Investigating bronchoalveolar lavage fluid cytology in relation to pathogen identification and consolidation depth in calves

BACKGROUND Small volume, non-bronchoscopic bronchoalveolar lavage (nBAL) is used for airway sampling in calves with respiratory tract infection (RTI). However, the usefulness of cytological analysis of this fluid to indicate bacterial RTI is unclear. Disease severity may influence these findings.

HYPOTHESIS/OBJECTIVES Investigate the associations among pathogen groups (none, viral, opportunistic bacterial infection [OB] with or without viral, Mycoplasmopsis bovis with or without OB or viral) and cytological findings, and the association between consolidation depth and cytological findings.

ANIMALS Eighty-seven calves showing at least 1 clinical sign of RTI from herds experiencing an outbreak of respiratory disease.

METHODS Cross-sectional study. Physical examination, thoracic ultrasonography, nBAL, bacteriology, nanopore sequencing, and cytology were performed. Bayesian hierarchical models were used to assess associations.

RESULTS The probability of belonging to a certain pathogen group was influenced by neutrophil percentage. Viral (median, 75.9%; interquartile range [IQR], 24.0) and M. bovis (median, 72.7%; IQR, 21.2) groups showed consistently high neutrophil percentages, whereas variability was higher in the OB group (median, 68.3%; IQR, 31.7%). Intracellular bacteria (≥1% of white blood cells, odds ratio [OR], 12.5; 95% credible interval [CrI], 1.46-142) increased the odds of OB isolation. The probability of different consolidation depths was affected by neutrophil percentage, with deeper lesions showing higher percentages (> 3 cm vs. < 1 cm: OR, 1.06; 95% CrI, 1.01-1.12), and by the presence of mast cells.

CONCLUSIONS AND CLINICAL IMPORTANCE The neutrophil percentage alone is likely insufficient to differentiate bacterial from viral infections. Intracellular bacteria might be useful to indicate OB. Evaluation of an optimal threshold could further evaluate this possibility.