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Pyothorax is a relatively rare but life-threatening dis-ease of dogs that is characterized by septic thoracic

effusion.1-6 Infection in affected dogs is typicallypolymicrobial, with frequent isolation of anaerobic orfilamentous bacteria.5 Reported sources of infectioninclude inhaled or ingested plant material, penetratingthoracic wounds, and esophageal perforation.Infection may also be a result of hematogenous ordirect spread of infection from elsewhere in the body ormay be iatrogenically induced.5 Dogs are rarely exam-ined during the acute stage of infection, and clinicalsigns are typically a result of chronic disease.6

Common clinical signs include lethargy, dyspnea, andcoughing.1-6

Actinomyces spp organisms have frequently beencultured from the pleural fluid of dogs with pyotho-

rax.1,3,4 It has been suggested that Actinomyces sppinfection is associated with plant material and that res-olution of the pyothorax without excision of this plantmaterial may be difficult.1 To the authors’ knowledge,however, the type of microbial organisms culturedfrom pleural fluid from dogs with pyothorax has notbeen associated with outcome.

Treatment of pyothorax in dogs involves adminis-tration of antimicrobials, intermittent or continuousthoracic drainage, thoracic lavage, and surgery.1-4

However, determining the most appropriate treatmentfor any individual dog is difficult at best. Surgical treat-ment has been recommended for other infectious dis-eases, such as peritonitis,7,8 septic arthritis,9 andabscesses. Therefore, even though surgery may be asso-ciated with high morbidity rates in dogs with pyotho-rax, it may potentially improve outcome. Althoughmortality rates ranging from 0 to 44% in dogs treatedmedically or surgically for pyothorax have been report-ed,1-4 to our knowledge, outcomes of dogs with pyotho-rax treated medically versus surgically have not beenevaluated. The purposes of the study reported herewere to compare outcomes of dogs with pyothoraxtreated medically versus surgically and to identifyprognostic indicators for dogs with pyothorax. Theoutcome of interest in this study was disease-freeinterval (DFI).

Criteria for Selection of CasesMedical records of dogs examined at the Colorado

State University Veterinary Teaching Hospital between1991 and 2001 because of pyothorax were reviewed.Dogs were eligible for inclusion in the study if bacteriawere evident during cytologic examination of a pleuralfluid sample or if bacterial culture of a sample of pleur-al fluid yielded bacterial growth. Dogs were excludedfrom the study if they had been treated prior to exam-ination at the veterinary teaching hospital or if theywere determined to have a systemic disease that couldalter survival time.

ProceduresMedical management was defined as having all of

the following: systemic intravenous antimicrobialtreatment, thoracic drainage (unilateral or bilateral)with aspiration (continuous or intermittent), andthoracic lavage. Dogs were included in the medicaltreatment group only if they had received at least 2days of medical treatment. Surgical management wasdefined as thoracotomy with excision of affected tis-sues and open pleural lavage; postoperative manage-ment followed the same protocol as medical manage-ment. All dog were treated with antimicrobials for along period.

Medical and surgical treatment of pyothorax in dogs: 26 cases (1991–2001)

Matthew B. Rooney, DVM, and Eric Monnet, DVM, PhD, DACVS

Objective—To compare outcomes of dogs withpyothorax treated medically versus surgically and toidentify prognostic indicators for dogs with pyothorax.Design—Retrospective study.Animals—26 dogs.Procedure—Medical records were reviewed toobtain information regarding signalment, results ofphysical and laboratory evaluations at the time of ini-tial examination, results of bacterial culture of pleuralfluid, radiographic abnormalities, treatment (surgicalvs medical), complications, whether the diseaserecurred, disease-free interval, survival time, andcause of death.Results—Calculated proportions of dogs free fromdisease 1 year after treatment were 25 and 78%,respectively, for dogs treated medically and surgical-ly. Treatment was 5.4 times as likely to fail in dogstreated medically as in dogs treated surgically. Tworegression models relating treatment group (medicalvs surgical) to disease-free interval were found to besignificant. The first contained terms for medicaltreatment and isolation of Actinomyces spp frompleural fluid; the second contained terms for medicaltreatment and radiographic detection of mediastinalor pulmonary lesions at the time of initial examination.Conclusions and Clinical Relevance—Results sug-gest that surgical treatment is associated with a bet-ter outcome than medical treatment in dogs withpyothorax. In addition, surgery should be consideredif radiographic evidence of mediastinal or pulmonarylesions is detected or if Actinomyces spp is isolatedfrom the pleural fluid. (J Am Vet Med Assoc 2002;221:86–92)

From the Department of Clinical Sciences, College of VeterinaryMedicine and Biomedical Sciences, Colorado State University, FortCollins, CO 80523.

Address correspondence to Dr. Rooney.

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Information obtained from the medical recordsincluded signalment (breed, age, sex, and bodyweight), clinical signs at the time of initial examinationat the veterinary teaching hospital (respiratory distress,gastrointestinal signs, rectal temperature, and concur-rent diseases), results of hematologic and biochemicalanalyses performed at the time of initial examination(absolute and differential WBC counts, platelet count,PCV, blood glucose concentration, activated clottingtime, albumin concentration, and alanine aminotrans-ferase [ALT] and alkaline phosphatase [ALP] activi-ties), results of bacterial culture of a sample of pleuralfluid collected at the time of initial examination,whether there was radiographic evidence of lungparenchymal disease, treatment (medical vs surgical),complications, whether the pyothorax recurred, causeof death, DFI, and survival time. The Cox proportionalhazard ratio was calculated for each variable to deter-mine whether it would be useful in predicting DFI.Data on antimicrobial susceptibility were tabulated.

Follow-up time was defined as time from initialexamination at the veterinary teaching hospital to timeof last contact with the owner or referring veterinarianor time of last entry in the medical record. Treatmentwas considered to have failed if the disease progresseddespite appropriate medical or surgical treatment, if thedisease recurred, or if the dog died of pyothorax.Treatment was considered successful if the dog was dis-charged from the hospital and the disease did not recur.

A response-adaptive method10-12 (ie, play-the-win-ner) study design was chosen to analyze the data inorder to more closely reflect the clinical setting andincrease the amount of information per patient. Dogsin which medical treatment was considered to havefailed because of progression of disease underwentsurgery. However, dogs initially entered in the medicaltreatment group and then in the surgical treatmentgroup were counted as single patients. Thus, whencomparing data between surgical and medical treat-ment groups, dogs that underwent surgery at any timeduring treatment were considered in the surgical treat-ment group. Dogs that had a successful outcome fol-lowing treatment, died of other causes, or were lost tofollow-up were censored in the analysis. Dogs in whichtreatment failed were not censored. Records werereviewed in an attempt to determine the attending clin-icians’ basis for deciding whether to perform surgery,but given the retrospective nature of the study, objec-tive interpretation was impossible.

Statistical analyses—The Kaplan-Meier product-limit method was used to construct DFI curves fordogs with pyothorax treated medically or surgically.Proportions of dogs free from disease 6 months and 1year after the initial diagnosis of pyothorax were deter-mined from the DFI curves. Mean DFI was defined asthe mean value of the cumulative survival function; theSE was defined as the SE about the estimated mean ofthe cumulative survival function. A log-rank test wasused to compare DFI curves.

Multivariate stepwise Cox regression analyseswere used to determine whether surgical treatment wasan independent predictor of DFI. Likelihood ratio tests

were used to determine whether the resulting modelswere associated with DFI. Confidence intervals (CI)were calculated to determine whether explanatory vari-ables included in the models were significant. The Coxmodel assumes proportionality of effects (ie, constanthazard rate ratios) across time. This assumption wasautomatically verified by the statistical packagea used forthe analyses and by comparing parallelism in log cumu-lative hazard plots. In each model, the reference group(eg, dogs in which Actinomyces spp was isolated frompleural fluid) was compared with all other dogs in thestudy (eg, dogs in which Actinomyces spp was not isolat-ed from pleural fluid). Frequencies of isolation of specif-ic bacteria were compared between medical and surgicaltreatment groups with χ2 analyses. Data were comparedbetween medical and surgical treatment groups withANOVA. Computer software packagesa,b were used toperform the statistical analyses. Values of P < 0.05 wereconsidered significant. Results are given as mean ± SE.

ResultsTwenty-six dogs met the criteria for inclusion in

the study. The cause of pyothorax was definitivelydetermined in only 1 dog in which plant material wasidentified during histologic examination of excisedtissue. None of the dogs had a history of penetratingwounds or previous medical or surgical treatment thatcould have caused nosocomial or iatrogenic pyotho-rax. There were 5 Labrador Retrievers, 3 GoldenRetrievers, 3 Border Collies, 2 Chesapeake BayRetrievers, 2 English Springer Spaniels, 1 GermanShepherd Dog, 1 Irish Wolfhound, 1 BerneseMountain Dog, 1 Samoyed, 1 Rottweiler, 1 Beagle, 1Treeing Walker Coonhound, 1 Weimaraner, 1 GreatPyrenees, 1 Saint Bernard, and 1 mixed-breed dog.There were 17 males and 9 females. Mean ± SE agewas 3.9 ± 0.56 years. Mean weight was 25.3 ± 2.4 kg(55.7 ± 1.1 lb). Mean rectal temperature at the time ofinitial examination was 39.6 ± 0.14 C (103.3 ± 0.25F). Seventeen of 26 dogs were examined because ofrespiratory distress; 6 of 26 had vomiting or diarrhea.Four of 26 had pneumothorax at the time of initialexamination, and 2 of 26 had thrombosis of the poste-rior vena cava, which was confirmed at surgery.

All dogs had radiographic evidence of pleural effu-sion. Fourteen dogs had signs of mediastinal or pul-monary lesions (ie, interstitial consolidation, alveolarconsolidation, atelectasis, or masses of soft tissue densi-ty). The right cranial lung lobe was involved in 5 dogs,the right middle lung lobe was involved in 8 dogs, theright caudal lung lobe was involved in 8 dogs, theaccessory lung lobe was involved in 3 dogs, the left cra-nial lung lobe was involved in 4 dogs, the left caudallung lobe was involved in 8 dogs, and the mediastinumwas involved in 3 dogs. Twelve dogs had lesions involv-ing > 1 anatomic location on thoracic radiographs.Twelve of the 14 dogs with radiographic evidence ofmediastinal or pulmonary lesions underwent surgery.Of these 12, 8 underwent surgery only after medicaltreatment failed, and 4 underwent surgery within 2days after initial examination. The remaining 2 dogswith radiographic evidence of mediastinal or pul-monary lesions had only medical treatment.

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For all dogs, results of bacterial culture of a sam-ple of pleural fluid collected at the time of initial exam-ination were positive (Table 1). Follow-up bacterialculture of pleural fluid samples collected during treat-ment did not yield any new organisms. For 12 of the 26dogs, multiple bacterial organisms were isolated; atotal of 69 isolates were cultured from pleural fluidfrom the 26 dogs. Forty-five of the isolates were gram-negative, and 12 were gram-positive; the remaining 12isolates were anaerobic filamentous organisms withvariable gram staining results. A combination of gram-

negative and gram-positive organisms was isolatedfrom 15 dogs. There were 39 aerobic and 30 obligateanaerobic organisms. A combination of aerobic andanaerobic organisms was isolated from 11 dogs.Antimicrobial susceptibility was determined for 19 aer-obic organisms (Table 2).

Actinomyces spp was isolated from pleural fluidcollected at the time of initial examination from 12dogs; 4 of the 12 were hunting breeds. One dog wastreated medically and died. Four were treated surgical-ly and survived; in all 4, pyothorax did not recur for atleast 1 year after surgery. Four dogs were treated med-ically for 4 to 14 days and subsequently underwentsurgery because of failure of medical treatment. All 4survived, but 1 had a recurrence 245 days after initialexamination. The remaining 3 dogs were successfullytreated medically.

Seven dogs received medical treatment alone andwere classified in the medical treatment group. Twelvedogs were treated medically for 2 to 14 days (mean ±SE, 4.14 ± 3.2 days) and then underwent surgery.Thus, these 12 dogs were classified as having beentreated surgically. The remaining 7 dogs underwentsurgery within 2 days after initial examination. Thus,19 dogs were classified in the surgical treatment group.

Unilateral thoracic drainage with intermittent orcontinuous suction was used in 3 dogs in the medicaltreatment group; in the remaining 23 dogs, bilateralthoracic drainage was used. In all 26 dogs, intermittent

Table 1—Results of bacterial culture of pleural fluid from dogswith pyothorax treated medically (n = 7) or surgically (19)

No. of isolates

Medical Surgical Bacteria treatment treatment P value

Acinobacter spp 1 0 NA Actinomyces spp 4 8 0.35 Bacteroides spp 3 4 0.54 Enterococcus faecium 1 1 NA Escherichia coli 7 7 0.99

Fusobacterium spp 4 5 0.99 Staphylococcus spp or

Streptococcus spp 5 3 0.58 Pasteurella spp 4 4 NA Peptostreptococcus spp 1 1 NA Prevotella spp 2 4 0.99

NA = Not applicable.

Table 2—Antimicrobial susceptibility of aerobic bacteria isolated from the pleural fluid of dogs with pyothorax

No. susceptible

P multocida E faecium Staphylococcus Streptococcus E coli Acinobacter TotalAntimicrobial (n = 7) (n = 2) spp (n = 2) spp (n = 2) (n = 5) spp (n = 1) (n = 19)

Amikacin 2 2 2 1 5 1 13 Amoxicillin-clavulanate 7 2 2 2 2 1 16 Ampicillin 7 2 1 2 3 1 16 Cefoxitin 7 1 2 2 5 1 18 Cephalothin 7 1 2 2 3 1 16

Chloramphenicol 7 1 2 2 4 1 17 Enrofloxacin 7 1 2 2 5 1 18 Erythromycin 7 2 2 2 1 1 11 Gentamicin 4 2 2 2 5 1 16 Tetracycline 7 2 2 1 4 1 17 Trimethoprim-sulfonamide 7 2 2 2 5 1 19

Antimicrobial susceptibility was determined with the disk diffusion method.

Table 3—Comparison of signalment and results of physical and laboratory evaluations performed atthe time of initial examination in dogs with pyothorax treated medically (n = 7) or surgically (19)

Variable Medical treatment Surgical treatment P value Power

Age (mo) 60.0 � 16.5 44.8 � 7.2 0.33 0.56 Weight (kg) 23.7 � 3.9 25.9 � 2.4 0.63 0.71 Rectal temperature (C) 39.6 � 0.27 39.3 � 0.16 0.64 0.72 WBC count (cells/µl) 24,800 � 6,500 22,800 � 3,000 0.74 0.80 Band neutrophil fraction (%) 4.00 � 2.20 7.60 � 2.70 0.50 0.59 Platelet count (platelets/µl) 356,900 � 81,700 235,300 � 25,500 0.77 0.82

PCV (%) 35.4 � 2.9 37.6 � 2.4 0.62 0.69 Blood glucose (mg/dl) 87.0 � 5.9 98.9 � 5.2 0.21 0.32 Activated clotting time (s) 137.0 � 42.5 131.1 � 6.3 0.76 0.81 Albumin (g/dl) 2.6 � 0.3 2.3 � 0.2 0.40 0.50 ALT (U/L) 31.3 � 13.1 44.7 � 12.0 0.55 0.64 ALP (U/L) 196.6 � 49.4 242.3 � 114.5 0.83 0.86

Data are given as mean � SE.ALT = Alanine aminotransferase. ALP = Alkaline phosphatase. To convert temperature in C to F, multiply by 9/5 and add 32.

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thoracic lavage was performed, and antimicrobialswere administered IV. All 19 dogs that underwentsurgery had a median sternotomy with partial excisionof the mediastinum. In 10 dogs, a pericardiectomy wasperformed. In 11 dogs, 1 or more lung lobes wereremoved. Histologic examination of the excised pul-monary tissue revealed suppurative inflammation in 6dogs and pyogranulomatous inflammation in 5. Dogsclassified in the surgical treatment group did not differfrom dogs classified in the medical treatment group inregards to any of the variables recorded at the time ofinitial examination (Table 3).

Four dogs were lost to follow-up before 1 yearafter the initiation of treatment. Mean ± SE follow-uptime for all dogs was 600 ± 111 days. Calculated pro-portions of dogs free from disease 6 months after treat-ment were 32 and 85% for the medical and surgicaltreatment groups, respectively (Fig 1); these valueswere significantly different (log rank test; P < 0.001).

The calculated proportions of dogs free from disease 1year after treatment were 25 and 78% for the medicaland surgical treatment groups, respectively (log ranktest; P < 0.001). After 1 year, the proportion of dogs freefrom disease did not change for either group. Mean DFIwere 61.8 ± 20.6 and 209.0 ± 22.1 days for dogs in themedical and surgical treatment groups, respectively.

Pyothorax recurred 40 days after initial examina-tion in 1 dog that underwent surgery, 245 days afterinitial examination in 1 dog that was treated medicallyand then underwent surgery, and 186 days after initialexamination in 1 dog that was treated medically. Thedog in which pyothorax had been treated medicallydied while being treated for recurrence of the disease.The other 2 dogs in which pyothorax recurred sur-vived; 1 was treated medically, and the other was treat-ed surgically.

Five dogs died of pyothorax. Two dogs treated sur-gically died of pyothorax 2 and 9 days after initialexamination. One dog died 9 days after initial exami-nation following 5 days of medical treatment andsurgery. Two dogs treated medically died of pyothorax5 and 186 days after initial examination.

The remaining 15 dogs, including 3 dogs treatedmedically, 7 dogs treated medically that underwentsurgery after medical treatment failed, and 5 dogs treat-ed surgically, all survived.

Complications were observed in 6 dogs treatedsurgically and included disseminated intravascularcoagulation (4 dogs) and postoperative abdominaleffusion (2). Two dogs treated medically had hemor-rhage from the site of chest tube placement andrequired blood transfusions.

Treatment group was the only factor significantlyassociated with DFI (Table 4). Treatment was 5.4 timesas likely to fail in dogs treated medically as in dogstreated surgically. Cox proportional hazard analysis

Figure 1—Disease free interval curves for dogs treated med-ically or surgically for pyothorax. Circles represent end points ofcensored data.

Table 4—Evaluation of factors potentially associated with disease-free interval in dogs with pyothorax

Variable P value Hazard ratio 95% CI Power

Age 0.827 1.001 0.989–1.014 0.84 Sex 0.318 0.139 0.160–2.921 0.37 Body weight 0.740 1.007 0.964–1.053 0.77 Respiratory distress 0.655 1.266 0.450–3.600 0.70 Rectal temperature 0.612 1.100 0.259–1.595 0.65 Pulmonary lesions 0.334 1.597 0.617–4.133 0.39 Gastrointestinal signs 0.582 1.298 0.511–3.295 0.63

WBC count 0.153 1.022 0.992–1.053 0.21 Band neutrophil fraction 0.828 0.994 0.946–1.045 0.85 Platelet count 0.345 0.999 0.996-1.001 0.41 PCV 0.535 0.983 0.932–1.037 0.59 Blood glucose 0.321 1.011 0.990–1.032 0.38 Activated clotting time 0.889 0.998 0.997–1.021 0.90 Albumin 0.347 0.742 0.398–1.383 0.41 ALT 0.175 1.007 0.997–1.017 0.23 ALP 0.884 1.000 0.999–1.001 0.90

Actinomyces spp cultured 0.176 0.425 0.123–1.468 0.23 E coli cultured 0.664 1.234 0.478–3.190 0.70 Anaerobic bacteria cultured 0.241 0.630 0.308–1.980 0.30 Aerobic bacteria cultured 0.231 2.487 0.559–11.065 0.29 Gram-negative bacteria cultured 0.615 0.376 0.210–1.801 0.66 Gram-positive bacteria cultured 0.363 0.617 0.490–4.198 0.42 Medical treatment 0.003 5.43 1.75–14.29 NA No. of thoracic lavages/d 0.754 1.014 0.934–1.100 0.78

CI = Confidence interval. See Tables 1 and 3 for remainder of key.

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could not be performed on gastrointestinal signs at thetime of initial examination and individual species ofbacteria cultured (except Escherichia coli andActinomyces spp), because there were not enough datapoints in each group to make valid comparisons.

Two Cox regression models relating treatmentgroup (medical vs surgical) to DFI were found to besignificant. The first model contained terms for med-ical treatment and isolation of Actinomyces spp (likeli-hood ratio test; P = 0.004). After adjusting for whetherActinomyces organisms were or were not isolated, dogsthat underwent medical treatment had a significantlydifferent DFI than did dogs treated surgically (hazardratio, 4.96; 95% CI, 1.56 to 15.78). The second modelcontained terms for medical treatment and detection ofmediastinal or pulmonary lesions at the time of initialexamination (likelihood ratio test; P < 0.001). Afteradjusting for whether mediastinal or pulmonarylesions were detected, dogs that underwent medicaltreatment had a significantly different DFI than diddogs treated surgically (hazard ratio, 6.97; 95% CI,2.16 to 22.46).

DiscussionResults of the present study suggest that surgical

treatment was associated with a better outcome thanmedical treatment in dogs with pyothorax. Presumably,this was because surgical debridement and resection ofaffected tissues reduced the amount of fibrotic andnecrotic tissue, decreased the number of bacteria, andallowed better penetration of antimicrobials. Surgicaldebridement likely also removed foreign material and tis-sue undergoing inflammatory changes, including fibro-sis. Consequently, it was not surprising that surgicaltreatment was associated with a more favorable outcome.

In the present study, 12 dogs that were initiallytreated medically underwent surgery because of a per-ceived failure of medical treatment. The decision topursue surgery in these dogs was made at the discre-tion of the attending clinician and was apparentlydetermined on the basis of results of daily cytologicevaluations of the pleural fluid, daily physical exami-nations, and daily or intermittent CBC. However, giventhe retrospective nature of the study, determiningattending clinicians’ criteria for pursuing surgery wasdifficult. In particular, many clinicians used differingterminology for recording results of daily cytologicevaluations, making objective interpretation and analy-sis difficult.

In previous studies1-4,13 of dogs with pyothorax,most dogs were treated medically, whereas in the pre-sent study, most dogs were treated surgically. In 12dogs in the present study, medical treatment failed, anddogs subsequently underwent surgery. Breed, age, andsex of dogs included in the present study and bacteriacultured from pleural fluid samples from these dogswere similar to those in previous reports.1-4,14,15 Medicalmanagement protocols used in this study were alsosimilar to those in previous reports. However, outcomeof dogs treated medically in the present study was quitedifferent from outcome of dogs in previous studies1-4,13

that were treated medically (56 to 100% success rate).It is possible that dogs in the present study had more

chronic disease, but this could not be confirmed giventhe retrospective nature of the study. Given the insidi-ous nature of the disease, differences between acuteand chronic pyothorax may not be clinically apparent.In a previous report2 of 9 hunting dogs with pyothorax,all 9 survived. However, Actinomyces spp was not iso-lated from any of the dogs, nor was plant materialrecovered.

In humans, empyema is commonly a result ofpneumonia, and medical management is advised forhuman patients with acute empyema.16-18 With chronicempyema, however, surgical intervention is advisedand has been associated with lower morbidity, recur-rence, and complication rates and shorter hospitaliza-tion times than medical treatment.19,20 In dogs withpyothorax, the disease is typically thought to be chron-ic at the time of diagnosis because of its insidiousnature and vague clinical signs.6 Thus, it appears logi-cal that surgical intervention may be warranted insome dogs with pyothorax. Given the nature of thestudy, it was not possible to accurately determine theduration of clinical signs prior to examination at theveterinary teaching hospital.

Results of the present study suggest that if medi-astinal or pulmonary lesions are seen on thoracic radi-ographs of dogs with pyothorax, surgical treatmentwill provide a better outcome than medical treatment.Although not confirmed in this study, radiographicallyapparent pulmonary lesions were likely associated withforeign body pneumonia, and surgical resection of theaffected lung lobes in dogs with foreign body pneumo-nia has been associated with a good prognosis.21 Inhuman patients with empyema, identification of radi-ographically apparent pulmonary lesions has not beendirectly determined to be a negative prognostic indica-tor. However, in a study22 of children with empyema,surgery was indicated with late referral to the hospital,empyema in the organizing phase, multiple loculationsunresponsive to thoracic drainage, and complicationsthat impaired lung expansion.

Pneumothorax was identified in 4 dogs in the pre-sent study. In these dogs, pneumothorax was presumedto be a result of rupture of the lung parenchyma sec-ondary to abscess formation or pneumonia. Given thatall dogs had radiographic evidence of pleural effusion,radiographic evidence of mild mediastinal or pul-monary disease may have been obscured.

Findings in this study also indicated that surgicaltreatment is warranted if Actinomyces organisms areisolated from the pleural fluid. Because Actinomycesspp is most frequently associated with the presence ofplant material (eg, grass awns), surgical debridement isrecommended.13,23 However, localization and removalof small foreign bodies in dogs with pyothorax can bedifficult, and a grass awn was identified in resected tis-sue from only 1 dog in this study. On the other hand,all 8 dogs in the present study for which Actinomycesspp were isolated from pleural fluid responded well toen bloc resection of affected tissue, suggesting that anyforeign body that may have been causing pyothorax inthese dogs likely was resected.

In the present study, neither the total WBC countnor the band neutrophil fraction was associated with

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outcome. In many dogs in this study, the percentage ofWBC that were band neutrophils was not excessive.Immature neutrophils have been reported as an indica-tor of outcome in dogs with sepsis.24 However, dogswith pyothorax rarely have acute septicemia at the timeof initial examination. It is possible that pyothorax isso slowly progressive that the body has time to height-en local and systemic immune responses. Thus, thebone marrow may maintain adequate production ofneutrophils without releasing excessive numbers ofpremature forms into the circulation.25

Results of bacterial culture of pleural fluid sampleswere analyzed individually in the present study, as wellas after grouping as aerobic or anaerobic and gram-pos-itive or gram-negative. This was done to determinewhether 1 group of bacteria was more pathogenic thananother, influencing outcome. Anaerobic bacteria arethought to increase the severity of mixed bacterialinfections,26 and gram-negative bacteria are consideredhighly pathogenic under certain conditions.27 However,no bacterial grouping was associated with DFI or out-come in the present study.

Results of antimicrobial susceptibility testing ofaerobic bacteria isolated from pleural fluid from dogsin the present study suggest that cefoxitin,enrofloxacin, and trimethoprim-sulfonamide would begood empirical choices for antimicrobial treatmentwhile results of bacterial culture and susceptibility test-ing are pending. Of these 3, enrofloxacin has the bestgram-negative spectrum and can be given IV.Antimicrobial susceptibility testing of anaerobic bacte-ria is difficult and expensive, but these bacteria gener-ally are susceptible to a variety of antimicrobials, suchas penicillin.26,28 Similarly, Actinomyces spp typically aresusceptible to ampicillin.29 Thus, a combination ofenrofloxacin and ampicillin may be considered theantimicrobial treatment of choice while results of bac-terial culture and susceptibility testing are pending indogs with pyothorax. Although enrofloxacin was com-monly given IV in this study, it is not approved for IVadministration in dogs.

There are inherent limitations to retrospectivestudies. In particular, inclusion of results for dogs thatwere initially treated medically and underwent surgeryafter medical treatment was considered to have failedlikely introduced some bias, so that dogs in the surgi-cal treatment group may have been more severelyaffected than dogs in the medical treatment group. Inaddition, treatment (medical vs surgical) was not stan-dardized but allocated at the attending clinician’s dis-cretion. Thus, confounding effects cannot be ruled out.

The play-the-winner study design has been usedmainly in prospective studies, and in these studies, cri-teria are established in advance to decide when tochange treatment. In the present study, we relied on theattending clinicians’ discretion to determine when tochange from medical to surgical treatment. We electedto use the play-the-winner design to analyze results ofthe present study because it more accurately reflectsthe clinical setting,10-12 where treatment is oftenchanged if the initial treatment fails. If a more tradi-tional study design had been used to analyze data inthe present study, data obtained after a change in treat-

ment would have been lost. Use of play-the-winneranalyses allowed inclusion of long-term follow-up datafor 12 dogs in which medical treatment failed andsurgery was undertaken.

In the present study, dogs that were euthanatizedbecause of recurrence of pyothorax or treatment failurewere classified as having died of pyothorax. This mayhave affected DFI, because different owners were like-ly committed to different levels of treatment. The issueof how, in these types of studies, to classify dogs thathave been euthanatized is controversial. Two othermethods of dealing with this problem have been pro-posed: exclusion of all animals that were euthanatizedand classification of all dogs that were euthanatized ashaving died of causes unrelated to the underlying dis-ease. Both of these methods are valid but have the dis-advantage of decreasing the number of animals includ-ed in the study or of disregarding the likelihood thatanimals were euthanatized because of a perceived poorquality of life secondary to the underlying disease. Forthe present study, animals were included only if theyhad undergone surgery or had been treated medicallyfor at least 2 days. Thus, dogs that were euthanatizedwithin 24 hours after the initial examination wereexcluded. In addition, 1 dog that was euthanatizedafter 20 days of medical treatment was excluded fromthe study because the decision was made on the basisof financial concerns and not quality of life reasons.

Given the low number of subjects and subsequentlow power of some results, we cannot conclude thatthere is not a difference between treatment groups inregard to some of the variables evaluated.30 A multi-center study would help increase the number of casesbut might increase the diversity of treatment modali-ties. A prospective multicenter study with standardizedtreatment protocols would be ideal in evaluating riskfactors and DFI following medical versus surgical treat-ment of pyothorax.

Data obtained at the time of the initial examina-tion cannot be relied on solely to determine whethermedical or surgical treatment should be performed,and overall clinical assessments should be considered.In addition, results of the present study suggest thatdogs with pyothorax that are treated surgically have abetter prognosis and longer DFI than dogs treatedmedically. Furthermore, surgical intervention shouldbe considered if there is radiographic evidence of medi-astinal or pulmonary lesions or if Actinomyces organ-isms are isolated from the pleural fluid.

aStatview, SAS Institute Inc, Cary, NC.bSolo power analysis, BMDP Statistical Software, Los Angeles, Calif.

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