IN-DEPTH: RESPIRATORY Diagnosis and Management of Bacterial Pneumonia in Adult Horses Harold C. McKenzie III, DVM, MS, Diplomate ACVIM Author’s address: Marion duPont Scott Equine Medical Center, VA/MD Regional College of Veter- inary Medicine, Virginia Polytechnic and State University, 17690 Old Waterford Road, Leesburg, VA 20176; e-mail: [email protected]. © 2011 AAEP. 1. Introduction and mitigate this exposure, thereby preventing dis- Lower respiratory infections in adult horses often ease. The normal defense mechanisms consist of have an initial viral component because the effects the barrier represented by the filtration provided by of lower respiratory viral infections on local clear- the upper respiratory tract, the physical barrier rep- ance and immune function may predispose to sec- resented by the epithelial lining fluid and the respi- ondary bacterial involvement. Bacterial lower ratory mucosa, and the innate and specific immune respiratory infections have greater clinical impact responses that eliminate or inactivate infectious or- than viral infections because of the substantial risk ganisms that reach the lower respiratory tract. The of complications ranging from focal abscessation to development of infectious lower respiratory tract infec- pleuropneumonia. The development of compli- tions requires that these respiratory tract defenses be cated pneumonia cannot always be prevented, but, overcome, and this may occur as a result of impair- once present, requires early and aggressive inter- ment of the defenses or by overwhelming exposure. vention to achieve acceptable outcomes. Bacterial Viral respiratory infections can impair mucocili- pneumonia can present substantial therapeutic ary clearance and suppress local immunity within challenges to the clinician that requires careful con- the lower respiratory tract, thereby creating an en- sideration of both patient factors and pharmacologic vironment favoring the development of secondary principles when formulating one’s therapeutic plan. infections.1 Transport represents a well-described risk factor for lower respiratory infections, due to 2. Development of Disease the combined effects of prolonged periods of head The respiratory tract is the largest mucosal surface elevation, which physically impairs lower respira- in the body, with an enormous surface area con- tory clearance, with the immunosuppressive effects stantly exposed to the external environment. The of physiologic stress.2 Management situations that respiratory mucosa is exposed to large amounts of result in the intermingling of large numbers of potentially infectious material because each breath horses increase the risk of exposure to potential carries thousands of microscopic particles and mi- pathogens and induce stress that impairs pulmo- croorganisms into the respiratory tract. There are nary immunity. Individuals undergoing anesthe- numerous mechanisms that function to minimize sia are at risk of lower respiratory infections due to NOTES 8 2011 ր Vol. 57 ր AAEP PROCEEDINGS IN-DEPTH: RESPIRATORY the temporary loss of the filtering function of the blood gas analysis may reveal hypoxia and hyper- upper respiratory tract and the immunosuppressive capnea in patients with diffuse lower respiratory effects of general anesthesia.3 Horses with dyspha- inflammation. Imaging studies may aid in the gia are also susceptible to lower respiratory tract staging and localization of lower respiratory infec- infections caused by aspiration of foreign material tions, with ultrasonography being particularly use- and large numbers of bacteria. ful in assessing the superficial pulmonary tissues Inhaled or aspirated bacteria from the upper re- and the pleural cavity. This can be performed in spiratory tract are the primary causes of bacterial the field, and even a linear reproductive probe can pneumonias in adult horses, unlike the situation in provide useful images when oriented lengthwise neonates, in which bacterial pneumonias are often of within the rib spaces. Thoracic radiographs are hematogenous origin. These infections develop ini- more challenging to obtain and are often not avail- tially on the surface of the respiratory mucosa but able in the field setting but provide a more global often progress to involve the pulmonary paren- assessment of pulmonary inflammation, allowing for chyma. Pulmonary abscessation can develop in sit- evaluation of the lung tissue below the pleural sur- uations in which physical and immunologic face. The sensitivity and specificity of radiographs clearance of the infectious organisms is incapable of can be low at times, however. Radiography is most completely resolving the infection. Pleuropneumo- useful in the detection of pathology deep within the nia develops when inflammation-associated injury lung and can be critically important in the diagnosis to the lung tissue secondary to bacterial broncho- of conditions such as pulmonary abscessation, neo- pneumonia breaks down the lung parenchyma and plasia, and equine multinodular pulmonary fibrosis. visceral pleura, allowing the infectious organisms Though not usually required in the initial evalu- access to the pleural space. When infection extends ation of patients with lower respiratory disease, air- into this space, it is difficult to resolve because it is way cytology is critical in the assessment of severe difficult for the immune system to mount an effec- or persistent lower respiratory infections. Airway tive immunologic response at this site, and the ac- cytology provides a clearer indication of the charac- cumulation of inflammatory cells and serous fluid ter of pulmonary inflammation, especially regarding within the pleural space provides an expanding res- the predominant type of inflammatory cells and the ervoir for infectious organisms. presence and type of bacteria, and is strongly indi- cated in patients that have undergone an unsuccess- 3. Diagnostic Evaluation ful course of antimicrobial therapy or those The clinical signs of pneumonia may include fever, presenting with evidence of severe lower respiratory cough, nasal discharge, tachypnea, dyspnea, depres- infection. Tracheal aspirates are easily obtained in sion, anorexia, and pain on palpation of the thoracic a sterile manner by either the percutaneous ap- wall (pleurodynia). The physical examination is key proach or the endoscopic approach, using a guarded in determining the extent and severity of lower respi- endoscopic aspiration catheter, allowing for culture ratory tract involvement, and this should include a of the sample. Bronchoalveolar lavage is less com- rebreathing examination in most cases. The re- monly used for the assessment of lower respiratory breathing test will greatly enhance the clinician’s abil- infections because of the potential for contamination ity to detect lower respiratory inflammation on occurring when the sampling device is advanced auscultation. This test should be avoided in patients through the upper respiratory tract that renders that are exhibiting severely increased respiratory ef- culture results suspect. Despite this limitation, the fort at rest. The presence of a cough on rebreathing is use of bronchoalveolar lavage can provide an impor- indicative of large airway inflammation/irritability. tant indication of small airway inflammation and Abnormal breath sounds indicate the presence of involvement, and the presence of suspected focal lower respiratory inflammation, with wheezes occur- pulmonary involvement is an indication for bron- ring as the result of small airway narrowing and choscopy and directed bronchial lavage.4 crackles being secondary to the presence of fluid ma- Thoracocentesis is indicated in cases in which terial in the small airways. The presence of very loud there is documented evidence of pleural fluid accu- airway sounds or the absence of airway sounds may mulation. This procedure can have both diagnostic indicate the presence of consolidated lung tissue or and therapeutic applications because it yields a ster- pleural effusion. The percussion test should not be ile sample for cytology and culture but also allows overlooked as a simple tool that can facilitate the iden- for the removal of fluid from the pleural cavity. tification of consolidated lung and/or pleural effusion. Draining of pleural effusion can be beneficial in de- Reluctance to move or the presence of pleural pain on creasing the degree of pulmonary dysfunction, in- palpation and/or percussion may indicate the presence creasing patient comfort, and removing large of pleural inflammation. amounts of inflammatory debris and large numbers Clinical pathology is important in evaluating the of bacterial organisms. Sterile lavage of the pleural animal with lower respiratory tract disease, with cavity through an indwelling thoracic drain can help to leukocytosis, neutrophilia, left shift, and hyperfi- remove additional inflammatory debris, and antimi- brinogenemia often accompanying the progression crobials can be added to the lavage fluid to achieve of lower respiratory bacterial infections. Arterial high local concentrations in the pleural cavity. AAEP PROCEEDINGS ր Vol. 57 ր 2011 9 IN-DEPTH: RESPIRATORY Repeated airway cytology can also be useful in into respiratory secretions but only to levels less documenting the status of lower respiratory inflam- than observed in the systemic circulation.12 Be- mation but should not be required in most cases cause anaerobic involvement is likely in cases of showing clinical resolution. Changes in clinical pleuropneumonia, the treatment regimen
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