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Dr Vanessa R Barrs discusses various forms of treatment of feline pyothorax. VANESSA R BARRS All small animal clinicians are likely to encounter feline pyothorax. Treatment of the condition carries a good prognosis with success rates of around 70 per cent (Waddell et al 2002). This article provides information on why and how pyothorax occurs, its diagnosis and treatment and a practical guide for choosing effective antibiotics. Pyothorax or thoracic empyema describes infection of the pleural space. Mechanisms of infection include haematogenous or lymphatic spread from a distant site (systemic sepsis), extension from an adjacent structure (bronchopneumonia, parapneumonic spread, oesophageal rupture, mediastinitis or subphrenic infection) or direct inoculation (penetrating trauma or foreign body, thoracocentesis or thoracic surgery). Most cases of feline pyothorax involve polymicrobial infections of obligate anaerobic and facultative bacteria, similar in composition to subcutaneous bite abscesses and to the bacterial flora of the normal feline oropharynx (Love et al 2000). Feline pyothorax is usually the result of contamination of the pleural space with oral flora. Organisms includeBacteroides sp,Fusobacterium sp,Peptostreptococcus sp,Clostridium sp,Actinomyces sp,Eubacterium sp,Propionibacterium sp,Prevotella sp,Filifactor villosus, Pasteurella multocida, Streptococcus sp andPorphyromonas sp. It was previously believed that direct inoculation of oral flora into the thorax from cat bite wounds was a common route of infection in young, free-roaming, entire male cats (Jonas 1983). However, given the current demographic of more neutered cats with restricted territories, this route of infection is now less common. In a large retrospective study of 80 cats with pyothorax, sexually intact males were not over-represented compared to a control population of 212 cats (Waddell et al 2002). In the same study, thoracic puncture wounds were identified in only four out of 25 cats autopsied. In most cases the mechanism of pleural space infection is not determined (Demetriou et al 2002, Waddell et al 2002). The author considers the most common mechanism of infection in polymicrobial pyothorax to be aspiration and colonisation of the lower respiratory tract by oropharyngeal flora. Direct extension of infection from the bronchi and lungs then results in pyothorax. Aspiration of oropharyngeal flora may occur subsequent to viral upper respiratory tract infection when local host defence mechanisms are compromised. Other risk factors for aspiration of oropharyngeal flora include ultrasonic tooth scaling procedures and general anaesthesia for routine procedures. One study has shown that cats with pyothorax are four times as likely to have come from multi-cat households compared to control cats (Waddell et al 2002). It was proposed that cats from multi-cat households were more likely to incur thoracic bite wounds than control cats. However, another equally likely scenario is that these cats were at greater risk of viral upper respiratory tract infection than control cats. Nine per cent of cats in the study had ocular or nasal discharges at presentation. The author has found that a recent history or clinical signs of upper respiratory tract infection are present in up to a third of cats diagnosed with pyothorax. Mycoplasma sp, also normal inhabitants of the feline oropharynx, may cause pyothorax after severe upper respiratory tract infections (Malik et al 1991). Less common causes of pyothorax include haematogenous spread of infection from septicaemic disease (for example, septic arthritis in young kittens); rupture or perforation of the oesophagus, trachea, or bronchi; migrating plant awns and parasitic migration. Not all cases of feline pyothorax are caused by oropharyngeal flora. Unusual pathogens include yeasts (Candida sp andCryptococcus sp),Staphylococcus sp,Rhodococcus equi ,Nocardia sp, enteric gram-negative organisms (E coli, Salmonella sp,Klebsiella sp,Proteus sp) and non-enteric gram-negative organisms (Pseudomonas sp). Current data indicates there is no breed or sex predisposition for feline pyothorax. Disease is most common in younger cats (mean age of four to five years), although cats of any age can be affected. (Walker et al 2000, Waddell et al 2002). Duration of clinical signs before presentation typically ranges from one to six weeks (Davies and Forrester 1996, Demetriou et al 2002). Anorexia or inappetences are the most common historical findings, followed by dyspnoea, lethargy and weakness. Less common historical findings include weight loss, signs of upper respiratory tract infection, coughing and vomiting. On presentation, over 90 per cent of affected cats are dehydrated and mentally dull. Over 80 per cent are dyspnoeic, tachypnoeic and have abnormal lung sounds on auscultation. Heart sounds may be muffled. The dyspnoea is inspiratory due to pleural effusion and compression atelectasis. Cats are usually in poor body condition and adopt a crouched sternally recumbent posture with elbows abducted to allow for maximal rib-cage expansion. In some cases, dyspnoea can be surprisingly subtle. While pyrexia is common, 15 per cent of cats are hypothermic. Hypothermic cats may be bradycardic, and this combination of clinical signs is indicative of severe sepsis (Brady et al 2000). Ocular and nasal discharges may also be present. 1.Diagnostic imaging In many cases, the clinician will be highly suspicious of pleural effusion based on a clinical examination. Confirmation is sought either by thoracic radiography or ultrasonography. Oxygen should be administered immediately to cats in severe respiratory distress using a mask or chamber. One dorsoventral radiographic view is adequate to confirm the presence of a pleural effusion in these cats since severe hypoxaemia may occur in cats with large volume effusions placed in lateral recumbency. Effusions are bilateral in 70 to 90 per cent of cases (Demetriou et al 2002, Waddell et al 2002). Radiographic signs of pleural effusion include retraction of the lobar borders from the thoracic wall, visibility of interlobar fissures and rounding or filling of the costophrenic angles. Thoracic ultrasonography is an expedient, non-invasive method for obtaining confirmation of pleural effusion. In contrast to transudates that are anechoic, the pleural fluid exudate in pyothorax is hypoechoic or complex echoic. The effusion is often septate due to fibrinous or fibrous tags extending between the parietal and visceral pleura. Pulmonary abscesses and restrictive pleuritis may also be identified ultrasonographically. 2. Thoracocentesis, cytology and culture Diagnostic thoracocentesis is performed (preferably using ultrasound-guidance) at the ventral third of the sixth, seventh or eighth intercostal space with the cat positioned in sternal recumbency. Care should be taken to avoid intercostal vessels and nerves located near the caudal rib margin. Cats usually tolerate thoracocentesis without sedation. A 21- or 23-gauge butterfly needle with extension tubing and three-way-tap is attached to a syringe for this purpose. Prior subcutaneous instillation of 1ml of local anaesthetic (eg two per cent lignocaine) at the thoracocentesis site helps facilitate the procedure. Mixed anaerobic infections are usually malodorous. Lack of odour should arouse suspicion for an unusual pathogen (eg aerobes, yeast orMycoplasma spp). Unless diagnostic imaging is indicative of a unilateral effusion, initial thoracocentesis should be bilateral to remove as much pleural fluid as possible prior to general anaesthesia and tube thoracostomy. Samples of pleural fluid should be collected in EDTA for cell counts and cytology, in a serum tube for biochemical analysis and in a sterile container for culture. Aerobic and anaerobic culture should be requested. For reliable anaerobic culture results, oxygen must be excluded from the transport specimen. Commercial anaerobic specimen collectors are available including the BD Vacutainer [superscript] Brand Anaerobic Specimen collector. This device allows collection and transport of liquid specimens with 72-hour viability of fragile anaerobic specimens. A built-in oxygen-eliminating system converts oxygen and hydrogen to water within the system to produce an anaerobic environment and an indicator changes colour to signal when anaerobiosis has been achieved within the device. Alternatively, some laboratories may accept syringes if they are plugged with a rubber bung and the plunger immobilised. It is essential to cytologically evaluate pleural fluid smears to determine the presence and morphology of bacteria. Pleural fluid culture will be negative in strictly anaerobic infections if laboratories use only routine aerobic culture techniques. In mixed infections, only the aerobic component of the infection will be cultured. The Gram stain is the most important tool for rapid assessment of micro-organisms in pleural fluid. In-house cytological examination of pleural fluid is very useful to determine empiric antimicrobial therapy prior to culture and susceptibility results. Modified Wright-Giemsa stains (Diff-Quik; Dade Shearing) are readily available to most practitioners. Polymicrobial infections of obligate anaerobes and facultative bacteria typically feature large numbers of degenerate neutrophils, a small proportion of mononuclear inflammatory cells and large numbers of pleomorphic, intracellular and/or extracellular bacteria. Cell types less commonly identified include erythrocytes, mesothelial cells and epithelial cells. Any combination of filamentous bacteria (egFilifactor villosus ) , cocci (egPeptostreptococcus) or rods may be present. Bacterial rods may be non-enteric facultative bacteria (egPasteurella sp), enteric facultative bacteria (egE coli ) or obligate anaerobes (egBacteroides, Prevotella, Porphyromonas orFusobacterium). 3. Haematology and biochemistry A neutrophilic leucocytosis with a left shift is the most common haematological finding. Some cats will have a mature neutrophilia alone. A low neutrophil count with a degenerative left shift occurs in cats with advanced sepsis and sequestration of neutrophils in the pleural space. Toxic changes in neutrophils are usually identified on examination of the peripheral blood film. Mild to moderate non-regenerative anaemia may be present in some cats. Abnormalities on serum biochemistry may include hypoalbuminaemia, hyperglobulinaemia, hypo- or hyperglycaemia, hyponatraemia, hypochloraemia, total hypocalcaemia, mildly elevated aspartate transaminase (AST) and mildly elevated total bilirubin. Hypoalbuminaemia is a common finding in sepsis, attributed to increased vascular permeability and decreased hepatic synthesis due to a shift towards acute phase protein synthesis. In one study, cholesterol concentrations were significantly lower in survivors than in non-survivors, although the significance of this finding was unclear (Waddell et al 2002). 1.Indwelling closed-tube thoracostomy Polymicrobial pyothorax is conceptually similar to a cat-bite abscess and the same principles of drainage and antimicrobial therapy apply. After emergency stabilisation with oxygen, diagnostic imaging, diagnostic and therapeutic thoracocentesis and intravenous crystalloid therapy, the next step is tube thoracostomy. The mortality rate for pyothorax treated by repeated thoracocenteses in the absence of an indwelling chest tube is up to 80 per cent. This method of treatment should be considered only when the other option is euthanasia. There are two methods for draining pyothorax using indwelling tube thoracostomy – closed-tube with intermittent suction or continuous water seal suction. Continuous suction is provided by a suction pump attached to a collection system that collects retrieved fluid, controls suction pressure and maintains a one-way closed system. Continuous suction offers the advantage of maximal drainage but does not greatly decrease the time needed to manage pyothorax. Leakage between the pleural cavity and the water seal can be fatal. Closed-tube thoracostomy with intermittent suction is simpler, less expensive, requires less monitoring and is adequate for most cases. The author recommends closed-tube thoracostomy in the first instance for treatment of feline pyothorax. To avoid obstruction with fibrin, the thoracostomy tube of greatest diameter that can fit between the intercostal spaces should be used. Commercially available paediatric thoracic trocar catheters are inserted under general anaesthesia. The chest tube should enter the skin two or more intercostal spaces (ICS) caudal to where the tube enters the thoracic cavity to minimise pneumothorax from leakage of air around the tube. The surgical site is prepared and a small stab incision is made in the dorsal-third of the tenth ICS. The trocar is advanced cranially through a subcutaneous tunnel and then driven into the pleural space through the eighth ICS. The tube is advanced over the trocar in a cranioventral direction parallel to the thoracic wall for a distance of 12 to 18cm. As the trocar is removed the end of the tube is clamped to prevent pneumothorax. The tube is secured to the thoracic wall by a purse-string suture to provide an airtight seal and a Chinese-finger trap suture is placed to prevent the tube from slipping. A plastic tube connector (‘Christmas tree’) attached to a three-way tap is placed in the end of the tube and any remaining exudate or air that entered during the procedure is evacuated. For safety, when the tube is not being drained it is sealed with a G-clamp. Where pyothorax is bilateral, the author strongly recommends placement of bilateral thoracostomy tubes. Bilateral chest tubes are more likely to provide effective drainage in cases of persistent loculation of fluid or where the mediastinum is complete. If complications arise with the use of one tube it can simply be removed and drainage continued with the other tube. If unilateral tube thoracostomy is considered, cats should be radiographed immediately after thoracic drain placement and the removal of as much fluid as possible through the tube. If there is minimal residual effusion, one tube may be sufficient for treatment. However, if effusion persists on the opposite side, a second thoracostomy tube should be placed immediately. In all cases radiographs should be taken after thoracostomy tube placement to assess drain position and the presence of any underlying bronchopulmonary disease. Tube complications can include pneumothorax, failure of drainage due to incorrect placement, kinking or adhesions, subcutaneous oedema or abscesses and thoracic wall abscess at the site of drain insertion. Nasal oxygen supplementation may be necessary for cats with concurrent severe pneumonia. Twenty-four hour monitoring is ideal while the thoracic drains are in place. After the initial thoracostomy tube placement, intermittent suction and lavage should be carried out every four hours for the first 24 to 48 hours. Thereafter, suction and lavage two to three times daily is usually adequate. For thoracic lavage 0.9 per cent NaCl or Hartmann’s solution can be safely instilled using volumes from 10 to 25ml/kg. Hypokalaemia may occur if large volumes of physiologic saline are used. Recovery of 75 per cent or more of instilled lavage solution is expected. If smaller volumes of fluid are recovered, radiology and/or ultrasonography are needed to investigate for thoracostomy tube complications or loculation of pockets of fluid due to adhesions. Indwelling thoracostomy tubes are generally removed after four to six days when the following criteria have been met: reduction of pleural effusion to approximately 2ml/kg/day; resolution of pleural effusion on thoracic radiographs; and cytological resolution of infection, as indicated by absence of micro-organisms, reduction of neutrophil numbers and loss of their degenerative appearance and appearance of macrophages. 2. Indications for surgery The persistence of pockets of pleural fluid after seven days or more of aggressive medical management and after the position of indwelling thoracostomy tubes has been optimised is an indication for exploratory thoracotomy. Surgery may also be indicated when imaging studies reveal intrathoracic masses including pulmonary abscesses. If a unilateral pulmonary lesion is identified on imaging, a lateral thoracotomy is performed to facilitate lung lobectomy. A median sternotomy is performed to allow thorough examination of both pleural cavities if no pulmonary lesion is identified. The aims of exploratory thoracotomy are to: Decortication refers to the removal of thick fibrinous material that forms on the pleural surfaces in response to inflammation. Although it may be necessary to remove some material from the visceral pleural surfaces to facilitate lung expansion, complete decortication (including the parietal pleural surfaces) is contraindicated due to the risk of severe haemorrhage. Thoracotomy is indicated in five to nine per cent of cases and carries a good prognosis for cure. Recurrence of pyothorax is uncommon. 3. Fibrinolytics Intrapleural fibrinolytics such as streptokinase and urokinase have been used as adjunctive therapy in human thoracic empyema for years. They enhance intercostal tube drainage and reduce the requirement for subsequent surgical mechanical debridement. Complications are rare and often result from allergic reactions to streptokinase. Urokinase is a safer but more expensive alternative. The efficacy of these agents in treating feline pyothorax has not been evaluated. 4. Antimicrobial therapy Initial antimicrobial therapy is empiric and based on cytology of the pleural fluid. Therapy can be modified, if necessary, after culture and susceptibility testing results. Factors to consider when choosing an antibiotic regimen for initial treatment are whether to use a bactericidal or bacteriostatic antimicrobial, spectrum of activity, combination therapy, dose, route and frequency of administration. Since the majority of cases are polymicrobial infections caused by oropharyngeal flora, antimicrobials need to be effective against both obligate anaerobes and facultative bacteria. It is important to remember that obligate anaerobes are inherently resistant to aminoglycosides and currently available fluorinated quinolones including enrofloxacin, orbifloxacin, marbofloxacin and ciprofloxacin. Antibiotics effective against non beta-lactamase producing obligate anaerobes include penicillin and its derivatives. First-generation cephalosporins have poor activity against anaerobes and their use is not recommended. Antibiotics effective against most beta-lactamase producing anaerobes such as from theBacteroides fragilis group include amoxicillin-clavulanic acid, ticarcillin-clavulanic acid, clindamycin and metronidazole. In a recent study of obligate anaerobes from dogs and cats all isolates were susceptible to amoxicillin-clavulanic acid and 98 per cent were susceptible to metronodazole (Jang et al 1997). Only 71 per cent ofBacteroides isolates were susceptible to ampicillin and only 83 per cent were susceptible to clindamycin. But in feline pyothorax, Bacteroides tectum is a more common isolate in pyothorax thanBacteroides fragilis and beta-lactamase producing strains of this species are uncommon, according to Australian studies (Love et al 1989; Love and Wigney 1997). Polymicrobial infections are often synergistic and concurrent facultative bacteria may scavenge oxygen and create an environment better suited for the proliferation of anaerobes. Therefore the combination of drainage and antibiotics effective against only non-beta-lactamase producing anaerobes and facultative bacteria is often adequate. It is often recommended that empiric therapy for the gram-negative facultative bacterial component of pyothorax should include either an aminoglycoside (gentamicin or amikacin) or a fluorinated quinolone (Hawkins and Fossum 2000; Walker et al 2000). This recommendation has erroneously occurred from considering canine and feline pyothorax as a single entity. In canine pyothorax, Enterobacteriaceae, especiallyE coli , are isolated relatively commonly (Walker et al 2000, Demetriou et al 2002). Enterobacteriaceae are uncommon in feline pyothorax andE coli is isolated in only up to 4 per cent of cases (Walker et al 2000, Demetriou et al 2002). The most common facultative gram-negative rod to be isolated from cats with pyothorax isPasteurella sp. They are susceptible to penicillin and its derivatives, quinolones and aminoglycosides. The addition of a quinolone or aminoglycoside is unnecessary in initial empiric therapy based on these findings. Aminoglycosides are nephrotoxic and many isolates ofE coli are not susceptible to quinolones. Of 50E coli isolates from cats examined at a veterinary teaching hospital, 75 per cent were susceptible to amoxicillin-clavulanate, but only 55 per cent were susceptible to enrofloxacin (Walker et al 2000). Parenteral antibiotics, preferably administered intravenously, should be given until the patient is alert and eating well. At this stage oral antibiotics may be substituted. Suitable antimicrobials (Table 1) include Penicillin G (benzylpenicillin) or ampicillin – either alone or in combination with metronidazole. Another alternative is parenteral monotherapy with amoxycillin-clavulanic acid or ticarcillin-clavulanic acid (Timentin, SmithKline Glaxo). These agents are effective against both b-lactamase producing anaerobes and <I>Pasteurella<I> sp. Ticarcillin-clavulanic acid can be administered intravenously. Preparations of amoxicillin-clavulanic acid are only available for subcutaneous or intramuscular use in Australia. In North America intravenous preparations of amoxycillin-clavulanic acid (Augmentin intravenous, SmithKline Glaxo) and ampicillin with sulbactam (Unasyn, Pfizer Animal Health) are also available. Monotherapy with clindamycin is not suitable ifPasteurella sp are present since clindamycin is ineffective against them. Clindamycin and penicillin G or ampicillin could be used in combination although the use of a beta-lactam antibiotic together with a protein synthesis inhibitor may result in antagonism both in vitro and in vivo. There is no advantage to adding antimicrobials such as penicillin to thoracic lavage solution because comparable tissue levels are attained with intravenous administration. Treating anaerobic infections generally requires high doses of antimicrobials administered for extended periods of time because they are associated with devitalised tissue and tend to relapse if therapy is prematurely discontinued. It is recommended that cats be treated for four to six weeks with oral antibiotics after discharge from hospital. Follow-up radiographs should be taken one to two weeks after discharge from hospital and at the end of antimicrobial therapy to ensure complete resolution of infection. Feline pyothorax can be successfully treated if a logical, step-wise approach to diagnosis and medical management is taken. The greatest risk of mortality comes during anaesthesia for tube thoracostomy and during placement of thoracostomy tubes. These risks can be minimised by thoracocentesis prior to anaesthesia and meticulous attention to avoiding lung penetration during tube placement. If medical treatment fails and surgical intervention is required the prognosis for resolving infection is good to excellent. 1. Brady CA, Otto CM, Van Winkle TJ, King, LG. Severe sepsis in cats: 29 cases (1986-1998).Journal of the American Veterinary Medical Association 217;531-535, 2000. 2. Davies C, Forrester SD (1996) Pleural effusion in cats: 82 cases (1987 to 1995).Journal of Small Animal Practice 37:217-224, 1996. 3. Demetriou JL, Foale RD, Ladlow J, McGrotty Y, Faulkner J, Kirby BM. Canine and feline pyothorax: a retrospective study of 50 cases in the UK and Ireland.Journal of Small Animal Practice 43; 388-394, 2002. 4. Jonas LD Feline pyothorax: a retrospective study of twenty cases. Journal of the American Animal Hospital Association 19; 865-871, 1983. 5. Hawkins EC, Fossum TW. Medical and surgical management of pleural effusion. In: Bonagura, JD.eds. Kirk’s Current Veterinary Therapy XIII. Small Animal Practice . Philadelphia, WB Saunders, 2000, pp 818-825. 6. Love DN, Johnson JL, Moore LVHBacteroides species from the oral cavity and oral-associated diseases of cats. <I>Veterinary Microbiology<I> 19; 275-271, 1989. 7. Love DN, Wigney DI. Evaluation of the E-test for antimicrobial susceptibility testing ofBacteroides tectum from soft tissue infections and normal mouths of cats. Australian Veterinary Practitioner 27:122-126, 1997. 8. Malik R, Love DN, Hunt GBH, Canfield PJ, Taylor V. Pyothorax associated with aMycoplasm species in a kitten.J Sm Anim Pract. 32:31-34, 1991. 9. Waddell LS, Brady CA, Drobatz KJ Risk factors, prognostic indicators, and outcome of pyothorax in cats: 80 cases (1986-1999).Journal of the American Veterinary Medical Association 221; 819-824, 2002. 10. Walker AL, Jang SS, Hirsch DC Bacteria associated with pyothorax of dogs and cats: 98 cases (1989-1998).Journal of the American Veterinary Medical Association 216; 359-363, 2000. |
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