Pleuropneumonia

CHAPTER 63 Pleuropneumonia



Marybeth Miskovic, Laurent Couëtil


Pleuropneumonia typically develops following bacterial colonization of the lower respiratory tract and extension to the pleural space. Bacterial colonization occurs when pulmonary defense mechanisms are compromised (especially the mucociliary clearance mechanism). This disease is common in any horses that are transported long distances. Thoroughbreds and Standardbreds are more predisposed to developing pleuropneumonia than other breeds, possibly because they are shipped more frequently and for longer distances than horses of other breeds with more comingling with other horses. Forty percent of horses with pleuropneumonia have a history of transport within the 3 weeks preceding development of clinical signs. As few as 12 hours of confinement with the head elevated is enough to increase purulent secretions and bacterial numbers in the lower respiratory tract through compromised mucociliary clearance. Twelve hours without head restraint are required after confinement to restore normal mucociliary clearance. Horses transported without head restraint do not have increases in inflammatory cell numbers or bacterial numbers in the lower respiratory tract, implying that decreased mucociliary clearance from head restraint is more important than the other stresses of transport in development of pleuropneumonia.


Several other risk factors may predispose a horse to bacterial pneumonia and pleuropneumonia. Horse-to-horse transmission of bacterial or viral respiratory disease is possible in environments where there is a high volume of horse traffic, such as racetracks or show barns. Strenuous exercise can result in inhalation of dirt into the lower trachea and increased bacterial numbers in the lower respiratory tract. Esophageal obstruction (choke) predisposes horses to developing aspiration pneumonia and pleuropneumonia. General anesthesia may also predispose to aspiration pneumonia and pleuropneumonia by suppressing laryngeal function during recovery from anesthesia. Penetrating chest wounds frequently result in pleuropneumonia from inoculation of bacteria directly into the pleural space.



CLINICAL SIGNS AND DIAGNOSIS


The physical examination is an important part of the diagnostic workup for pleuropneumonia. Horses with pleuropneumonia usually have an abnormal respiratory pattern consisting of short, shallow respirations. Depending on the duration of disease, horses with pleuropneumonia are usually depressed, dehydrated, febrile, tachycardic, and tachypneic. These horses are often reluctant to move and can appear to be in considerable pain. Pleuropneumonia must be differentiated from other painful conditions such as colic, laminitis, and rhabdomyolysis. Rectal examination, nasogastric intubation, application of hoof testers, muscle palpation, and a complete blood count (CBC) and chemistry panel are useful in ruling out other causes of the clinical signs.


Auscultation and percussion are important components of the physical examination. Sound does not travel well through fluid, so auscultation of the lungs reveals dull or absent lung sounds ventrally, where fluid has accumulated. Lung sounds dorsal to the fluid may sound normal, or bronchovesicular sounds may be louder than normal. Crackles or wheezes may also be heard in the dorsal lung fields. The heart sounds may be muffled by accumulated fluid. Percussion can help determine the volume of fluid accumulated; however, this technique is less sensitive than auscultation. With the use of a reflex hammer and a spoon, tapping on the intercostal spaces reveals the level of fluid accumulation. When the hammer is tapped on the spoon over areas of normal, aerated lung, the resulting sound is resonant. When the same procedure is repeated over a more ventral region where fluid has accumulated, the sound is dull. The point where the sound changes from resonant to dull is the dorsal extent of fluid accumulation in the pleural space. Percussion must be performed in a quiet area, as the sounds generated are soft and the change in pitch can be subtle.



Laboratory Diagnosis


Abnormalities in CBC and chemistry panel values are not specific for pleuropneumonia. The CBC findings change with the stage of disease. With severe, acute infection, leukopenia and neutropenia with a left shift may be detected. As the disease becomes chronic, leukocytosis and mature neutrophilia develop. Hyperfibrinogenemia is a common finding in horses with pleuropneumonia. Common chemistry panel abnormalities are hypoalbuminemia and hyperglobulinemia. Other abnormalities reflect underlying or secondary problems such as high blood urea nitrogen and creatinine concentrations in horses that are dehydrated.



Ultrasonographic Examination


Ultrasonographic examination is the most sensitive diagnostic and monitoring tool for detecting pleuropneumonia. In the healthy horse, only the body wall and parietal pleura can be assessed with ultrasound. In the horse with pleuropneumonia, the dorsal extent of fluid accumulation, depth and echogenicity of fluid, visceral pleura, underlying consolidated lung, and superficial pulmonary abscesses can be identified (Figures 63-1 and 63-2).


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Figure 63-1 Sonographic appearance of pleural abscess and aerated lung in a horse with pleuropneumonia.


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Figure 63-2 Sonographic appearance of fibrin on the visceral pleura in a horse with pleuropneumonia.



Thoracocentesis


Thoracocentesis is performed for both diagnostic and therapeutic purposes. Cytologic evaluation and bacterial culture and sensitivity should be performed on pleural fluid to attempt to define the cause of the disease. On cytologic evaluation, pleural fluid in horses with pleuropneumonia typically has a high total protein concentration (more than 3.0 g/dL) and variable increases in total nucleated cell (1600 to 300,000 cells/μL) and red blood cell counts. The primary cell type is usually degenerative neutrophils. Extracellular or intracellular bacteria are often identified. Culture and sensitivity of the pleural fluid are essential in tailoring antimicrobial therapy to each case. Periodic cytologic evaluation of the pleural fluid can also be used to follow the progression of disease. As the disease resolves, the total protein concentration and cell counts should decrease. With appropriate antimicrobial therapy, extracellular and intracellular bacteria should no longer be detectable.


A tracheal wash should also be performed for cytologic evaluation and bacterial culture and sensitivity. Tracheal wash may be performed percutaneously or through an endoscope by use of a guarded catheter. This is an important diagnostic tool, as different bacteria may be cultured from the tracheal wash fluid than are cultured from the pleural fluid. Bronchoalveolar lavage (BAL) is not usually a useful diagnostic tool for pleuropneumonia. BAL is more useful for evaluation of horses with diffuse pulmonary disease. Lung changes with pleuropneumonia can be localized, with discrete areas of diseased tissue interspersed among areas of healthy tissue. If an area ofnormal lung is sampled, results of cytologic analysis of the resulting BAL fluid will also be normal. Samples of BAL fluid are not suitable for culture, as nasopharyngeal contamination occurs when the endoscope or catheter is passed.



Bacterial Isolates


Common aerobic bacteria that are cultured from either the pleural fluid or trachea of horses with pleuropneumonia include Streptococcus equi zooepidemicus, other β-hemolyticStreptococcus

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May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Pleuropneumonia

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