The most popular serological test method is ELISA (Enzyme-linked Immunosorbent Assay), which is used to demonstrate antibodies against micro-organisms (e.g. bacteria, viruses and parasites). ELISAs are based on the principle that the antibodies in question, or parts thereof, attach to a carrier (indirect ELISAs) or compete with a specific labelled antibody (blocking ELISAs), with attached antibodies then being made visible by staining. GD Animal Health carries out millions of ELISA tests every year. To do this, GD Animal Health uses ELISA robots, though ELISAs are also carried out by hand, with a capacity of 15,000 tests per day. In addition to ELISA, the GD Animal Health laboratory carries out other serological tests, such as various agglutination tests (HI tests), agar-gel immunodiffusion tests and serum neutralisation tests. Several commercially available ELISAs are used, but when kits are not available, GD Animal Health develops in-house ELISAs for routine testing.
Haemagglutination Inhibition tests
Haemagglutination Inhibition tests (HI tests) are used for viruses that can cause haemagglutination: the virus forms complexes with erythrocytes, resulting in a characteristic clotting. If specific antibodies against these viruses are present in the test sample, they attach to the virus and haemagglutination is prevented or inhibited.
Complement Fixation test
In case of the Complement Fixation test, haemolysis of erythrocytes is inhibited when specific antibodies in the test sample form complexes with antigen and complement in the test system. In the absence of specific antibodies, the complement remains in free solution, then binds to the erythrocytes after which lysis of the erythrocytes occurs. CFT is used for detecting antibodies against Brucella and M. paratuberculosis, for example.
For Serum Neutralisation (SN) we use a several test systems with various cells. Specific virus antibodies in the blood test sample are first given time to react with the virus in the test system, after which cells are added with the result that the virus is neutralised or is no longer capable of infecting cells. In the absence of specific antibodies, infection occurs. The effect of such an infection is visible under the light microscope because infected cells are destroyed (cytopathogenic effect), or infection is made visible by IPMA.