Diagnosis of disease can be accomplished by the combined evaluation of flock history, clinical signs, serology and isolation and identification of the pathogen. Serological monitoring can be particularly helpful in establishing the timing and nature of a field infection. For unvaccinated flocks, the presence of IBV field challenge can simply be demonstrated by detecting positive IBV serology. However, for IBV vaccinated flocks, identification of a field challenge is more complicated and will requirethe knowledge of expected vaccination titers (baselines). Every user of an Elisa system should develop his or her own vaccination baselines depending on placement programmes, specific vaccines used and type of birds examined. These factors, in combination with periodic flock profiling, can establish a serological history in order to determine if serological results are normal or abnormal. Unexpected rising titers, significantly higher than the expected vaccination titers, may then indicate the presenceof a field challenge. It should be stressed, however, that titers by themselves cannot be used to establish diagnosis. Diagnosis can only be established when one combines serology with clinical symptoms and isolation of the pathogen. Until true diagnosis is established, it is recommended to refer to titers as "suspect of infection" rather then "infection titers".
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