Blood Samples - When and Why
Examination of blood samples taken from poultry flocks can provide a great deal of information about the health status of the birds. When birds are exposed to viral or bacterial infections they produce antibodies against these infectious agents. The same thing happens when birds are given vaccinations against these infections. These antibodies can be measured in blood samples. We may take blood samples for various reasons:
To assess whether birds have responded to vaccination by measuring antibody titres in the blood. The blood sample results in this case can tell us not only that the birds have responded to vaccination but how well they have responded and how even the response is, is as it is important that all birds in the flock have a similar level of protection.
Blood samples can be taken to look for changes in antibody level which might indicate a disease challenge. This might be a change from a negative to a positive result, for example, in the case of Mycoplasma or Salmonella infection or an increase in existing antibody levels, for example, to IB which might suggest recent exposure to this virus.
Blood samples are routinely taken from poultry breeding stock to ensure that antibody titres in the parent bird are sufficient to protect the chick in the early days of life, for example, against Gumboro disease.
Blood sample results are only a "snapshot in time" of the antibody status of the bird on a particular day. In order to gain the maximum information from blood sampling, it is always useful to examine a series of samples taken over a period of time to look at changes in antibody levels or changes from a negative result to a positive result which could indicate infection. If you have only one blood sample with high antibody titres and no previous sample results to compare with, it is difficult to judge when birds might have been exposed to infection as, following exposure to some diseases, birds may have detectable antibodies for the rest of their lives. If, however, you have a series of blood samples, changes in antibody titres or changes from a positive to a negative result between one set of samples and the next will enable you to pinpoint when a disease problem may have occurred and perhaps link this to problems seen on farm.
When should we be sampling our pullets?
Pullet rearers will take blood samples several times during the rearing period to assess response to live vaccination. Further samples can be taken on the laying site to assess antibody levels, for example, against Avian Pneumovirus (TRT), IB and Newcastle disease. Blood samples can also be taken at several points during the laying flock's life (for example every 10 weeks) and serum can be stored frozen for retrospective examination if a problem develops. This reduces the costs associated with blood sampling as if the flock shows no disease problem, then the only cost incurred is the minimal cost of storing the sera for the lifetime of the flock. Whole blood samples cannot be satisfactorily frozen. It is important to sample a reasonable number of birds from the flock to gain a meaningful insight into what is happening. A minimum of 20 bloods per flock is preferable.
Serological examination can be a very useful diagnostic tool to try and pinpoint disease problems and to monitor flocks to determine response to vaccine. Identifying a disease problem in one flock will enable you to plan a suitable vaccination programme for any subsequent flock. Obviously to obtain the best results from samples taken, blood samples should be taken correctly. This will be covered in the next issue.
