Airway Sampling and Microbiologic Diagnosis in Canine and Feline Bacterial Pneumonia

Introduction 

While imaging and laboratory findings may strongly suggest bacterial pneumonia, definitive diagnosis requires identification of infectious organisms within the lower respiratory tract. Empirical antimicrobial therapy without sampling may be appropriate in unstable patients, but failure to obtain diagnostic specimens can result in inappropriate treatment, relapse or missed underlying disease. 

Airway sampling, cytology and culture remain the gold standard for confirming bacterial pneumonia and guiding targeted antimicrobial therapy. This article reviews practical sampling techniques and interpretation strategies for small animal clinicians. 

1. Why Airway Sampling Matters 

Definitive diagnosis requires^{1, 2}: 

• Cytologic evidence of inflammation 

• Identification of septic inflammation 

• Microbiologic culture 

Optimal submission includes: 

• Cytology 

• Aerobic culture 

• Anaerobic culture 

• Mycoplasma culture 

Airway cytology in bacterial pneumonia typically reveals neutrophilic inflammation. Sepsis is observed in approximately 75% of dogs with bronchopneumonia¹. 

However, 25% of dogs with culture-confirmed infection may lack visible intracellular bacteria on cytology¹. 

Clinical implication: 
Culture should be submitted even when intracellular bacteria are not observed. 

2. Fine Needle Lung Aspiration 

Indicated for¹: 

• Lobar consolidation 

• Nodular radiographic lesions 

Advantages: 

• Minimal equipment 

• Suitable for focal disease 

Limitations: 

• Risk of pneumothorax 

• Typically limited cytologic yield 

Ultrasound guidance and small-gauge needles minimise complications. 

3. Tracheal Wash Techniques 

Endotracheal Wash (ETW) 

Suitable for¹: 

• Small to medium dogs (<15 kg) 

• Cats 

• Puppies and kittens 

Requires brief intravenous anaesthesia for intubation. 

Transtracheal Wash (TTW) 

More appropriate for: 

• Larger dogs (>15 kg) 

• Cases requiring greater cellular yield 

This technique is more invasive but often provides improved sample quality³. 

Only 1–2 mL of fluid is required for cytology and culture³. Turbid, mucus-laden fluid improves diagnostic adequacy. 

4. Bronchoscopy and Bronchoalveolar Lavage (BAL) 

Bronchoscopy allows: 

• Segment-specific sampling 

• Visual assessment of airway pathology 

• Removal of foreign bodies 

It requires specialised equipment and general anaesthesia¹. 

In cats, non-bronchoscopic “blind” BAL techniques have been described¹. 

BAL is particularly valuable in: 

• Recurrent pneumonia 

• Suspected foreign body disease 

• Complex or non-responsive cases 

5. Interpreting Cytology and Culture 

Typical cytologic findings include: 

• Neutrophilic inflammation 

• Degenerate neutrophils 

• Intracellular bacteria 

Interpretation of culture must consider: 

• Cytologic findings 

• Potential oropharyngeal contamination 

• Presence of squamous cells 

• Mixed light growth of upper airway flora 

Importantly: 

• PCR of oropharyngeal swabs rarely provides clinically relevant information 

• Blood culture does not adequately substitute for airway lavage cytology and culture⁴ 

Molecular techniques and sequencing are expanding understanding of lower airway microbiology but are not yet standard in clinical practice. 

6. Recurrent or Antibiotic-Responsive Pneumonia 

In young animals with recurrent, antibiotic-responsive pneumonia, primary ciliary dyskinesia should be considered^1,5. 

Practical screening in intact males includes sperm motility evaluation, as dynein arm defects affect all cilia in the body. Electron microscopy of respiratory epithelium may confirm diagnosis¹. 

Conclusion 

Airway sampling remains the cornerstone of definitive diagnosis in bacterial pneumonia. While empirical therapy may be necessary in unstable patients, failure to obtain diagnostic specimens increases the risk of inappropriate antimicrobial use, treatment failure and recurrence. 

Cytology and culture must be interpreted together, and absence of visible intracellular bacteria does not exclude infection. In recurrent or non-responsive cases, advanced sampling techniques such as bronchoscopy and BAL are strongly indicated. 

Ultimately, microbiologic confirmation not only guides antimicrobial selection but also provides critical information about disease chronicity, severity and underlying predisposing conditions — ensuring more durable clinical success. 

References  

1. Dear JD, Hulsebosch SE, Johnson LR. Recognition and diagnosis of underlying disease processes in bacterial pneumonia. Animals. 2024 May 29;14(11):1601. https://www.mdpi.com/2076-2615/14/11/1601  

2. Dear JD, Vernau W, Johnson EG, Hulsebosch SE, Johnson LR. Clinicopathologic and radiographic features in 33 cats with aspiration and 26 cats with bronchopneumonia (2007-2017). Journal of veterinary internal medicine. 2021 Jan;35(1):480-9. https://academic.oup.com/jvim/article-pdf/35/1/480/66650014/jvim16005.pdf  

3. Graham AM, Tefft KM, Stowe DM, Jacob ME, Robertson JB, Hawkins EC. Factors associated with clinical interpretation of tracheal wash fluid from dogs with respiratory disease: 281 cases (2012-2017). Journal of veterinary internal medicine. 2021 Mar;35(2):1073-9. https://academic.oup.com/jvim/article-pdf/35/2/1073/66651533/jvim16052.pdf  

4. Vientós-Plotts AI, Ericsson AC, Rindt H, Reinero CR. Blood cultures and blood microbiota analysis as surrogates for bronchoalveolar lavage fluid analysis in dogs with bacterial pneumonia. BMC veterinary research. 2021 Mar 23;17(1):129. https://link.springer.com/content/pdf/10.1186/s12917-021-02841-w.pdf  
5. Anderegg L, Im Hof Gut M, Hetzel U, Howerth EW, Leuthard F, Kyöstilä K, Lohi H, Pettitt L, Mellersh C, Minor KM, Mickelson JR. NME5 frameshift variant in Alaskan Malamutes with primary ciliary dyskinesia. PLoS genetics. 2019 Sep 3;15(9):e1008378. https://journals.plos.org/plosgenetics/article/file?id=10.1371/journal.pgen.1008378&type=printable