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The diagnostic approach of bovine respiratory disease (BRD) in cattle, due to the recent developments in the field of molecular biology, has become more and more sophisticated. The clinical examination of the diseased animals, however, should not be omitted as it can deliver important information needed for the selection of suitable sampling procedures in order to come to an etiological diagnosis of BRD. Although, clinical findings in the respiratory tract are not always too evident in the initial stage of BRD, the clinical examination can deliver valuable information on the involvement of either the upper or the lower respiratory tract or both. In addition, auscultation and percussion deliver information on the extent of pneumonic and emphysematous alterations being present in the lung. Among a great number of in part highly sophisticated methods such as blood gas analysis, ultrasonographic examination and lung function testing, the respiratory rates and the kind and intensity of dyspnoea occurring in the course of the disease have been shown to give the most reliable results with respect to the outcome of BRD.
A number of sampling procedures are available for the collection of materials from the respiratory tract and blood that are used for virological, bacteriological or mycological examination in specialized laboratories. These comprise: obtaining serum samples to test for seropositivity and ? in case of paired samples ? for detection of seroconversion, nasal swabs, tracheal swabs, transtracheal washes (TTW), bronchoalveolar lavages (BAL), brush biopsies, lung biopsies as well as necropsy. The success of the latter procedures expressed in the likelihood to recover agents involved in BRD depends on the stage of disease, the suitability of animals selected as indicators of the herd problem, the choise of the material, the proper sampling procedure itself, the package and transport of the samples. Deficits with respect to sampling and posting have been shown to deliver disappointment results. Consequently, in some farmers? opinion the sampling of material from diseased calves does not deliver any useful information and for that reason it is considered as useless. Before starting the sampling procedure in BRD a clear idea should exist about the results to be expected and the subsequent consequences drawn from these. The sampling procedure should take place in accordance with the guidelines of the laboratory that will receive the samples. Most likely viruses involved in BRD can be recovered from materials obtained at the initial phase of disease. In later stages of disease obtaining paired serum samples should be preferred. Besides the time point of sampling with respect to the first occurrence of disease symptoms, the kind of sampling method selected for bacteriological examination will influence the result. Some of the agents involved in BRD are normal inhabitants of the mucous membranes of the nose, but colonize the lower respiratory tract only in the course of BRD, when the first line defence mechanisms of the upper respiratory tract have been damaged by virus infection. However, sampling materials from the lower respiratory tract by TTW, BAL, brush biopsies or lung biopsy does not have to mean that the material obtained represents the situation in the parts of the lung affected by pneumonia. The cranial lobes of the lung, where pneumonic lesion often are observed, are inaccessible for sampling by BAL in vivo. The catheter for obtaining BAL enters the main bronchi (preferentially the right one) and subsequently passes into the caudal lobe of the lung, where flushing takes place. In a later stage of BRD, the disease process is driven more by dysfunction of the respiratory mechanism than by original agents. Due to the deficient first line defence and the increased velocity of the inspired air caused by stenosis of the airways bacteria from the environment of the animal reach the lower airways. To this end , the bacterial examination of BAL and TTW obtained at different results. However, the isolation of Arconabacterium pyogenes from TTW-fluid indicates abscess formation in the lung and due this finding a poor prognosis.