CLINICAL INTERPRETATION

The emphasis in this dissertation is on serology but it must not be forgotten that the serological results form only one part of the mosaic of information available to the poultry clinician. Serology will be most powerful and accurate when used in association with other sources of information such as:

1. Productivity data - levels and patterns of mortality and culling, egg production, egg quality, hatchability, growth rate and uniformity as measured by weighing birds, feed conversion ratios etc.

2. Condemnations data generated at slaughter

3. Clinical examination of affected flocks

4. Post-mortem examinations, backed up by appropriate microbiological, histological and parasitological tests.

5. Routine micro-biological testing - farms after clean-out and disinfection, feed and raw-materials, cull chicks etc.

The above list is not exhaustive. In diagnosing a disease problem and planning and implementing a control strategy it will often be necessary to achieve the cooperation of many different people. The key to cost-efficient use of serology lies in the planning of sample collection and choice of assays appropriate to each specific problem or objective. The samples made available and the non-serological information will often have a considerable bearing on the interpretation of the serological results.

There are 3 main strategies of serological sampling :

A. Routine random sampling at periodic intervals with a view to detecting response to infection. The tests used will normally be qualitative, resulting in negative, suspect and positive results. Difficulties in interpretation in this case tend to be related to false-positive reactions which may be related to technical problems with the test materials or cross reactions with non-target organisms.

B. Sampling at a standard interval after vaccination in order to quantify the response.

Unfortunately there is not always a direct relationship between the titre of circulating antibody and how birds will behave when subject to challenge. In view of the complex nature of the immune response it would be surprising if such a rigid relationship did exist. Birds with quite low titres of circulating antibody may be quite well protected by local immunity, and, on the other hand, birds with high titres may, in the absence of clinical signs, excrete large quantities of virulent virus, due to a lack of local immunity.

Routine monitoring of serum antibody response to vaccination is most appropriate for comparison between flocks immunized with the same type and route of vaccine application. Rather than relying on published data to interpret the results obtained, it is recommended that each organization establish a normal base-line curve for response to vaccination using its laboratory and normal flocks immunized with the customary programme. This entails intensive initial testing to determine the absolute values for antibody titre post-vaccination, and the degree of variability obtainable within and between flocks. It is , however, sometimes necessary to interpret the results of small groups of samples in relative isolation. In this case it is best to compare the titres with those obtained in the same laboratory with samples obtained from other flocks with a similar vaccination program.

C. Targeted sampling in order to investigate a specific disease incident. It is preferable that the results of two paired samples of sera are available to confirm a rise in antibody titre coincident with the suspected disease or exposure to infection. The first set of samples should be taken, ideally, before the first signs of disease or at least within a few days of the beginning of the problem. Normally the second, or convalescent, set would be taken 14 days later.

In the case of Avian Influenza it is recommended that a third sample be taken 28 days later because in some diseases, (such as some Avian Influenza infections) antibody titres do not rise until after 21 days (Easterday and Bela-Tumova, 1972). What is considered a significant rise in titre in a particular circumstance will depend on previous experience of both disease problems and fluctuations of antibody response unrelated to clinical disease. In HI tests, for instance, a rise in titre of 2 logs is often used as a rule of thumb.

Examples of clinical interpretation of serological results are given in Section 3 of this dissertation and in the accompanying case-histories