Pleural effusion is a relatively common cause of respiratory distress in the dog and cat. Both species are affected by
several types of effusion, with numerous causes and variable prognosis. Pleural effusion may be discovered incidentally or
may cause respiratory distress resulting in presentation of the pet for veterinary care. In the case of pyothorax, the animal
may present with signs of sepsis rather than respiratory compromise. Small amounts of effusion may not result in changes on
physical examination. If fact, it may require approximately 10 ml/kg of effusion to result in radiographic detection of pleural
fluid, and more than 30 ml/kg of effusion to result in altered physical examination. Respiratory distress may not be severe
until at least 50-60 ml/kg of effusion have accumulated.
When clinical signs related to pleural effusion are present, they may include tachypnea, respiratory distress (primarily on
inspiration), shallow respiration, decreased bronchovesicular lung sounds in dependant portions of the thorax and/or increased
bronchovesicular sounds in the remainder of the thorax, and hyporesonance on percussion of the dependant portions of the thorax
(detection of "fluid line"). Cough is uncommonly associated with pleural disease, and is typically found in association with
disease extension to or from the lungs or airways. Because pleural effusion may be associated with systemic illness, clinical
findings may be related to systems other than the respiratory system or may be related to an underlying respiratory pathology
(eg, sepsis associated with bacterial pyothorax).
Confirmation of pleural effusion may be obtained radiographically or via thoracocentesis. Animals presenting with inspiratory
distress and quiet dependent lung sounds may be harmed by the restraint required to obtain radiographs; in such cases, thoracocentesis
may prove life-saving as well as providing crucial diagnostic information. Even if radiographs are obtained first to document
pleural fluid, an aliquot of the fluid will be required for further diagnosis. Analysis of the collected fluid varies with
differential diagnosis. In general, samples should be submitted for fluid and cytologic analysis with aliquots saved for
aerobic and anaerobic culture if needed. Other tests may be appropriate depending on signalment, clinical signs, and ancillary
evidence of disease.
Pleural fluid may be classified as hemorrhagic, transudative, or exudative. Frank hemorrhage in the pleural space is most
often associated with trauma or defects in secondary hemostasis (eg, vitamin K antagonist rodenticide exposure). Transudate
type fluids are poorly cellular fluids (<500 TNC/ul) with low protein content (<3 g/dl); modification of these fluids (often
with time) may increase either cell number (500-5,000 TNC/ul) or protein content (3-5 g/dl). Exudates have higher cell counts
and protein content than transudates or modified transudates but vary tremendously in type. Pyothorax and chylothorax are
types of exudative effusions.
Pyothorax (aka, empyema)
Bacterial infection of the pleural space leading to accumulation of purulent fluid occurs in both dogs and cats. In dogs,
infection often follows the entry of foreign material such as grass awns into the thoracic cavity (this may be more or less
common as a cause of pyothorax depending on the area in which the dog lives). Traditionally, pyothorax in cats was associated
with cat fight injuries. More recently, an association has been made in cats between pyothorax and upper respiratory infection.
Often, no cause is ever identified in either cats (C) or dogs (D). Animals with pyothorax are often systemically ill and may
demonstrate lethargy, anorexia, fever, and other non-specific signs with or without respiratory compromise. It is not uncommon,
especially in dogs, for signs to have been present for many weeks prior to diagnosis of pyothorax.
The purulent fluid which accumulates in animals with pyothorax is usually off white, beige, pink, or red ("cream of tomato
soup" color) and malodorous. When Nocardia or Actinomyces infections are present, the fluid may contain white or yellow granular material (sulfur granules). Neutrophils are the predominate
cell type and are often degenerate and typically contain intracellular bacteria. Bacteria are often observed cytologically
both inside and outside the neutrophils. In Nocardia or Actinomyces infection granular material should be squashed and examined, and acid fast stains applied; in these cases degenerate neutrophilic
changes may not be pronounced. Both Nocardia and Actinomyces are Gram positive filamentous bacterial but Nocardia is acid fast while Actinomyces is not.
The pleural effusion in pyothorax is typically acidic and has a low glucose content. Both aerobic and anaerobic cultures should
be requested from the fluid. The most common pathogens identified in pyothorax are Pasteurella (C), Bacteroides (D&C), Actinomyces (D&C), Clostridium (C), Nocardia (D); infections are often mixed.
Animals with pyothorax are usually systemically ill and may have complications of sepsis. Although aggressive, broad spectrum
antibiotics including anaerobic coverage is mandatory for therapy of pyothorax, it is not adequate. The purulent fluid must
be drained. Some argument exists as to the ideal method of dealing with effusion; success has been documented after drainage
a single time, after intermittent drainage, and after continuous evacuation. It is the author's preference to use continuous
(or at least frequent) evacuation via chest tubes for at least several days, or until < 1 ml/kg day fluid is produced from
the tube. Thoracic lavage (with or without antibiotics) has not been demonstrated to provide additional benefit over simple
drainage of the purulent fluid but has not been thoroughly investigated in pet animals.
Reasonable initial choice of antibiotics must include anaerobic coverage. Gram stain and acid fast stain can provide more
timely results than pending culture, but once culture results are available coverage can be changed. A combination of ampicillin
and metronidazole are frequently prescribed for initial therapy, unless acid fast or Gram negative bacterial are identified
on stains. Sulfonamides are the antimicrobials of choice for acid fast Nocardia. Gram negative organism may or may not respond to ampicillin, so fluoroquinolones are often used in combination with another
antibiotic to provide additional Gram negative spectrum of action. Antibiotics are typically continued for an extended period
of 4 to 6 weeks, and at least one full week past apparent radiographic resolution of effusion.
In a single study in dogs, surgical thoracotomy was demonstrated to provide a survival benefit over medical management. However,
medical management can also be successful without surgery, provided that evidence does not support Actinomyces or Nocardiosis (non-granular effusion) and there is no known foreign material, organized abscess, or mass within the chest. Thoracic ultrasound
or CT scan may be used to help determine if it is reasonable to forgo surgery. Thus far, there is no evidence that thoracotomy
provides an advantage over medical management of pyothorax in cats.
In addition to treatment of the thoracic infection, animals with pyothorax require supportive care. Electrolyte abnormalities,
diminished oncotic pressure, anemia, malnutrition, and other metabolic disorders may complicate the care of these patients.
The prognosis for treatment of pyothorax is generally considered guarded to fair, but may vary with the severity of illness