Inherent Limitations of Serological Tests

Having obtained the sample it is subjected to one or more serological tests. Although these tests vary widely in their mechanism all of them depend on the detection and sometimes quantification of circulating antibodies against the infectious agents. Most serological tests have the following limitations :

1. Birds remain serologically negative for 4-20 days after infection. This period may vary according to the test used and may be very much longer when latent (inactive) infection occurs.

2. The commonly used tests only evaluate circulating antibody, and thus take no account of mucosal antibody, cytotoxic T cells, and cell-mediated immunity.

3. Without repeated sampling it is impossible to tell whether the antibody level is rising, stable, or falling.

4. Antibodies to field strains of infectious agents are usually qualitatively very similar to those of vaccinal or naturally avirulent strains.

5. Antibodies to antigenically-related agents may cause confusion due to false-positive reactions.

We can add to this list the risk of false positive and false negative reactions which is inherent to any test system in\par clinical pathology. Careful and repeated applications of the simple serological techniques is capable of overcoming most if not all of the above limitations. However we cannot expect that serology will give us the right answers if we ask of it the wrong questions!

Solving Problems

Any routine monitoring program will sooner or later turn up flocks which have a very poor response in terms of circulating antibody post-vaccination. In considering what to do about these flocks it should be remembered that the response of the birds to vaccination may be modified by three main types of factor :

1. Factors related to the vaccine, such as antigenic concentration, contaminating agents, virulence of vaccine strain, nature and quality of adjuvants.

2. Factors relating to the manner in which the vaccine is applied, such as the skill of the vaccinators, number of vaccines applied on one occasion, the route of\par application and sometimes interfering factors such as poor water quality.

3. Factors relating to the physiology and the pathology of the immunity producing systems of the bird, such as early exposure to Infectious Bursal Disease, or Marek's disease virus, and probably stress and a number of other diseases and intoxications.