Systematic Veterinary Virology

INTRODUCTION

• Many viruses attach to molecules present on the surface of RBCs. A consequence of this is that at certain concentrations, a viral suspension may bind together (agglutinate) the RBCs, thus preventing them from settling out of suspension. By serially diluting a virus suspension into an assay tray (a series of wells of uniform volume) and adding a standard amount of blood cells, an estimation of the number of virus particles can be made. It is cheaper and quicker (taking just 30-40 minutes). This assay may be modified to include the addition of an antiserum. By using a standard amount of virus, a standard amount of blood cells, and serially diluting the antiserum, one can identify the minimum inhibitory concentration of the antiserum (the greatest dilution which inhibits hemagglutination).The HI test is simple to perform and requires inexpensive equipment and reagents. The HI test may be complicated by the presence of non-specific inhibitors of viral haemagglutination and naturally occurring agglutinins of the erythrocytes.
• The advantages of HI tests are that they are relatively easy and inexpensive to perform. The disadvantages are that HI tests are not as sensitive as ELISAs or RIAs, the actual reading of results is subjective and the reagents should be fresh or else abnormal agglutination patterns may arise which makes the reading and interpretation of the test very difficult. As a result the HI test is being replaced by more sensitive and reliable ELISA and RIA tests in many virus diagnostic laboratory