BALf cytology and (quantitative) bacteriology in calves with respiratory infection

Respiratory infections are the main indication for antimicrobial use in calves. A shift from antimicrobial group treatments to individual treatment is needed to rationalize and reduce antimicrobial use. Using on-farm thoracic ultrasonography (TUS) and a clinical score card allows definition of four categories: healthy animals, upper respiratory tract infections (URTIs), subclinical and clinical pneumonia. Whether this practical categorisation coincides with (quantitative) bacterial culture results and cytological findings in broncho-alveolar lavage fluid (BALf) is currently unknown. Therefore, we compared BALf cytological and (quantitative) culture results between healthy calves, URTIs, subclinical and clinical pneumonia as diagnosed by clinical and TUS findings. Materials and methods: Case-control study, 305 indoor group-housed calves from 62 farms without recent epidemic respiratory disease. Clinical examination and TUS were performed. Cytological and bacteriological analysis on non-endoscopic BALf was carried out. Results: Bacterial isolation rates and quantitative BALf culture results did not differ between the diagnostic groups. In animals with clinical pneumonia, a positive culture result and presence of neutrophil phagocytosis a significantly higher BALf neutrophil percentage (59.0%) was observed compared to healthy calves (37.7%; P = 0.03) and URTIs (35.4%; P = 0.015). Inversely, lymphocyte percentage was lower in these calves (1.8% vs. 5.3% in healthy calves, P = 0.003). Within the clinical pneumonia group, neutrophil percentage was significantly higher in the phagocytosis group (59.0%) compared to the no-phagocytosis group (36.5%; P = 0.021). Conclusions: Care should be taken not to overinterpret isolation of opportunistic pathogens in calves with various respiratory signs and ultrasonographic lesions. Phagocytosis by neutrophils in BALf potentially is an interesting diagnostic criterion for active respiratory bacterial infection in calves