Etiology and effusion characteristics in 29 cats and 60 dogs with pyothorax (2010‐2020)

Abstract Background Pyothorax, an accumulation of inflammatory fluid in the pleural space, is often caused by foreign body inhalation in dogs, whereas the etiology in cats can be more difficult to discern. Objective Compare clinical, microbiologic findings, and etiology in cats and dogs with pyothorax. Animals Twenty‐nine cats and 60 dogs. Methods Medical records of cats and dogs diagnosed with pyothorax from 2010 to 2020 were reviewed. Clinical findings, fluid analysis, and microbiologic results were retrieved. Results Antimicrobials had been administered to equal proportions of cats and dogs before fluid sampling (45% and 47%). Groups did not differ in age or total protein concentration or percentage neutrophils in pleural fluid, but effusion cell count was significantly higher in cats than in dogs (P = .01). Neutrophils containing intracellular bacteria were identified in more cats (27/29, 93%) than dogs (44/60, 73%; P = .05). Penetrating damage to the thorax was implicated as the cause of pyothorax in equal percentages of cats (76%) and dogs (75%). Etiology could not be determined in 2 cats and 1 dog. Cats had higher numbers of bacterial isolates per patient (median, 3) than dogs (median, 1; P = .01) and anaerobes were isolated more often in cats (23/29, 73%) than in dogs (27/60, 45%; P = .003). Conclusions and Clinical Importance Pyothorax had similar etiologies in cats and dogs. Cats had higher fluid cell counts, higher numbers of bacterial isolates identified per patient, and intracellular bacteria detected more commonly than did dogs.

infections in dogs and cats are polymicrobial, 1,2 but some studies have found single organism infection. 3,4 Diagnosis of bacterial pyothorax is often readily achieved because of the presence of intracellular bacteria on cytology in 68% of dogs and 91% of cats. 2 However, empirical use of antimicrobial agents before sample collection could impact the diagnostic utility of both cytology and culture.
The underlying cause of pyothorax varies based on geography, environment, and the species involved. Hunting dogs in rural environments are most commonly affected by grass awn migration, [5][6][7] which causes pyothorax as well as pneumothorax and often requires surgical intervention. 8 Cats do not inhale foreign material commonly, 7,9 and various studies have indicated poor identification of etiology in cats. 10,11 Bite wounds and other penetrating injuries as a cause for pyothorax are supported by the findings that outdoor access and multi-cat households are common in affected cats, 10,11 but parapneumonic spread also has been suggested as a frequent cause of pyothorax. 12 Retrospective studies are limited in ability to determine the etiology of disease. Hence, our primary aim was to comprehensively evaluate historical findings and combined cytological and microbiological features of pyothorax in cats compared to dogs and assess the ability to confidently determine a cause for empyema in the 2 species. We hypothesized that penetrating damage to the thorax (e.g., bite wounds, foreign bodies, traumatic injuries) would be the most common causes in both dogs and cats and that evidence for underlying pleuropneumonia would be uncommon. Our secondary aim was to summarize antimicrobial susceptibility data for infecting bacteria.

| MATERIALS AND METHODS
Ours was a retrospective study that collated data from review of medical records. Using the search terms pyothorax, empyema, and pleural infection, the medical record database at the William R. Pritchard Veterinary Medical Teaching Hospital at the University of California-Davis was searched between 2010 and 2020 for cats and dogs with a clinical diagnosis of pyothorax. Pyothorax was defined as a suppurative exudative pleural effusion with protein concentration ≥2.5 g/dL or cell count ≥5000/μL. 13 Thoracic fluid was described as septic when bacteria were identified on cytologic assessment by a veterinary clinical pathologist, regardless of microbial culture findings. Inclusion criteria for the study included contemporaneous collection of pleural fluid for cytology and microbiologic culture (aerobic and anaerobic).
Animals with specimens collected at disparate times, those with surgically collected swabs, or those with culture results from only tissue specimens were excluded from analysis. Animals with aseptic chylothorax or neoplastic effusions identified on cytology by a board-certified clinical pathologist were not included in the study.
Cats considered likely to have effusion related to feline infectious peritonitis (FIP) based on very high fluid protein concentration (above the exudative range) with cell counts ≤5000/μL or positive immunohistochemistry on pleural fluid also were excluded. Fluid specimens were obtained by thoracocentesis or from an indwelling thoracic catheter that had been placed using aseptic technique. Thoracic fluid was placed in both EDTA and nonanticoagulant tubes for transport to the laboratory, and any sample obtained after midnight was stored at 4 C for <8 hours before processing.
Fluid analysis was performed by staff of the clinical pathology laboratory and cytologic assessment was completed by a board-certified pathologist. Total nucleated cell count was performed using an automated cell counter (Advia 120, Siemens, Deerfield, IL) and a 100-cell differential cell count was performed. Slides were prepared using an automated cell stainer (Model 7151 Wescor Aerospray Hematology Pro, ELITech Bio-Medical Systems, Logan, UT) and Wright-Giemsa stain. One milliliter of fluid was transferred to a disposable tube and centrifuged for 5 minutes at 2400 rpm. A drop of the supernatant was applied to the refractometer for assessment of total protein concentration. Total protein concentration in pleural fluid (g/dL), total cell count (cells/μL), and differential cytology (%) were recorded for all cases and cytologic reports were reviewed for intracellular bacteria (neutrophils containing phagocytosed bacteria), which was taken as evidence of septic pleural effusion.
Fluid specimens were submitted for aerobic and anaerobic bacterial cultures in all cases, and selected cases were submitted for isolation of Mycoplasma species at the discretion of the attending clinician.
One drop of pleural fluid was plated onto 5% defibrinated sheep blood and MacConkey agars (Hardy Diagnostics, Santa Maria, CA) and incubated in the presence of 5% CO 2 at 35 C for isolation of aerobic organisms. Pre-reduced anaerobic Brucella blood agar (Anaerobe Systems, Morgan Hill, CA) was used for anaerobic culture with incubation at 35 C under anaerobic conditions. Pleuropneumonia-like organism base with thallium acetate and penicillin G (University of California-Davis Media Laboratory, Davis, CA) was used for Mycoplasma species isolation with incubation at 35 C in 5% CO 2 . Standard biochemical methods were used to identify cultured bacteria. The number and species of isolates obtained from each cat and dog were recorded under aerobic and anaerobic conditions. Cases were designated as polymicrobial culture if ≥2 species of bacteria were isolated. Bacterial susceptibility testing was performed according to standards established by the Clinical Laboratory Standards Institute using broth microdilution. 14 Susceptibility interpretations of minimum inhibitory concentration (MIC) testing were based on animal standards when available and, when not available, interpretative criteria established for human medicine were used. Multidrug resistance (MDR) was defined as resistance to at least 1 antimicrobial in ≥3 classes of antimicrobial drugs in which the wild type bacteria could be susceptible to (eg, no intrinsic resistance).
Interviews with owners, physical examination findings, thoracic imaging studies, and results of surgical, bronchoscopic, or histologic investigations were reviewed. Patient age and information about administration of antimicrobials within 1 week before presentation were collected from the medical record. In cats, FeLV/FIV status was determined from the medical record. All thoracic imaging studies (e.g., radiography, ultrasonography, computed tomography) were interpreted by a board-certified radiologist, and results were used by the clinician of record in case management. Imaging was retrospectively evaluated by 1 of the authors (Lynelle R. Johnson) as part of record review in determining etiology and to assess the side of the effusion.
Contemporaneous notes made by the primary clinician along with the assessment of results were used to assign the probable cause of pyothorax, which was defined as penetrating damage to the thorax (e.g., foreign body inhalation, bite wound, traumatic injury), iatrogenic pyothorax, suspected pneumonic spread of infection, or systemic disease-associated pyothorax according to criteria presented in Table 1.
Animals were placed into specific categories when ≥2 or were met within that category and other causes could be rationally excluded.
Cases were assigned the etiology of foreign body inhalation when foreign material was identified by surgery, bronchoscopy, computed tomography or magnetic resonance imaging, or when histopathology indicated birefringent material, inflammatory tracts, or focal pyogranulomatous inflammation. Bite injuries were documented by evaluation of the clinical history and physical examination findings. In cats, a known history of fighting, outdoor access, living in a multi-cat household, history of cat bite abscess or thoracic wound in the past 6 months contributed to the probable diagnosis of a bite wound as the cause of pyothorax. Animals with pyothorax were assigned the likely cause of traumatic injury (outside-in or inside-out) when no surgical intervention had been performed or histopathology was available and there was no history of an abscess but at least 2 of the following criteria were met: the animal had outdoor access, there was known exposure to plant awns, a thoracic wound was present, focal radiographic infiltrates resolved with treatment, coincident pneumothorax was identified, or Actinomyces was isolated from pleural fluid. Animals in this category also had no historical, clinical, or imaging evidence of pneumonia, systemic disease, or iatrogenic causes.
Pneumonia or pulmonary disease as an underlying cause for pyothorax was assigned to animals lacking airway foreign bodies that had radiographic or computed tomographic evidence of parenchymal infiltrates with air bronchograms and without volume loss, or those with histologic evidence of parenchymal inflammation or pneumonia.
Aspiration pneumonia was included in this group and was considered a probable diagnosis if previously identified risk factors including vomiting, recent anesthesia, laryngeal disease or enteral nutrition were reported. 15,16 Systemic disease was considered the underlying cause of pyothorax in animals with multiorgan infectious disease or systemic inflammatory response syndrome, the loss of localized control of disease resulting in a circulating inflammatory response likely mediated by cytokines. 17 Evidence of immunosuppression as a contributor to disease was examined in these cases by assessing coincident administration of immunosuppressive medications.
Iatrogenic causes were identified in animals with procedural interventions potentially involving the pleural space such as thoracocentesis, thoracotomy, or esophageal feeding tube placement that preceded the onset of pyothorax. Finally, the etiology was considered unknown if a reasonable clinical explanation for pyothorax could not be obtained from information available in the medical record.
Statistical analysis: Clinical data were assessed for normality using the D'Agostino and Pearson test and are presented as mean ± SD for normally distributed data or median and range for nonparametric data.
Age, duration of illness, circulating white blood cell count, pleural fluid cell counts, pleural fluid protein concentration, and fluid percentage of neutrophils were compared between cats and dogs using Student's ttest for normally distributed data or Mann-Whitney U test for nonparametric data. Isolation of anaerobes, presence of sepsis in pleural fluid, and presampling exposure to antimicrobials were compared between cats and dogs using Fisher's exact test. Significance was set at P < .05. (GraphPad Prism v 9.5.1, San Diego, CA).
T A B L E 1 Probable etiology of pleural effusion in cats and dogs was determined by interrogation of owner history, physical examination findings, and diagnostic imaging as well as interventional procedures.   (22/29) and mixed breed dogs (12/60) were affected most commonly.
Labrador retrievers (n = 10) and German Shepherds (n = 6) were over-represented compared to the hospital population. The FeLV/FIV status was assessed in all cats with 1 cat testing positive for FeLV.
Groups did not differ in duration of illness or circulating white blood cell count between cats and dogs or across causes of disease (P > .05; Table 2). Thoracic radiographs available for review in 27/29 cats identified unilateral effusion in 2 cats and bilateral effusion in the remaining cats. In dogs, thoracic radiographs were reviewed for 53 cases and    One cat and 5 dogs were known to be receiving corticosteroids.
history of abscess or thoracic wounds. Both dogs were from multi-pet households and had thoracic wounds evident.
An additional 9 cats and 12 dogs were presumed to have traumatic injury to the thorax (outside-in or inside-out) as the cause for pyothorax. Foreign body migration could not be distinguished from bite wounds in these animals because surgical intervention was not pursued, and no histopathology was available for review.
All animals had outdoor access with possible exposure to plant awns, and most were from multi-animal household. Five cats and 3 dogs had focal radiographic infiltrates that resolved as pyothorax was treated, and concurrent pneumothorax was documented in   Susceptibility data for bacteria isolated most frequently from cats and dogs with pyothorax are listed in Table 5  We confirmed our primary hypothesis that penetrating injury to the thorax would be the most common cause of pyothorax in both dogs and cats. Inhalation of grass awns was the most common cause for pyothorax in dogs, and likely was the reason that twice as many dogs were affected by pyothorax as cats, because cats rarely inhale foreign material. 9 Depending on environmental conditions and geographic location, airway foreign bodies can be a common 6,7 or uncommon 18  pleuritis, 19 and isolation of Actinomyces sp. 20 added to confidence in foreign body migration as the etiology in some cases.
Bite wounds were more commonly identified as an etiologic diagnosis in cats compared to dogs in our study. Although outdoor status was common in dogs and many were from multi-animal households in both species, dogs lacked a common history of fighting or bite wounds in comparison to cats. Most cases of pyothorax in cats in our study were from multi-cat households (61%), which has been reported to result in a 3.8-fold increase in the risk of pyothorax, compared with cats from single-cat households. 10 In our study, pyothorax was diagnosed in 2 housemates that routinely fought. In a previous study of pyothorax, 10 recent history of a bite or other external wound could be documented in only 11/76 cats (14.5%) that had a complete medical history recorded, and findings could be confirmed by external wounds in only 2 cats. However, lack of external wounds is not surprising because bites from dogs failed to leave a detectable wound in 29% of cats that had been bitten in the thorax. 21 In another study of pyothorax, 75% of cats came from multi-cat households, but those cats did not have a history of fighting or bite wounds and the authors could only determine an etiology for pyothorax in 3/55 cases. 11 Most cats with pyothorax in our study had access to the outdoors (59%), similar to findings in large studies of pyothorax from Europe. 3,11 Several studies have failed to discover an underlying cause for pyothorax in cats, 1,11 whereas only 2/29 cats did not have a cause of pyothorax identified in our study.
Using the methodology described in our study, essentially equal percentages of cats and dogs had pyothorax ascribed to suspected thoracic trauma, defined as inside-out (e.g., unrecognized foreign body, esophageal penetration) or outside-in (e.g., thoracic trauma, bite wound) lesions as a probable cause of pyothorax.
These etiologies were considered probable rather than definitive given the lack of histopathologic assessment in these cases. Isolation of anaerobes in conjunction with aerobes was common in cats with a penetrating injury as cause of pyothorax. These bacteria also are isolated from cats with pneumonia, 16  been isolated from bite wounds in humans, 25 and was found in 1 cat in our study.
Detection of oral flora in cases of pyothorax also has been taken as evidence that pyothorax in cats is related to oropharyngeal aspiration resulting in pleuropneumonia. 1,4,12 Although aspiration pneumonia is increasingly recognized in cats, 15 Limitations of our study include those common to retrospective studies, which lack a prescribed method of data collection and assessment, particularly because the criteria for inclusion in our study were based on clinical diagnoses. Although our clinical pathology service has specific cut-off values for protein concentration and cell counts in exudates, 13 some animals met only 1 of those criteria. Many cases were excluded from analysis because pleural fluid was not submitted contemporaneously for both culture and cytology, and it is likely that our study was underpowered to determine significant differences between variables for specific etiologies. Although some animals fit well into categories of bite wounds or foreign bodies based on defined criteria, a number of animals had less definitive findings, which resulted in the category of suspected penetrating injury (inside-out or outside-in). These issues could be more appropriately addressed in a prospective study.
We cannot exclude the possibility that pleuropneumonia developed in some animals with foreign bodies or bite wounds as a cause for pyothorax, although these are not reported as risk factors for aspiration. 38,39 The occurrence in some dogs of pyothorax in conjunction with systemic disease remains intriguing. It seems unlikely that immunosuppressive drug treatment led to empyema or predisposed dogs to this condition. Multiorgan dysfunction and translocation of bacteria might have played a role in some dogs, but this possibility could not be discerned in a retrospective study. Finally, cats and dogs were assigned a cause for pyothorax based on evidence in the medical record, which is subject to interpretation. Theoretically, a cat with pyothorax could be assigned an etiology of bite wound based on multi-pet household and history of fighting only. Although not the case here, the uncertainties of defining an etiology based on retrospective data must be considered when interpreting our results.
In summary, in our study, pyothorax in dogs was most commonly associated with foreign body inhalation whereas pyothorax in cats was related to bite wounds or traumatic injury to the thorax. Interestingly, we found pyothorax in conjunction with various systemic diseases in some dogs, which represents a unique finding. In many studies, the etiology of pyothorax remains obscure in cats and in dogs. 10,11,18 However, comprehensive medical record review here yielded confident assessment of the probable cause of pyothorax in all but 3/89 cases, even though almost 25% of all cases were narrowed down to penetrating injury alone. The inability to assign a precise cause represents a limitation of our study, but represents an opportunity for future prospective studies of pyothorax. Improved scrutiny for upper respiratory tract signs and confirmation of lower airway bacterial species that match pleural organisms would provide more confidence in a role for parapneumonic spread as a cause for pyothorax in cats.

ACKNOWLEDGMENT
No funding was received for this study.