Edited by: Rachel Jean Derscheid, Iowa State University, United States
Reviewed by: Dong Chan Moon, Korea National Institute of Health, Republic of Korea
Jennifer Granick, University of Minnesota Twin Cities, United States
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Transthoracic fine-needle aspiration of the lungs (TFNA) combined with positive bacterial culture is used less frequently than airway lavage in diagnosing bacterial pneumonia in dogs and cats. This retrospective study evaluated TFNA results and bacterial culture findings in 14 dogs (1 dog twice) and two cats with bacterial pneumonia. Bacterial culture yielded positive results in 9/16 (56.3%) of samples. Only 1/11 (9.1%) of isolated bacterial species showed resistance to empiric treatment (amoxicillin with clavulanate). Six patients without prior antimicrobial treatment had negative bacterial cultures, despite the presence of intracellular bacteria on cytology samples. For patients with suspected bacterial pneumonia, we should consider performing both cytological examination and bacterial culture of lung aspirates, with careful interpretation of the results. Bacterial culture should be considered regardless of ongoing or previous antibiotic treatment, and even in cases where intracellular bacteria are not identified on cytology. Based on our antimicrobial susceptibility findings, first-line empirical treatment with amoxicillin with clavulanate might be an appropriate choice in most of cases regardless of the previous treatment.
Bacterial pneumonia in dogs and cats is traditionally diagnosed through a combination of airway lavage cytology showing intracellular bacteria and positive bacterial culture results (
TFNA offers several potential advantages over airway lavage techniques, including the ability to directly sample affected lung regions, and the option to perform the procedure without general anesthesia. However, the relationship between cytological findings and bacterial culture results from TFNA samples has not been studied extensively in clinical cases of canine or feline bacterial pneumonia.
The primary aim of this retrospective study was to evaluate the diagnostic utility of TFNA by comparing cytological findings with bacterial culture results in dogs and cats suffering from bacterial pneumonia. Additionally, we assessed the antimicrobial susceptibility patterns of isolated bacterial species to provide insights into potential antimicrobial resistance.
Medical records of dogs and cats diagnosed with bacterial pneumonia between 2009 and 2021 at a small animal referral hospital in Germany were reviewed retrospectively. The cases were included if they met the following criteria: TFNA of the lungs was performed, and bacterial pneumonia was confirmed either by obtaining positive bacterial culture from the aspirate, or by identifying intracellular bacteria on cytology samples. Dual-view thoracic radiographs were performed for all patients. Data collected included patient signalment, prior antimicrobial treatment, clinical signs, cytological findings, results of aerobic and anaerobic bacterial culture with antimicrobial susceptibility testing, and details of antimicrobial treatment (
Clinical characteristics of the assessed dogs and cats with bacterial pneumonia, including signalment, clinical signs, prior antimicrobial treatment, cytological results, aerobic and anaerobic bacterial culture,
| Nr | Species | Sex | Age | Clinical signs | Antimicrobial treatment prior to the presentation | Intracellular bacteria | Aerobic bacterial culture | Anaerobic bacterial culture | Antimicrobial resistance | Antibiotic treatment | Outcome |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Dog | MN | 3 | Nasal discharge, vomiting | No | Yes | Beta-hemolytic |
Not performed | Tetracycline (R), clindamycin (R) | Amoxicillin with clavulanate | Euthanasia after 2 days of treatment |
| Amoxicillin (R), ampicillin (R), tetracycline (R), doxycycline (R) | |||||||||||
| 2 | Dog | M | 5 | Fever, cough | Yes, not documented | Yes | Negative | No information about antimicrobial resistance | Amoxicillin with clavulanate | Discharged | |
| 3 | Cat | MN | 2 | Fever, cough | No | Yes | Not performed | Cefalexin (R) | Amoxicillin with clavulanate | Euthanasia due to cavernous lung lesions and thoracic effusion after 3 weeks | |
| 4 | Dog | FS | 9 | Respiratory distress, cough | No | Yes | Not performed | Cefalexin (R) | Amoxicillin with clavulanate | Discharged | |
| 5 | Dog | MN | 3 | Fever, cough | No | Yes | Not performed | Amoxicillin (R), ampicillin (R), cefalexin (R), tetracycline (R), doxycycline (R), sulfamethoxazole with trimethoprim (R) | Amoxicillin with clavulanate | Euthanasia due to sepsis | |
| 6 | Dog | MN | 14 | Fever, cough | Enrofloxacin | Yes | Negative | Penicillin (R), amoxicillin (R), ampicillin (R), pradofloxacin (I) | Amoxicillin with clavulanate | Discharged | |
| 7 | Dog | FS | 8 | Edema of the limbs, fever, cough | Marbofloxacin, amoxicillin with clavulanate | Yes | Not performed | Penicillin (R), amoxicillin (R), ampicillin (R), tetracycline (R), doxycycline (R), pradofloxacin (I), enrofloxacin (I) | Amoxicillin with clavulanate | Discharged | |
| 8 | Dog | MN | 7 | Weakness, cough | No | Yes | Negative | Negative | n.a. | Amoxicillin with clavulanate | Discharged |
| 9 | Dog | M | 13 | Respiratory distress, Cough | No | Yes | Negative | Not performed | n.a. | Amoxicillin with clavulanate | Discharged |
| 10 | Dog | F | 4 | Vomiting | No | Yes | Negative | Not performed | n.a. | Amoxicillin with clavulanate | Discharged |
| 11 | Dog | M | 13 | Vomiting, diarrhea | No | Yes | Negative | Negative | n.a. | Amoxicillin with clavulanate | Discharged |
| 12 | Dog | M | 11 | Dyspnea, cough | No | Yes | Negative | Negative | n.a. | Amoxicillin with clavulanate | Discharged |
| 13 | Dog | MN | 13 | Cough, respiratory distress, vomiting | Enrofloxacin | Yes | Negative | Not performed | n.a. | Amoxicillin with clavulanate | Discharged |
| 14 | Cat | M | 4 weeks | Respiratory distress | Amoxicillin with clavulanate | Yes | Not performed | Not performed | n.a. | Amoxicillin with clavulanate | Discharged |
| 15 | Dog | FS | 13 | Vomiting, diarrhea | No | No | Not performed | Penicillin (R), amoxicillin (R), ampicillin (R) | Amoxicillin with clavulanate | Discharged | |
| 16A | Dog | F | 6 | Regurgitation | No | Yes | Negative | Negative | n.a. | Amoxicillin with clavulanate | Discharged |
| 16B | Dog | F | 6 | Cough | Amoxicillin with clavulanate | Yes | Negative | Penicillin (R), amoxicillin (R), ampicillin (R), pradofloxacin (I) | Enrofloxacin | Discharged | |
| Penicillin (R), amoxicillin (R), ampicillin (R), amoxicillin with clavulanate (R), cefalexin (R), tetracycline (R), doxycycline (R) |
F, female intact; FS, female spayed; M, male intact; MN, male neutered; R, resistant, I, intermediate; R, resistant; n.a., not applicable.
TFNA was performed within 6 h of admission. Lesion localization was guided by thoracic radiographs and ultrasound examination. When visible on ultrasound, consolidated lung regions appearing as hypoechoic areas (“shred sign”) adjacent to the pleural surface were targeted. In cases where peripheral lesions were not ultrasonographically visible, TFNA was performed based on radiographic findings. The applied method of the TFNA (blindly performed or ultrasound-guided) was not recorded in all cases. 25G hypodermic needle was used for all aspiration procedures Flow-by oxygen supplementation was provided during the procedure for all the patients. Sedation with butorphanol (Dolorex® 10 mg/mL; MSD; 0.2–0.3 mg/kg IM or IV) was administered only to uncooperative patients. Samples for cytology and bacterial culture were collected from the same anatomical site.
Cytological samples were air-dried, stained with Diff-Quick (LABOR+TECHNIK Eberhard Lehmann GmbH, Germany), and evaluated by a board-certified small animal internal medicine specialist. A diagnosis of bacterial pneumonia was made when samples showed increased numbers of neutrophils with intracellular bacteria. Cases with increased neutrophils, but no visible bacteria were classified as purulent inflammation.
Samples with increased neutrophils were submitted to the external laboratory (IDEXX Laboratories, Germany) for bacterial culture within 24 h of collection. Both aerobic and anaerobic cultures were performed when sample volume was sufficient. In only 7/17 cases (41%), the TFNA yielded adequate volume for both aerobic and anaerobic bacterial culture procedures. Antimicrobial susceptibility testing was conducted on all bacterial isolates. The exact MIC breakpoints for each testing usually were not provided by the laboratory.
All the animals were monitored in the intensive care unit for at least 24 h following TFNA to assess for potential complications. Clinical outcome (discharge or euthanasia) was recorded for all patients.
The study included 16 animals (14 dogs and 2 cats), with one dog (case nr 16) undergoing TFNA twice, due to recurrent aspiration pneumonia associated with megaesophagus, resulting in 17 samples. The median age was 8.5 years (range: 3–14 years) for the dogs, whereas the two cats were 4 weeks and 2 years old, respectively.
6/17 of samples (35.3%) were obtained from patients that had received prior antimicrobial treatment, including enrofloxacin (
Cytological examination revealed intracellular bacteria in 16/17 samples (94.1%). Bacterial culture was performed on 16/17 samples (94.1%), with both aerobic and anaerobic cultures conducted in 8/16 cases (50%). Only the sample obtained from 4-week-old kitten was insufficient for performing bacterial culture. 9/16 cultured samples (56.3%) yielded positive results, including the presence of
Antimicrobial susceptibility testing revealed widespread resistance to pure
Among the 16 animals, 14 (87.5%) were discharged (12 dogs and both cats). Two dogs were euthanized: one shortly after sampling due to severe pneumonia and sepsis, and another one after 2 days of treatment. The 2-year-old cat was readmitted 3 weeks post-discharge with thoracic effusion and cavitary lung lesions (suspected pulmonary abscess). No immediate complications were reported following TFNA procedures in any patient.
This study demonstrated that TFNA of the lungs is a valuable diagnostic tool for bacterial pneumonia in dogs and cats. In patients with negative bacterial cultures, we frequently observed intracellular bacteria on TFNA cytology. Patients that received antimicrobial therapy before the TFNA, had often positive bacterial culture results.
Bacterial species (
We found polymicrobial infections in 2/17 (11.8%) samples, in dogs with either confirmed or suspected aspiration pneumonia. Polymicrobial infections were reported frequently in other studies (
The discrepancies we observed between cytology and bacterial culture results (negative cultures with positive cytology and vice versa) have been reported previously, though are relatively uncommon (
The discrepancy between cytology and bacterial culture could be caused by several factors. Firstly, bacterial growth could be compromised by prior antimicrobial treatment, or the causative agent could be difficult to culture. Secondly, separate aspirations for cytology and culture could sample different areas of the lungs. Thirdly, culture cannot differentiate between intra-and extracellular bacteria, whereas our cytological criterion for bacterial pneumonia was the presence of intracellular bacteria. Lastly, although samples were taken aseptically, there was always some risk of contamination with opportunistic pathogens.
The 35.3% incidence of prior antimicrobial treatment in our population resulted in relatively low bacterial resistance (9.1%), compared to another study reporting 57.4% resistance to previously administered antimicrobials (
Study limitations include its retrospective nature with incomplete documentation of prior treatment, small sample size, potentially introducing bias, and inconsistent use of anaerobic culture potentially underestimating anaerobic infections.
While, as demonstrated in several studies (
This study demonstrated that TFNA is an effective diagnostic tool for bacterial pneumonia in dogs and cats. For patients with suspected bacterial pneumonia, we should consider performing both cytological examination and bacterial culture of lung aspirates, with careful interpretation of the results. Bacterial culture should be considered regardless of ongoing or previous antibiotic treatment, and even in cases where intracellular bacteria are not identified on cytology. Based on our antimicrobial susceptibility findings, first-line empirical treatment with amoxicillin with clavulanate might be an appropriate choice in most of cases regardless of the previous treatment.
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
Ethical approval was not required for the studies involving animals in accordance with the local legislation and institutional requirements because it was a retrospective study and only gathered data were analyzed. Written informed consent was not obtained from the owners for the participation of their animals in this study because it was a retrospective study and only gathered data were analyzed.
AP: Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing. CS: Supervision, Writing – review & editing.
The author(s) declare that no financial support was received for the research and/or publication of this article.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors declare that Gen AI was used in the creation of this manuscript. We acknowledge the use of Claude 3.5 Sonnet (New) (Anthropic,
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