Rapid tests for the detection of Newcastle disease virus in vaccinated birds and the detection of antibodies in unvaccinated birds are outlined below:
Several rapid test that directly detect Newcastle-disease antigens have been employed in the diagnosis of infections in vaccinated birds. Those most commonly used to date are fluorescent antibody tests on longitudinal sections of the trachea and peroxidase antibody tests on the brain. There seems no reason to doubt that other direct antigen detection tests could be applied to Newcastle-disease virus infections.
The drawback to such tests is that it is impracticable to examine all the potential sites of replication of Newcastle-disease virus in the vaccinated birds. So that, for example, absence of evidence of virus in the trachea does not preclude virus replication in the gut. No direct detection method is recommended for routine use in the diagnosis of Newcastle disease, although in specific circumstances such tests may have a useful role.
The majority of laboratories involved in Newcastle disease diagnosis are familiar with the haemagglutination inhibition test and the recommendation described below relate to this test for the measurement of antibodies to the virus. However, enzyme-linked immunosorbent assays (Elisa) may be successfully used to detect antibodies to the virus. It is suggested that if there is a wish to employ an Elisa test at regional laboratory level the test should be monitored by the national laboratory referred to in Annex II.
Blood samples should be taken from all birds if the flock size is less than 20 and from 20 birds from larger flocks (this will give a 99 % probability of detecting at least one positive serum if 25 % or more of the flock is positive, regardless of flock size). The blood should be allowed to clot and serum removed for testing.
Individual serum samples should be tested for their ability to inhibit Newcastle disease virus haemagglutinating antigen in standard haemagglutination inhibition tests as defined in Chapter 6.
There is some debate as to whether 4 or 8 haemagglutinin units should be used for the HI test. It would appear that either is valid and the choice should be left to the discretion of the national laboratories. However, the antigen used will affect the level at which a serum is considered positive: for 4 HAU a positive serum is any showing a titre of 24 or greater and for 8 HAU a positive serum is any showing a titre of 23 or greater.