Diagnosis and Management of Bacterial Pneumonia in Adult Horses

1. Introduction

Lower respiratory infections in adult horses often have an initial viral component because the effects of lower respiratory viral infections on local clearance and immune function may predispose to secondary bacterial involvement. Bacterial lower respiratory infections have greater clinical impact than viral infections because of the substantial risk of complications ranging from focal abscessation to pleuropneumonia. The development of complicated pneumonia cannot always be prevented, but, once present, requires early and aggressive intervention to achieve acceptable outcomes. Bacterial pneumonia can present substantial therapeutic challenges to the clinician that requires careful consideration of both patient factors and pharmacologic principles when formulating one’s therapeutic plan.

2. Development of Disease

The respiratory tract is the largest mucosal surface in the body, with an enormous surface area constantly exposed to the external environment. The respiratory mucosa is exposed to large amounts of potentially infectious material because each breath carries thousands of microscopic particles and microorganisms into the respiratory tract. There are numerous mechanisms that function to minimize and mitigate this exposure, thereby preventing disease. The normal defense mechanisms consist of the barrier represented by the filtration provided by the upper respiratory tract, the physical barrier represented by the epithelial lining fluid and the respiratory mucosa, and the innate and specific immune responses that eliminate or inactivate infectious organisms that reach the lower respiratory tract. The development of infectious lower respiratory tract infections requires that these respiratory tract defenses be overcome, and this may occur as a result of impairment of the defenses or by overwhelming exposure.
     
Viral respiratory infections can impair mucociliary clearance and suppress local immunity within the lower respiratory tract, thereby creating an environment favoring the development of secondary infections.¹ Transport represents a well-described risk factor for lower respiratory infections, due to the combined effects of prolonged periods of head elevation, which physically impairs lower respiratory clearance, with the immunosuppressive effects of physiologic stress.² Management situations that result in the intermingling of large numbers of horses increase the risk of exposure to potential pathogens and induce stress that impairs pulmonary immunity. Individuals undergoing anesthesia are at risk of lower respiratory infections due to the temporary loss of the filtering function of the upper respiratory tract and the immunosuppressive effects of general anesthesia.³ Horses with dysphagia are also susceptible to lower respiratory tract infections caused by aspiration of foreign material and large numbers of bacteria.
     
Inhaled or aspirated bacteria from the upper respiratory tract are the primary causes of bacterial pneumonias in adult horses, unlike the situation in neonates, in which bacterial pneumonias are often of hematogenous origin. These infections develop initially on the surface of the respiratory mucosa but often progress to involve the pulmonary parenchyma. Pulmonary abscessation can develop in situations in which physical and immunologic clearance of the infectious organisms is incapable of completely resolving the infection. Pleuropneumonia develops when inflammation-associated injury to the lung tissue secondary to bacterial bronchopneumonia breaks down the lung parenchyma and visceral pleura, allowing the infectious organisms access to the pleural space. When infection extends into this space, it is difficult to resolve because it is difficult for the immune system to mount an effective immunologic response at this site, and the accumulation of inflammatory cells and serous fluid within the pleural space provides an expanding reservoir for infectious organisms.

3. Diagnostic Evaluation

The clinical signs of pneumonia may include fever, cough, nasal discharge, tachypnea, dyspnea, depression, anorexia, and pain on palpation of the thoracic wall (pleurodynia). The physical examination is key in determining the extent and severity of lower respiratory tract involvement, and this should include a rebreathing examination in most cases. The rebreathing test will greatly enhance the clinician’s ability to detect lower respiratory inflammation on auscultation. This test should be avoided in patients that are exhibiting severely increased respiratory effort at rest. The presence of a cough on rebreathing is indicative of large airway inflammation/irritability. Abnormal breath sounds indicate the presence of lower respiratory inflammation, with wheezes occurring as the result of small airway narrowing and crackles being secondary to the presence of fluid material in the small airways. The presence of very loud airway sounds or the absence of airway sounds may indicate the presence of consolidated lung tissue or pleural effusion. The percussion test should not be overlooked as a simple tool that can facilitate the identification of consolidated lung and/or pleural effusion. Reluctance to move or the presence of pleural pain on palpation and/or percussion may indicate the presence of pleural inflammation.
      
Clinical pathology is important in evaluating the animal with lower respiratory tract disease, with leukocytosis, neutrophilia, left shift, and hyperfibrinogenemia often accompanying the progression of lower respiratory bacterial infections. Arterial blood gas analysis may reveal hypoxia and hypercapnea in patients with diffuse lower respiratory inflammation. Imaging studies may aid in the staging and localization of lower respiratory infections, with ultrasonography being particularly useful in assessing the superficial pulmonary tissues and the pleural cavity. This can be performed in the field, and even a linear reproductive probe can provide useful images when oriented lengthwise within the rib spaces. Thoracic radiographs are more challenging to obtain and are often not available in the field setting but provide a more global assessment of pulmonary inflammation, allowing for evaluation of the lung tissue below the pleural surface. The sensitivity and specificity of radiographs can be low at times, however. Radiography is most useful in the detection of pathology deep within the lung and can be critically important in the diagnosis of conditions such as pulmonary abscessation, neoplasia, and equine multinodular pulmonary fibrosis.
      
Though not usually required in the initial evaluation of patients with lower respiratory disease, airway cytology is critical in the assessment of severe or persistent lower respiratory infections. Airway cytology provides a clearer indication of the character of pulmonary inflammation, especially regarding the predominant type of inflammatory cells and the presence and type of bacteria, and is strongly indicated in patients that have undergone an unsuccessful course of antimicrobial therapy or those presenting with evidence of severe lower respiratory infection. Tracheal aspirates are easily obtained in a sterile manner by either the percutaneous approach or the endoscopic approach, using a guarded endoscopic aspiration catheter, allowing for culture of the sample. Bronchoalveolar lavage is less commonly used for the assessment of lower respiratory infections because of the potential for contamination occurring when the sampling device is advanced through the upper respiratory tract that renders culture results suspect. Despite this limitation, the use of bronchoalveolar lavage can provide an important indication of small airway inflammation and involvement, and the presence of suspected focal pulmonary involvement is an indication for bronchoscopy and directed bronchial lavage.⁴
      
Thoracocentesis is indicated in cases in which there is documented evidence of pleural fluid accumulation. This procedure can have both diagnostic and therapeutic applications because it yields a sterile sample for cytology and culture but also allows for the removal of fluid from the pleural cavity. Draining of pleural effusion can be beneficial in decreasing the degree of pulmonary dysfunction, increasing patient comfort, and removing large amounts of inflammatory debris and large numbers of bacterial organisms. Sterile lavage of the pleural cavity through an indwelling thoracic drain can help to remove additional inflammatory debris, and antimicrobials can be added to the lavage fluid to achieve high local concentrations in the pleural cavity. [...]