Inflammatory conditions of the pleura may be dry, serofibrinous, pyogranulomatous, or purulent. Dry pleuritis often precedes inflammatory pleural effusions. Dry pleuritis may be caused by bacteria, viruses, or trauma. A diagnosis of
dry pleuritis is suggested by clinical findings of a rapid and shallow respiratory pattern, obscure thoracic pain, nonproductive
cough, and auscultation of a pleural friction rub.Serofibrinous pleuritis is reported with canine hepatitis, canine leptospirosis, canine distemper, canine and feline upper respiratory viruses, and
parasitic diseases such as Aelurostrongylus in cats and Spirocerca lupi in dogs. Bile and tuberculosis are unusual causes of severe serofibrinous pleuritis.
Pyogranulomatous pleuritis is associated with feline infectious peritonitis. The effusion is secondary to virus-induced vasculitis affecting all serous
membranes.Purulent pleuritis, also referred to as pyothorax or empyema, is invariably the result of bacterial or fungal sepsis of the pleural space.
Sources of bacterial contamination include penetrating thoracic wounds, extension of bacterial pneumonia, migrating foreign
bodies, esophageal perforations, extension of cervical, lumbar or mediastinal infections, and hematogenous spread. Thoracic
bite wounds are frequently implicated in feline pyothorax. Inhalation and migration of a grass awn often is suspected in field
dogs with pyothorax. Anaerobic bacteria and Nocardia asteroides are isolated most often from dogs with pyothorax. Nocardia and Actinomyces are very commonly associated to a foreign body. Pleural infections are almost always polymicrobic in nature There is a high
incidence of obligate anaerobic bacteria as sole pathogens or in combination with aerobic-facultative and anaerobic bacteria.
Obligate anaerobic bacteria (Bacteroides, Fusobacterium) and Gram positive filamentous organism such as Nocardia and Actinomyces are most commonly isolated from dogs with pyothorax. Pasteurella multocida and anaerobes are the most prevalent isolates in cats.
Pleuritis and pyothorax frequently have an insidious course and presentation may be delayed. Pyothorax occurs most commonly
in young adult, male, non purebred cats and adult large breed dogs. Clinical signs result from restrictive disease, including
increased respiratory rate, shallow respiration, dyspnea and orhtopnea. Other clinical signs include exercise intolerance,
lethargy, anorexia, and fever. Physical examination reveals muffled heart sounds, decreased lung sounds, and hypersonant (dull)
percussion sounds, especially over the ventral portions of the thorax. Chronic or severe infection result in a patient in
septic shock with dehydration debilitation or hypothermia.
A diagnosis of pyothorax is confirmed by hematology, thoracic radiography and thoracocenthesis with cytologic evaluation and
culture of the pleural fluid. Neutrophilic leukocytosis with or without a left shift is the most common hematologic finding.
Leukogram results do not correlate with the severity of the underlying infection. Radiographic signs of free pleural fluid
include hazy density of the lung fields which obscures the cardiac silhouette, retraction of the lung lobes from the chest
wall, visibility of the interlobular fissures and rounding of the costophrenic angles.
Cytologic evaluation of the pleural is consistent with a septic or a nonseptic exudate. Degenerate neutrophils and mixed population
of bacteria are usually seen. If degenerative neutrophils are observed an anaerobic-anaerobic culture of the pleural fluid
should be performed. Inflammatory exudates typically exhibit a total protein greater than 3.0 gm/dl, a specific gravity greater than 1.018, and a total cell count greater
than 30 X 103 cells/ul. Inflammatory exudates may be nonseptic or septic. Nonseptic exudates usually have a serofibrinous or serosanguineous appearance. Feline infectious peritonitis produces a nonseptic exudative
pleural effusion that appears yellow, translucent, and viscous on gross examination. Total protein values will approach serum
levels ranging from 4 to 8 gm/dl. Electrophoresis will reveal an elevated gamma globulin fraction. The predominant cell types
present in nonseptic exudates are non-degenerative neutrophils and macrophages. Total cell counts are generally not high,
ranging from 5 to 15 X 103 cells/ul. Septic exudates are characteristic of pyothorax. The fluid is viscous, opaque, and varies in color from white or yellow to green or red.
The fluid may clot or exhibit fibrinous debris, and often produces a foul odor. Cell counts range from 30 to 200 X103 cells/ul, although accurate cell counts are difficult due to extensive cellular degeneration. Degenerate neutrophils predominate
and bacteria are often visualized. Gram stains may give an early indication of the types of bacteria present. Fluid should
be cultured for aerobic and anaerobic bacteria. Macrophages and plasma cells increase as an exudative process becomes longstanding.