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How to treat pyothorax in dogs and cats | VETgirl Veterinary CE Podcasts

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How do you treat patients with pyothorax in your practice? Do you have a different protocol for dogs versus cats? Do you know they are likely to have different causes of pyothorax?

In today’s VETgirl online veterinary continuing education podcast, we review pyothorax in dogs and cats. Pyothorax is defined as the accumulation of septic purulent fluid within the pleural space, and it can cause life-threatening respiratory distress and sepsis. While the keys to therapy are generally agreed upon and consist of some type of thoracic drainage along with antimicrobial therapy, debate exists in human and veterinary medicine about medical versus surgical management. So, Stillion et al wanted to review and summarize the physiology and pathophysiology of the pleural space and the development of pyothorax, but also focus on the different etiologies in dogs versus cats as well as the recommended treatments. This was published in JVECC in 2015 in the paper “A clinical review of the pathophysiology, diagnosis, and treatment of pyothorax in dogs and cats.”

The pleural space is a potential space in the thoracic cavity lined by the parietal and visceral pleura. While some controversy exists as to whether the left and right sides of the pleural space are completely or incompletely separated by the mediastinum in dogs and cats, diseases of the pleural space are frequently bilateral, and the membrane can easily be disrupted or occluded secondary to inflammation. Normally, the pleural space has a small amount of fluid present to allow lubrication of the lungs and prevent friction during normal breathing. Starling’s forces (based on hydrostatic pressures, oncotic pressures, and tissue permeabilities) govern fluid flux in the pleural space so that in the normal thoracic cavity, a small amount of fluid is always being filtered into the pleural space and drained via the lymphatics.

When an exudative effusion begins to develop in the pleural space, secondary inflammatory changes occur that may worsen the effusion. Release of inflammatory cytokines and vasoactive mediators can affect capillary permeability and lymphatic flow, raise body temperature and cause endothelial damage, and thicken the pleura, preventing fluid resorption. Bacteria can enter the pleural space via damage anywhere along the respiratory tract (thoracic wall, trachea, bronchi, lung parenchyma) or via the esophagus.

In human patients, pyothorax develops in 5-15% of patients with bacterial pneumonia from parapneumonic spread.2 While the underlying cause of pyothorax in dogs and cats is often difficult to determine (only 2-22% of canine cases and 35-67% of feline cases have a determined route of infection), the most common etiologies in each species differ. In dogs, the most commonly documented cause is migrating grass awns or plant material. In cats, interestingly, bite wounds from other cats causing bacterial inoculation of the pleural space were thought to be most common, but more recently, parapneumonic spread has also been suggested.2 Since previous studies have shown that cats with pyothorax are more likely to live in multi-cat households, an increased risk for bite wounds was suspected. However, only 15.6% of cases in the literature (20/128 cats) had identifiable bite wounds, leading to the suggestion that cats in multi-cat households may have a higher incidence of upper respiratory infections, and thus risk of pneumonia and secondary pyothorax.

Clinical findings in patients with pyothorax may include signs of underlying respiratory disease (tachypnea, dyspnea, cough, restrictive breathing pattern) but may also be nonspecific and include things like weight loss, lethargy and fever. Thoracic auscultation will reveal dull lung sounds and muffled heart sounds in most cases, and patients may also have signs of SIRS or sepsis. Clinicopathologic findings are usually non-specific but may reveal evidence of underlying inflammation (for example, neutrophilic leukocytosis with or without a left shift). Patients presenting with signs of respiratory distress and pleural space disease should have thoracocentesis performed immediately – it is therapeutic and diagnostic! The presence of intracellular bacteria in the fluid on cytologic exam is diagnostic for pyothorax. Fluid should be sterilely collected (ideally before beginning antimicrobial therapy) and submitted for aerobic and anaerobic cultures. The most commonly isolated organisms in dogs and cats are E. coli, Pasteurella spp., Actinomyces spp., Nocardia spp., Streptococcus spp., Staphylococcus spp., and Corynebacterium spp. Polymicrobial infections and anaerobes are common, with Fusobacterium spp., Peptostreptococcus anaerobius and Bacteroides spp. often cultured.  Actinomyces and Nocardia spp. are often associated with inhalation of grass awns.

In human patients, further testing of pleural fluid, including evaluation of protein concentrations, lactate dehydrogenase, and pH has proven helpful in diagnosing pyothorax; these criteria have not been fully investigated in veterinary patients. Some biomarkers are also under investigation, and may prove helpful in the future. Imaging for cases of pyothorax can include ultrasound, radiographs or CT. Radiographs should be performed after thoracocentesis, and the most common signs of pleural effusion include retraction of the lung lobes from the chest wall, atelectasis, fissure lines and loss of the cardiac silhouette.

The two main components of treatment for pyothorax are antimicrobial therapy and some type of thoracic drainage. Antibiotics should be broad-spectrum to start, with tailoring once culture and sensitivity results are available. Potentiated penicillins in combination with a fluoroquinolone are reasonable; aminoglycosides should be avoided as they have poor penetration into the pleural space. While the ideal route of administration is unknown, IV therapy is recommended in human patients until they are afebrile; oral antibiotics should then be given, although the ideal duration has not been determined. Generally in veterinary medicine, antibiotics are continued two weeks beyond radiographic resolution of pleural effusion. Thoracic drainage is recommended in all cases of pyothorax to relieve the patient’s symptoms, minimize the potential for further procedures, remove as much infected fluid as possible and optimize imaging. Some type of continued drainage (either via placement of chest tubes with intermittent aspiration, or continuous suction) is recommended. Intermittent thoracocentesis is not recommended due to increased morbidity and risk. Chest tubes may be large bore (placed surgically) or small bore (placed via Seldinger technique) as both seem to be effective in draining fluid, and may be unilateral or bilateral depending on the distribution of fluid. Radiographs should always be performed following placement of chest tubes to ensure correct placement. There is no evidence to support flushing of the chest tubes, but possible benefits of lavage with warm saline include reduction of pleural fluid viscosity, facilitation of drainage, prevention of tube obstruction and dilution of bacteria. Little evidence exists to support intrapleural fibrinolytics or heparin. Chest tubes can be removed depending on clinical improvement, but can generally be considered when fluid production is less than 2 ml/kg/day, cytologic evidence of infection has resolved, and radiographs are improving.

The debate regarding medical versus surgical management of pyothorax in dogs and cats has been long-standing, and some evidence exists to support both therapies. In human patients, surgical intervention is recommended if patients are persistently septic after 5-7 days of medical therapy with antibiotics and chest tubes. In small animal patients, this could be extrapolated to mean that surgery is indicated if there is persistence of pleural effusion or infection after 3-7 days. If there is evidence of pulmonary or mediastinal lesions or foreign material based on imaging, surgical intervention is recommended in dogs and cats. Surgical options include open thoracotomy (median sternotomy) or video-assisted thoracic surgery, which is less invasive. Neither method has been shown to be better than the other, and the choice is largely dependent upon surgeon skill and experience. The prognosis for patients with pyothorax is good with appropriate treatment, and survival on average is 83% in dogs and 62% in cats based on a number of studies. Infection with Nocardia or Actinomyces and inhalation or migration of plant material may be risk factors for recurrence.

So, what can we take from this VETgirl podcast? This article is a nice review of the clinical aspects of pyothorax in veterinary patients, with some interesting comparisons to human medicine. Dogs and cats with pyothorax have a pretty good prognosis if they are treated appropriately with the two mainstays of therapy: antimicrobials and thoracic drainage. The medical versus surgical debate continues, but implementing medical management and monitoring for resolution in patients without an obvious surgical indication seems reasonable.

References:
1. Davies HE, Davies RJO, Davies CWH. Management of pleural infection in adults: British Thoracic Society pleural disease guideline 2010. Thorax 2010;65(Suppl 2):ii41-ii53.
2. Davies C, Forrester SD. Pleural effusion in cats: 92 cases (1987-1995). J Small Anim Pract 1996;37(5):217-224.
3. Epstein SE. Exudative pleural diseases in small animals. Vet Clin North Am Small Anim Pract 2014;44(1):161-180.
4. Stillion JR, Letendre J. A clinical review of the pathophysiology, diagnosis, and treatment of pyothorax in dogs and cats. J Vet Emerg Crit Care 2015;25(1):113-129.

Suggested reading:
Waddell LS, Brady CA, Drobatz KJ. Risk factors, prognostic indicators, and outcome of pyothorax in cats: 80 cases (1986-1999). J Am Vet Med Assoc 2002;15;221(6):819-24.

Monnet E. Medical treatment, surgical treatment, or both? CVC Proceedings, 2008.

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