Medical and surgical treatment of pyothorax in dogs_26 cases

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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 with pyothorax treated medically versus surgically and to identify prognostic indicators for dogs with pyothorax.

Design—Retrospective study.

Animals—26 dogs.

Procedure—Medical records were reviewed to obtain information regarding signalment, results of physical and laboratory evaluations at the time of initial examination, results of bacterial culture of pleural fluid, radiographic abnormalities, treatment (surgical vs medical), complications, whether the disease recurred, disease-free interval, survival time, and cause of death.

Results—Calculated proportions of dogs free from disease 1 year after treatment were 25 and 78%, respectively, for dogs treated medically and surgically. Treatment was 5.4 times as likely to fail in dogs treated medically as in dogs treated surgically. Two regression models relating treatment group (medical vs surgical) to disease-free interval were found to be significant. The first contained terms for medical treatment and isolation of Actinomyces spp from pleural fluid; the second contained terms for medical treatment and radiographic detection of mediastinal or pulmonary lesions at the time of initial examination.

Conclusions and Clinical Relevance—Results suggest that surgical treatment is associated with a better outcome than medical treatment in dogs with pyothorax. In addition, surgery should be considered if radiographic evidence of mediastinal or pulmonary lesions is detected or if Actinomyces spp is isolated from the pleural fluid. (J Am Vet Med Assoc 2002; 221:86–92)

P

yothorax is a relatively rare but life-threatening disease of dogs that is characterized by septic thoracic effusion.1-6 Infection in affected dogs is typically polymicrobial, with frequent isolation of anaerobic or filamentous bacteria.5 Reported sources of infection include inhaled or ingested plant material, penetrating thoracic wounds, and esophageal perforation.

Infection may also be a result of hematogenous or direct spread of infection from elsewhere in the body or may be iatrogenically induced.5 Dogs are rarely examined during the acute stage of infection, and clinical signs are typically a result of chronic disease.6 Common clinical signs include lethargy, dyspnea, and coughing.1-6 Actinomyces spp organisms have frequently been cultured from the pleural fluid of dogs with pyothoFrom the Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

Address correspondence to Dr. Rooney.

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rax.1,3,4 It has been suggested that Actinomyces spp infection is associated with plant material and that resolution of the pyothorax without excision of this plant material may be difficult.1 To the authors’ knowledge, however, the type of microbial organisms cultured from pleural fluid from dogs with pyothorax has not been associated with outcome.

Treatment of pyothorax in dogs involves administration of antimicrobials, intermittent or continuous thoracic drainage, thoracic lavage, and surgery.1-4 However, determining the most appropriate treatment for any individual dog is difficult at best. Surgical treatment has been recommended for other infectious diseases, such as peritonitis,7,8 septic arthritis,9 and abscesses. Therefore, even though surgery may be associated with high morbidity rates in dogs with pyothorax, it may potentially improve outcome. Although mortality rates ranging from 0 to 44% in dogs treated medically or surgically for pyothorax have been reported,1-4 to our knowledge, outcomes of dogs with pyothorax treated medically versus surgically have not been evaluated. The purposes of the study reported here were to compare outcomes of dogs with pyothorax treated medically versus surgically and to identify prognostic indicators for dogs with pyothorax. The outcome of interest in this study was disease-free interval (DFI).

Criteria for Selection of Cases Medical records of dogs examined at the Colorado State University Veterinary Teaching Hospital between 1991 and 2001 because of pyothorax were reviewed.

Dogs were eligible for inclusion in the study if bacteria were evident during cytologic examination of a pleural fluid sample or if bacterial culture of a sample of pleural fluid yielded bacterial growth. Dogs were excluded from the study if they had been treated prior to examination at the veterinary teaching hospital or if they were determined to have a systemic disease that could alter survival time.

Procedures Medical management was defined as having all of the following: systemic intravenous antimicrobial treatment, thoracic drainage (unilateral or bilateral) with aspiration (continuous or intermittent), and thoracic lavage. Dogs were included in the medical treatment group only if they had received at least 2 days of medical treatment. Surgical management was defined as thoracotomy with excision of affected tissues and open pleural lavage; postoperative management followed the same protocol as medical management. All dog were treated with antimicrobials for a long period.

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Statistical analyses—The Kaplan-Meier productlimit method was used to construct DFI curves for dogs with pyothorax treated medically or surgically.

Proportions of dogs free from disease 6 months and 1 year after the initial diagnosis of pyothorax were determined from the DFI curves. Mean DFI was defined as the mean value of the cumulative survival function; the SE was defined as the SE about the estimated mean of the cumulative survival function. A log-rank test was used to compare DFI curves.

Multivariate stepwise Cox regression analyses were used to determine whether surgical treatment was an independent predictor of DFI. Likelihood ratio tests JAVMA, Vol 221, No. 1, July 1, 2002

were used to determine whether the resulting models were associated with DFI. Confidence intervals (CI) were calculated to determine whether explanatory variables included in the models were significant. The Cox model assumes proportionality of effects (ie, constant hazard rate ratios) across time. This assumption was automatically verified by the statistical packagea used for the analyses and by comparing parallelism in log cumulative hazard plots. In each model, the reference group (eg, dogs in which Actinomyces spp was isolated from pleural fluid) was compared with all other dogs in the study (eg, dogs in which Actinomyces spp was not isolated from pleural fluid). Frequencies of isolation of specific bacteria were compared between medical and surgical treatment groups with χ2 analyses. Data were compared between medical and surgical treatment groups with ANOVA. Computer software packagesa,b were used to perform the statistical analyses. Values of P < 0.05 were considered significant. Results are given as mean ± SE. Results Twenty-six dogs met the criteria for inclusion in the study. The cause of pyothorax was definitively determined in only 1 dog in which plant material was identified during histologic examination of excised tissue. None of the dogs had a history of penetrating wounds or previous medical or surgical treatment that could have caused nosocomial or iatrogenic pyothorax. There were 5 Labrador Retrievers, 3 Golden Retrievers, 3 Border Collies, 2 Chesapeake Bay Retrievers, 2 English Springer Spaniels, 1 German Shepherd Dog, 1 Irish Wolfhound, 1 Bernese Mountain Dog, 1 Samoyed, 1 Rottweiler, 1 Beagle, 1 Treeing Walker Coonhound, 1 Weimaraner, 1 Great Pyrenees, 1 Saint Bernard, and 1 mixed-breed dog.

There were 17 males and 9 females. Mean ± SE age was 3.9 ± 0.56 years. Mean weight was 25.3 ± 2.4 kg (55.7 ± 1.1 lb). Mean rectal temperature at the time of initial examination was 39.6 ± 0.14 C (103.3 ± 0.25 F). Seventeen of 26 dogs were examined because of respiratory distress; 6 of 26 had vomiting or diarrhea.

Four of 26 had pneumothorax at the time of initial examination, and 2 of 26 had thrombosis of the posterior vena cava, which was confirmed at surgery.

All dogs had radiographic evidence of pleural effusion. Fourteen dogs had signs of mediastinal or pulmonary lesions (ie, interstitial consolidation, alveolar consolidation, atelectasis, or masses of soft tissue density). The right cranial lung lobe was involved in 5 dogs, the right middle lung lobe was involved in 8 dogs, the right caudal lung lobe was involved in 8 dogs, the accessory lung lobe was involved in 3 dogs, the left cranial lung lobe was involved in 4 dogs, the left caudal lung lobe was involved in 8 dogs, and the mediastinum was involved in 3 dogs. Twelve dogs had lesions involving > 1 anatomic location on thoracic radiographs.

Twelve of the 14 dogs with radiographic evidence of mediastinal or pulmonary lesions underwent surgery.

Of these 12, 8 underwent surgery only after medical treatment failed, and 4 underwent surgery within 2 days after initial examination. The remaining 2 dogs with radiographic evidence of mediastinal or pulmonary lesions had only medical treatment.

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Information obtained from the medical records included signalment (breed, age, sex, and body weight), clinical signs at the time of initial examination at the veterinary teaching hospital (respiratory distress, gastrointestinal signs, rectal temperature, and concurrent diseases), results of hematologic and biochemical analyses performed at the time of initial examination (absolute and differential WBC counts, platelet count, PCV, blood glucose concentration, activated clotting time, albumin concentration, and alanine aminotransferase [ALT] and alkaline phosphatase [ALP] activities), results of bacterial culture of a sample of pleural fluid collected at the time of initial examination, whether there was radiographic evidence of lung parenchymal disease, treatment (medical vs surgical), complications, whether the pyothorax recurred, cause of death, DFI, and survival time. The Cox proportional hazard ratio was calculated for each variable to determine whether it would be useful in predicting DFI. Data on antimicrobial susceptibility were tabulated.

Follow-up time was defined as time from initial examination at the veterinary teaching hospital to time of last contact with the owner or referring veterinarian or time of last entry in the medical record. Treatment was considered to have failed if the disease progressed despite appropriate medical or surgical treatment, if the disease recurred, or if the dog died of pyothorax.

Treatment was considered successful if the dog was discharged from the hospital and the disease did not recur.

A response-adaptive method10-12 (ie, play-the-winner) study design was chosen to analyze the data in order to more closely reflect the clinical setting and increase the amount of information per patient. Dogs in which medical treatment was considered to have failed because of progression of disease underwent surgery. However, dogs initially entered in the medical treatment group and then in the surgical treatment group were counted as single patients. Thus, when comparing data between surgical and medical treatment groups, dogs that underwent surgery at any time during treatment were considered in the surgical treatment group. Dogs that had a successful outcome following treatment, died of other causes, or were lost to follow-up were censored in the analysis. Dogs in which treatment failed were not censored. Records were reviewed in an attempt to determine the attending clinicians’ basis for deciding whether to perform surgery, but given the retrospective nature of the study, objective interpretation was impossible.

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negative and gram-positive organisms was isolated from 15 dogs. There were 39 aerobic and 30 obligate anaerobic organisms. A combination of aerobic and anaerobic organisms was isolated from 11 dogs.

Antimicrobial susceptibility was determined for 19 aerobic organisms (Table 2).

Actinomyces spp was isolated from pleural fluid collected at the time of initial examination from 12 dogs; 4 of the 12 were hunting breeds. One dog was treated medically and died. Four were treated surgically and survived; in all 4, pyothorax did not recur for at least 1 year after surgery. Four dogs were treated medically for 4 to 14 days and subsequently underwent surgery because of failure of medical treatment. All 4 survived, but 1 had a recurrence 245 days after initial examination. The remaining 3 dogs were successfully treated medically.

Seven dogs received medical treatment alone and were classified in the medical treatment group. Twelve dogs 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 been treated surgically. The remaining 7 dogs underwent surgery within 2 days after initial examination. Thus, 19 dogs were classified in the surgical treatment group.

Unilateral thoracic drainage with intermittent or continuous suction was used in 3 dogs in the medical treatment group; in the remaining 23 dogs, bilateral thoracic drainage was used. In all 26 dogs, intermittent

Table 1—Results of bacterial culture of pleural fluid from dogs with pyothorax treated medically (n = 7) or surgically (19) No. of isolates Medical treatment

Surgical treatment

P value

Acinobacter spp Actinomyces spp Bacteroides spp Enterococcus faecium Escherichia coli

1 4 3 1 7

0 8 4 1 7

NA 0.35 0.54 NA 0.99

Fusobacterium spp Staphylococcus spp or Streptococcus spp Pasteurella spp Peptostreptococcus spp Prevotella spp

4

5

0.99

5 4 1 2

3 4 1 4

0.58 NA NA 0.99

Bacteria

NA = Not applicable.

For all dogs, results of bacterial culture of a sample of pleural fluid collected at the time of initial examination were positive (Table 1). Follow-up bacterial culture of pleural fluid samples collected during treatment did not yield any new organisms. For 12 of the 26 dogs, multiple bacterial organisms were isolated; a total of 69 isolates were cultured from pleural fluid from the 26 dogs. Forty-five of the isolates were gramnegative, and 12 were gram-positive; the remaining 12 isolates were anaerobic filamentous organisms with variable gram staining results. A combination of gram-

Table 2—Antimicrobial susceptibility of aerobic bacteria isolated from the pleural fluid of dogs with pyothorax No. susceptible

P multocida (n = 7)

E faecium (n = 2)

Staphylococcus spp (n = 2)

Streptococcus spp (n = 2)

E coli (n = 5)

Acinobacter spp (n = 1)

Total (n = 19)

Amikacin Amoxicillin-clavulanate Ampicillin Cefoxitin Cephalothin

2 7 7 7 7

2 2 2 1 1

2 2 1 2 2

1 2 2 2 2

5 2 3 5 3

1 1 1 1 1

13 16 16 18 16

Chloramphenicol Enrofloxacin Erythromycin Gentamicin Tetracycline Trimethoprim-sulfonamide

7 7 7 4 7 7

1 1 2 2 2 2

2 2 2 2 2 2

2 2 2 2 1 2

4 5 1 5 4 5

1 1 1 1 1 1

17 18 11 16 17 19

Antimicrobial

Antimicrobial susceptibility was determined with the disk diffusion method.

Table 3—Comparison of signalment and results of physical and laboratory evaluations performed at the 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) Weight (kg) Rectal temperature (C) WBC count (cells/µl) Band neutrophil fraction (%) Platelet count (platelets/µl)

60.0 16.5 23.7 3.9 39.6 0.27 24,800 6,500 4.00 2.20 356,900 81,700

44.8 7.2 25.9 2.4 39.3 0.16 22,800 3,000 7.60 2.70 235,300 25,500

0.33 0.63 0.64 0.74 0.50 0.77

0.56 0.71 0.72 0.80 0.59 0.82

35.4 2.9 87.0 5.9 137.0 42.5 2.6 0.3 31.3 13.1 196.6 49.4

37.6 2.4 98.9 5.2 131.1 6.3 2.3 0.2 44.7 12.0 242.3 114.5

0.62 0.21 0.76 0.40 0.55 0.83

0.69 0.32 0.81 0.50 0.64 0.86

PCV (%) Blood glucose (mg/dl) Activated clotting time (s) Albumin (g/dl) ALT (U/L) ALP (U/L)

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 antimicrobials were administered IV. All 19 dogs that underwent surgery had a median sternotomy with partial excision of the mediastinum. In 10 dogs, a pericardiectomy was performed. In 11 dogs, 1 or more lung lobes were removed. Histologic examination of the excised pulmonary tissue revealed suppurative inflammation in 6 dogs and pyogranulomatous inflammation in 5. Dogs classified in the surgical treatment group did not differ from dogs classified in the medical treatment group in regards to any of the variables recorded at the time of initial examination (Table 3).

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

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

Hazard ratio

95% CI

Power

Age Sex Body weight Respiratory distress Rectal temperature Pulmonary lesions Gastrointestinal signs

0.827 0.318 0.740 0.655 0.612 0.334 0.582

1.001 0.139 1.007 1.266 1.100 1.597 1.298

0.989–1.014 0.160–2.921 0.964–1.053 0.450–3.600 0.259–1.595 0.617–4.133 0.511–3.295

0.84 0.37 0.77 0.70 0.65 0.39 0.63

WBC count Band neutrophil fraction Platelet count PCV Blood glucose Activated clotting time Albumin ALT ALP

0.153 0.828 0.345 0.535 0.321 0.889 0.347 0.175 0.884

1.022 0.994 0.999 0.983 1.011 0.998 0.742 1.007 1.000

0.992–1.053 0.946–1.045 0.996-1.001 0.932–1.037 0.990–1.032 0.997–1.021 0.398–1.383 0.997–1.017 0.999–1.001

0.21 0.85 0.41 0.59 0.38 0.90 0.41 0.23 0.90

Actinomyces spp cultured E coli cultured Anaerobic bacteria cultured Aerobic bacteria cultured Gram-negative bacteria cultured Gram-positive bacteria cultured Medical treatment No. of thoracic lavages/d

0.176 0.664 0.241 0.231 0.615 0.363 0.003 0.754

0.425 1.234 0.630 2.487 0.376 0.617 5.43 1.014

0.123–1.468 0.478–3.190 0.308–1.980 0.559–11.065 0.210–1.801 0.490–4.198 1.75–14.29 0.934–1.100

0.23 0.70 0.30 0.29 0.66 0.42 NA 0.78

Variable

CI = Confidence interval.

See Tables 1 and 3 for remainder of key.

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Figure 1—Disease free interval curves for dogs treated medically or surgically for pyothorax. Circles represent end points of censored data.

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

Pyothorax recurred 40 days after initial examination in 1 dog that underwent surgery, 245 days after initial examination in 1 dog that was treated medically and then underwent surgery, and 186 days after initial examination in 1 dog that was treated medically. The dog in which pyothorax had been treated medically died while being treated for recurrence of the disease.

The other 2 dogs in which pyothorax recurred survived; 1 was treated medically, and the other was treated surgically.

Five dogs died of pyothorax. Two dogs treated surgically died of pyothorax 2 and 9 days after initial examination. One dog died 9 days after initial examination following 5 days of medical treatment and surgery. Two dogs treated medically died of pyothorax 5 and 186 days after initial examination.

The remaining 15 dogs, including 3 dogs treated medically, 7 dogs treated medically that underwent surgery after medical treatment failed, and 5 dogs treated surgically, all survived.

Complications were observed in 6 dogs treated surgically and included disseminated intravascular coagulation (4 dogs) and postoperative abdominal effusion (2). Two dogs treated medically had hemorrhage from the site of chest tube placement and required blood transfusions.

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

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could not be performed on gastrointestinal signs at the time of initial examination and individual species of bacteria cultured (except Escherichia coli and Actinomyces spp), because there were not enough data points in each group to make valid comparisons.

Two Cox regression models relating treatment group (medical vs surgical) to DFI were found to be significant. The first model contained terms for medical treatment and isolation of Actinomyces spp (likelihood ratio test; P = 0.004). After adjusting for whether Actinomyces organisms were or were not isolated, dogs that underwent medical treatment had a significantly different DFI than did dogs treated surgically (hazard ratio, 4.96; 95% CI, 1.56 to 15.78). The second model contained terms for medical treatment and detection of mediastinal or pulmonary lesions at the time of initial examination (likelihood ratio test; P < 0.001). After adjusting for whether mediastinal or pulmonary lesions were detected, dogs that underwent medical treatment had a significantly different DFI than did dogs treated surgically (hazard ratio, 6.97; 95% CI, 2.16 to 22.46).

Discussion Results of the present study suggest that surgical treatment was associated with a better outcome than medical treatment in dogs with pyothorax. Presumably, this was because surgical debridement and resection of affected tissues reduced the amount of fibrotic and necrotic tissue, decreased the number of bacteria, and allowed better penetration of antimicrobials. Surgical debridement likely also removed foreign material and tissue undergoing inflammatory changes, including fibrosis. Consequently, it was not surprising that surgical treatment was associated with a more favorable outcome.

In the present study, 12 dogs that were initially treated medically underwent surgery because of a perceived failure of medical treatment. The decision to pursue surgery in these dogs was made at the discretion of the attending clinician and was apparently determined on the basis of results of daily cytologic evaluations of the pleural fluid, daily physical examinations, and daily or intermittent CBC. However, given the retrospective nature of the study, determining attending cliniciansâ&#x20AC;&#x2122; criteria for pursuing surgery was difficult. In particular, many clinicians used differing terminology for recording results of daily cytologic evaluations, making objective interpretation and analysis difficult.

In previous studies1-4,13 of dogs with pyothorax, most dogs were treated medically, whereas in the present study, most dogs were treated surgically. In 12 dogs in the present study, medical treatment failed, and dogs subsequently underwent surgery. Breed, age, and sex of dogs included in the present study and bacteria cultured from pleural fluid samples from these dogs were similar to those in previous reports.1-4,14,15 Medical management protocols used in this study were also similar to those in previous reports. However, outcome of dogs treated medically in the present study was quite different 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 90

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chronic disease, but this could not be confirmed given the retrospective nature of the study. Given the insidious nature of the disease, differences between acute and 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 isolated from any of the dogs, nor was plant material recovered.

In humans, empyema is commonly a result of pneumonia, and medical management is advised for human patients with acute empyema.16-18 With chronic empyema, however, surgical intervention is advised and has been associated with lower morbidity, recurrence, and complication rates and shorter hospitalization times than medical treatment.19,20 In dogs with pyothorax, the disease is typically thought to be chronic at the time of diagnosis because of its insidious nature and vague clinical signs.6 Thus, it appears logical that surgical intervention may be warranted in some dogs with pyothorax. Given the nature of the study, it was not possible to accurately determine the duration of clinical signs prior to examination at the veterinary teaching hospital.

Results of the present study suggest that if mediastinal or pulmonary lesions are seen on thoracic radiographs of dogs with pyothorax, surgical treatment will provide a better outcome than medical treatment.

Although not confirmed in this study, radiographically apparent pulmonary lesions were likely associated with foreign body pneumonia, and surgical resection of the affected lung lobes in dogs with foreign body pneumonia has been associated with a good prognosis.21 In human patients with empyema, identification of radiographically apparent pulmonary lesions has not been directly determined to be a negative prognostic indicator. However, in a study22 of children with empyema, surgery was indicated with late referral to the hospital, empyema in the organizing phase, multiple loculations unresponsive to thoracic drainage, and complications that impaired lung expansion.

Pneumothorax was identified in 4 dogs in the present study. In these dogs, pneumothorax was presumed to be a result of rupture of the lung parenchyma secondary to abscess formation or pneumonia. Given that all dogs had radiographic evidence of pleural effusion, radiographic evidence of mild mediastinal or pulmonary disease may have been obscured.

Findings in this study also indicated that surgical treatment is warranted if Actinomyces organisms are isolated from the pleural fluid. Because Actinomyces spp is most frequently associated with the presence of plant material (eg, grass awns), surgical debridement is recommended.13,23 However, localization and removal of small foreign bodies in dogs with pyothorax can be difficult, and a grass awn was identified in resected tissue from only 1 dog in this study. On the other hand, all 8 dogs in the present study for which Actinomyces spp were isolated from pleural fluid responded well to en bloc resection of affected tissue, suggesting that any foreign body that may have been causing pyothorax in these dogs likely was resected.

In the present study, neither the total WBC count nor the band neutrophil fraction was associated with JAVMA, Vol 221, No. 1, July 1, 2002

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ment would have been lost. Use of play-the-winner analyses allowed inclusion of long-term follow-up data for 12 dogs in which medical treatment failed and surgery was undertaken.

In the present study, dogs that were euthanatized because of recurrence of pyothorax or treatment failure were classified as having died of pyothorax. This may have affected DFI, because different owners were likely committed to different levels of treatment. The issue of how, in these types of studies, to classify dogs that have been euthanatized is controversial. Two other methods of dealing with this problem have been proposed: exclusion of all animals that were euthanatized and classification of all dogs that were euthanatized as having died of causes unrelated to the underlying disease. Both of these methods are valid but have the disadvantage of decreasing the number of animals included in the study or of disregarding the likelihood that animals were euthanatized because of a perceived poor quality of life secondary to the underlying disease. For the present study, animals were included only if they had undergone surgery or had been treated medically for at least 2 days. Thus, dogs that were euthanatized within 24 hours after the initial examination were excluded. In addition, 1 dog that was euthanatized after 20 days of medical treatment was excluded from the study because the decision was made on the basis of financial concerns and not quality of life reasons.

Given the low number of subjects and subsequent low power of some results, we cannot conclude that there is not a difference between treatment groups in regard to some of the variables evaluated.30 A multicenter study would help increase the number of cases but might increase the diversity of treatment modalities. A prospective multicenter study with standardized treatment protocols would be ideal in evaluating risk factors and DFI following medical versus surgical treatment of pyothorax.

Data obtained at the time of the initial examination cannot be relied on solely to determine whether medical or surgical treatment should be performed, and overall clinical assessments should be considered.

In addition, results of the present study suggest that dogs with pyothorax that are treated surgically have a better prognosis and longer DFI than dogs treated medically. Furthermore, surgical intervention should be considered if there is radiographic evidence of mediastinal or pulmonary lesions or if Actinomyces organisms are isolated from the pleural fluid. a

Statview, SAS Institute Inc, Cary, NC. Solo power analysis, BMDP Statistical Software, Los Angeles, Calif.

b

References 1. Frendin J. Pyogranulomatous pleuritis with empyema in hunting dogs. Zentralbl Veterinarmed [A] 1997;44:167–178.

2. Piek CJ, Robben JH. Pyothorax in nine dogs. Vet Q 2000;22:107–111.

3. Robertson SA, Stoddart ME, Evans RJ, et al. Thoracic empyema in the dog: a report of twenty-two cases. J Small Anim Pract 1983;24:103–119.

4. Turner WD, Breznock EM. Continuous suction drainage for management of canine pyothorax—a retrospective study. J Am Anim Hosp Assoc 1988;24:485–494.

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outcome. In many dogs in this study, the percentage of WBC that were band neutrophils was not excessive.

Immature neutrophils have been reported as an indicator of outcome in dogs with sepsis.24 However, dogs with pyothorax rarely have acute septicemia at the time of initial examination. It is possible that pyothorax is so slowly progressive that the body has time to heighten local and systemic immune responses. Thus, the bone marrow may maintain adequate production of neutrophils without releasing excessive numbers of premature forms into the circulation.25 Results of bacterial culture of pleural fluid samples were analyzed individually in the present study, as well as after grouping as aerobic or anaerobic and gram-positive or gram-negative. This was done to determine whether 1 group of bacteria was more pathogenic than another, influencing outcome. Anaerobic bacteria are thought to increase the severity of mixed bacterial infections,26 and gram-negative bacteria are considered highly pathogenic under certain conditions.27 However, no bacterial grouping was associated with DFI or outcome in the present study.

Results of antimicrobial susceptibility testing of aerobic bacteria isolated from pleural fluid from dogs in the present study suggest that cefoxitin, enrofloxacin, and trimethoprim-sulfonamide would be good empirical choices for antimicrobial treatment while results of bacterial culture and susceptibility testing are pending. Of these 3, enrofloxacin has the best gram-negative spectrum and can be given IV. Antimicrobial susceptibility testing of anaerobic bacteria is difficult and expensive, but these bacteria generally are susceptible to a variety of antimicrobials, such as penicillin.26,28 Similarly, Actinomyces spp typically are susceptible to ampicillin.29 Thus, a combination of enrofloxacin and ampicillin may be considered the antimicrobial treatment of choice while results of bacterial culture and susceptibility testing are pending in dogs with pyothorax. Although enrofloxacin was commonly given IV in this study, it is not approved for IV administration in dogs.

There are inherent limitations to retrospective studies. In particular, inclusion of results for dogs that were initially treated medically and underwent surgery after medical treatment was considered to have failed likely introduced some bias, so that dogs in the surgical treatment group may have been more severely affected than dogs in the medical treatment group. In addition, treatment (medical vs surgical) was not standardized but allocated at the attending clinician’s discretion. Thus, confounding effects cannot be ruled out.

The play-the-winner study design has been used mainly in prospective studies, and in these studies, criteria are established in advance to decide when to change treatment. In the present study, we relied on the attending clinicians’ discretion to determine when to change from medical to surgical treatment. We elected to use the play-the-winner design to analyze results of the present study because it more accurately reflects the clinical setting,10-12 where treatment is often changed if the initial treatment fails. If a more traditional study design had been used to analyze data in the present study, data obtained after a change in treat-

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5. Green CE. Pleural infections. In: Green CE, ed. Infectious diseases of the dog and cat. 2nd ed. Philadelphia: WB Saunders Co, 1998;592–594.

6. Holmberg DL. Management of pyothorax. Vet Clin North Am Small Anim Pract 1979;9:357–362.

7. Greenfield CL, Walshaw R. Open peritoneal drainage for treatment of contaminated peritoneal cavity and septic peritonitis in dogs and cats: 24 cases (1980–1986). J Am Vet Med Assoc 1987;191:100–105.

8. King LG. Postoperative complications and prognostic indicators in dogs and cats with septic peritonitis: 23 cases (1989–1992).

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9. Smith MM. Orthopedic infections. In: Slatter D, ed.

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10. Hallstrom A, Brooks MM, Peckova M. Logrank, play the winner, power and ethics. Stat Med 1996;15:2135–2142.

11. Thall PF, Millikan RE, Sung HG. Evaluating multiple treatment courses in clinical trials. Stat Med 2000;19:1011–1028.

12. Yao Q, Wei LJ. Play the winner for phase II/III clinical trials. Stat Med 1996;15:2413–2423.

13. Walker AL, Jang SS, Hirsh DC. Bacteria associated with pyothorax of dogs and cats: 98 cases (1989–1998). J Am Vet Med Assoc 2000;216:359–363.

14. Frendin J, Obel N. Catheter drainage of pleural fluid collections and pneumothorax. J Small Anim Pract 1997;38:237–242.

15. Frendin J, Greko C, Hellmen E, et al. Abdominal wall swellings in dogs caused by foreign bodies. J Small Anim Pract 1994;35:499–508.

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