SELECTION OF ANIMALS

To evaluate the prevalence of a disease or to assess the presence or absence of a disease in an animal population, a survey protocol should be prepared in association with an epidemiologist to insure that the sampling frame will allow the detection of the disease or its prevalence according to statistical models and epidemiology of the disease. 

When an exotic disease is suspected the purpose of selecting animals for specimen submission is to confirm the cause of the disease. For this purpose sick animals are selected in order to maximise the chance of finding the disease. Preferably, many animals should be sampled instead of only one. In practice 5 to10 animals should be selected at different stage s of the disease. If possible animals should be sacrificed to collect fresh post-mortem samples as carcases quickly decompose in warm climates and this is most applicable for small animal production such as poultry.

To chose animals, the following should be considered:

• Animals with characteristic signs of the disease are more likely to permit diagnosis (!),
• Early stages of the disease are usually associated with viremia or bacteremia peaks and are more suitable for pathogen identification,
• Late stages of the disease are usually associated with higher antibody levels and are more suitable for serological diagnosis,
• Very young animals may cause a problem because of maternal antibodies persistence.
  
  It is always best to ask advice animal selection from the laboratory that will perform the analysis when this is possible .