Section 1 - Background Information

INTRODUCTION

Serological monitoring is sometimes regarded as the principal means of evaluating infectious disease in poultry. Clearly we are going to concentrate on the benefits of serology in this dissertation. However, it seems appropriate first of all to place serology in the context of the other things which may be going on. There are at least four main types of approach which can help us evaluate infectious disease in poultry. These are ;

1. Recording and analysis of productivity data (especially mortality and culling, including age patterns, and egg production data).

2. Recording and analysis of disease data (clinical signs, post-mortem results).

3. Detection and measurement of serological response.

4. Isolation, identification (and sometimes quantification) of pathogenic infectious agents.

These groups are listed in order of increasing specificity but also, unfortunately, of increasing effort and cost. Often it may be activities within groups 1 and 2 which warn of a problem, but it will be groups 3 and 4 which, hopefully, tell what the problem is. However, if we are to obtain full benefit from the time and money invested in testing for disease it is important to understand clearly what the results obtained mean. This, in turn, depends on the theoretical basis of the test system (what it measures, and what it does not measure) and the frequency of its application. A serological response, for example, may confirm that the birds have been exposed to the test organism, or a closely related one, but not that it is the cause of a particular problem. Most test systems will only provide reliable information if applied regularly. This will usually mean that they are applied fairly intensively during the period of introduction of the disease control program, and are thereafter relaxed to provide a periodic check on the results. This is perfectly acceptable provided that the systems used are sufficiently flexible to allow activation at short notice. Such activation may, for instance, be needed to deal quickly with suspected break-downs of the control programme.

There is a frequent misconception that the more expensive the method used to evaluate infectious disease the "better" it must be. In fact the opposite is usually true. This is because the more expensive the laboratory test the fewer samples are usually tested. All test systems have their weaknesses. Whatever the scientific weaknesses of a given test system, the results obtained will almost always have a beneficial psychological side-effect - the maintenance of staff interest in disease control.

Most of the techniques which we use to identify the presence of a specific antibody in blood or secretions with a known antigen can also be used to detect antigen. If an antibody solution of known potency and specificity is available we simply use the test in reverse. This dissertation will, however, deal only with the uses of serological techniques for the detection and quantification of antibody.