PCR as a diagnostic test method for deduction of H. somni on trans-tracheal aspirated bronchoalveolar fluid from clinically normal calves and calves with pneumonia

The aim of the present field study was to evaluate the diagnostic value of a species specific PCR test against Histophilus somni (earlier Haemophilus somnus). Trans-tracheal aspirated bronchoalveolar fluid (BAL) from 56 clinically normal healthy calves and 36 calves that developed pneumonia were collected in 6 different herds during September and November 2002. All 92 aspirations were analysed for Bovine Respiratory Syncytial virus (BRSV), Parainfluenza-3 virus, Bovine Coronavirus by antigen ELISA. Bacteria were detected by cultivation and H. somni additionally also by PCR. The results showed that 63% of the clinically normal calves harboured pathogenic bacteria (mainly P. multocida, M. haemolytica and H. somni) in the lower respiratory tract, and in 60% of these cases bacteria were found either in pure culture or as the predominant flora, something that could be expected to cause clinical disease. Among diseased calves, 97% contained bacteria in the lower respiratory tract, all of them classified as pure culture or many pathogenic bacteria in mixed culture. BRSV was detected in 53% of the diseased calves. A comparison of H. somni specific PCR test to cultivation showed PCR to be most sensitive test. No significant relation between pneumonia and detection of H. somni by PCR could be demonstrated, whereas a clear significant correlation between pneumonia and detection of H. somni by cultivation was found.