood cells. To demonstrate that the haemagglutinating agent is Newcastle disease virus, it is necessary to use a specific Newcastle disease virus antiserum to inhibit the haemagglutinating activity.

Substances that agglutinate red blood cells are referred to as haemagglutinins.

Note:

The abbreviation HA is used in this manual for haemagglutinin and haemagglutination.

Two tests are described, the rapid test which takes one minute and the micro test which takes 45 minutes.

Figure 19: Principle of the haemagglutination test

Figure 19. - A. Negative control well (no haemagglutinin)

Figure 19 - B. Positive control well (contains haemagglutinin)

Red blood cell control in the haemagglutination test

Every time a haemagglutination test is carried out, it is necessary to test the settling pattern of the suspension of red blood cells. This involves mixing diluent with red blood cells and allowing the cells to settle.

1. Dispense diluent.

2. Add red blood cells and mix by gently shaking.

3. Allow the red blood cells to settle and observe the pattern.

4. Observe if the cells have a normal settling pattern and there is no auto-agglutination. This will be a distinct button of cells in the micro test and an even suspension with no signs of clumping in the rapid test.

Note:

The diluent used for haemagglutination tests in this manual is PBS.

There should be no signs of haemolysis in the red blood cell suspension. If there are signs of haemolysis, a fresh suspension must be prepared.

There should not be any sign of auto-agglutination in the red blood cell control. If an agglutination pattern is observed, discard the suspension of red blood cells. Prepare a fresh suspension and test again.

Control allantoic fluid samples

Negative and positive control samples are tested in both the rapid and micro haemagglutination tests to ensure the validity of the test.

Negative control allantoic fluid is harvested from 14-day old embryonated eggs that have not been inoculated with Newcastle disease virus. It should always test negative for the presence of haemagglutinins. There should not be any sign of haemagglutination.

Positive control allantoic fluid is known to contain a high infectivity titre of Newcastle disease virus. It should always test positive for the presence of haemagglutinins. Haemagglutination should be visible.

Rapid haemagglutination test

This test can determine the presence of a haemagglutinating agent in one minute. If testing many samples at the same time, it is necessary to test the negative and positive control samples only once.

Materials

Clean glass microscope slide or a clean white ceramic tile.
10 percent suspension of washed chicken red blood cells. See Section 8.
Micropipette and tips, glass Pasteur pipette or a wire loop.
PBS.
Negative and positive control allantoic fluid samples.
Sample to be tested for the presence of Newcastle disease virus, for example allantoic fluid.

Method

1. Place 4 separate drops of 10 percent chicken red blood cells onto a glass slide or a white tile.

2. To each drop of blood, add one drop of the control and test samples as follows. Use separate tips, pipettes or a flamed loop to dispense each sample.

Drop 1 PBS
Drop 2 Negative control allantoic fluid (no haemagglutinin)
Drop 3 Positive control allantoic fluid (contains haemagglutinin)
Drop 4 Unknown sample to be tested
3. Mix by rotating the slide or tile for one minute.

4. Observe and record results. Compare results of the test samples with the control samples.

Results

Agglutinated red blood cells in suspension have a clumped appearance distinct from non-agglutinated red blood cells.

The red blood cells mixed with the positive control allantoic fluid will clump within one minute.

The red blood cells mixed with the PBS and negative control allantoic fluid remain as an even suspension and do not clump.

Judge the results of the test sample by comparison with the positive and negative controls.

The PBS and negative allantoic fluid controls are used to detect clumping of the red blood cells in the absence of virus. This is unlikely to occur. If it does occur, the test is invalid.

Figure 20: Rapid haemagglutination test

Micro haemagglutination test in a V-bottom microwell plate

This method is convenient when testing allantoic fluid from a large number of embryonated eggs for the presence or absence of haemagglutinin. A 1 percent solution of red blood cells is used. The cells settle faster in V-bottom plates and there is a better contrast between positive and negative results than observed in U-bottom plates. The method for preparing eggs for and harvesting of allantoic fluid for this test is described in detail in Section 9.

Materials

Inoculated eggs, chilled for at least 2 hours, preferably overnight
Negative and positive control samples
V-bottom microwell plate and lid
Micropipette and tips to measure 50 µL
1 percent suspension of red blood cells
70 percent alcohol solution
Cotton wool
Forceps and/or small scissors
Absolute alcohol
Discard tray
Microwell plate recording sheet. See Appendix 9

Method

1. Fill in the details of samples being tested on a recording sheet. Samples and controls will be distributed into the wells as indicated on this sheet.

2. Use a micropipette to remove 50 mL of allantoic fluid from each egg and dispense into a well of the microwell plate. Use a separate tip for each sample.

3. Include negative and positive control allantoic fluid samples on one of the plates.

4. Dispense 50 mL of PBS into two wells. These wells will be the red blood cells controls for auto-agglutination.

5. Add 25 mL of 1 percent red blood cells to each well.

6. Gently tap sides of the plate to mix. Place a cover on the plate.

7. Allow the plate to stand for 45 minutes at room temperature.

8. Observe and record the results.

Results

The settling patterns of single and agglutinated red blood cells are different. Single cells roll down the sides of the V-bottom well and settle as a sharp button. Agglutinated cells do not roll down the sides of the well to form a button. Instead they settle as a diffuse film.

Negative HA result = a sharp button

Positive HA result = a diffuse film

Red blood cell control = a sharp button

Mark the HA results on microwell recording sheet.

See Appendix 10 for an example completed microwell plate recording sheet.

Table 2: Summary of Haemagglutination tests

Tests	Result	Interpretation
Rapid HA, Micro HA	Positive	Presence of viral particles that may or may not be infectious.
Rapid HA, Micro HA	Negative	Absence of viral particles or presence of viral particles in levels too low to detect.

Titration to establish haemagglutinin (HA) titre of a suspension of virus

The haemagglutination test is used to quantify the amount of Newcastle disease virus in a suspension. This is done by carrying out two-fold serial dilutions of the viral suspension in a microwell plate and then testing to determine an end point. This result can then be used to determine the amount of haemagglutinin in the suspension and is expressed as a HA titre.

Materials

96 well V-bottom microwell plate and cover
25 mL single and multi-channel micropipettes and tips
PBS
1 percent chicken red blood cells
Sample to be titrated
Reagent troughs
Microwell plate recording sheet. See Appendix 9.

Method

1. Record on recording sheets how samples will be dispensed in microwell plate.

2. Dispense 25 µL of PBS into each well of the microwell plate.

3. Place 25 µL of test samples in first well of each row of column 1. Samples can be tested in duplicate or triplicate if necessary.

4. Use a multichannel pipette to carry out two-fold serial dilutions across the plate until Column 11. See Appendix 4 for instructions on carrying out two-fold serial dilutions.

5. Add 25 µL of PBS to each well.

6. Add 25 µL of 1 percent red blood cells to each well including Column 12. The wells in this column are control wells that contain only PBS and red blood cells.

7. Gently tap sides of the plate to mix. Place a cover on the plate.

8. Allow the plate to stand for 45 minutes at room temperature.

9. Read and record the results in each well. All the control wells should be HA negative.

10. HA negative: A sharp button of red blood cells at the bottom of the V-bottom well.

11. HA positive: A hazy film of red blood cells, no button or a very a small button of red blood cells at the bottom of the V-bottom well.

12. Identify the end point. This will be the last well to show complete haemagglutination and contains one haemagglutinating unit.

Figure 21: Titration to determine HA titre of allantoic fluid sample

Definition of one HA unit

One HA unit in the haemagglutinin titration is the minimum amount of virus that will cause complete agglutination of the red blood cells. The last well that shows complete agglutination is the well that contains one HA unit.

Calculation of the HA titre of the test sample

The HA titre is the reciprocal of the dilution that produces one HA unit.

Example of HA titration shown in Figure 21.

A 1 in 64 (1/64) dilution contains 1 HA unit.

The HA titre of the test sample is therefore the reciprocal of 1/64 = 64 = 2⁶

The titre of the suspension of Newcastle disease virus can be expressed as 64 or 2⁶ HA units in 25 mL.