Although under appropriate circumstances any pathogen may produce an abscess, the commonly isolated organisms include aerobic and anaerobic streptococci, Staphylococcus aureus, and a host of gram-negative organisms. Mixed infections occur often because of the important casual role that inhalation of foreign material plays. Anaerobic organisms normally found in the oral cavity, including members of the Bacteroides, Fusobacterium and Peptococcus spices, are the exclusive isolates in about 60% of cases. The causative organisms are introduced by the following mechanisms:

Aspiration of infective material (the most frequent cause):

This is particularly common in acute alcholism, coma, anesthesia, sinusits, gingivodental sepsis, and debilitation in which the cough reflexes are depressed. Aspiration of gastric contents is serious because the gastric acidity adds to the irritant role of the food particles, and in the course of aspiration mouth organisms are inevitably introduced.

Antecedent primary bacterial infection:

Postpneumonic abscess formations are usually associated with S.aureus,K.Pneumoniae, and the type 3 pneumococcus. Fungal infections and bronchiectasis and additional antecedents to lung abscess formation. Post-transplant or otherwise immunosuppressed individuals are at apecial risk for this complication.

Septic embolism

Infected emboli from thrombophlebitis in any portion of the systemic venous circulation or from the vegetations of infective bacterial endocarditis on the right side of the heart are trapped in the lung

Neoplasia

Secondary infection is particularly common in the bronchopulmonary segment obstructed by a primary or secondary malignancy (postobstructive pneumonia)

Miscellaneous

Direct traumatic pnetrations of the lungs; spread of infections from a neighboring organ, such as suppuration in the esophagus, spine, subphrenic space, or pleural cavity; and hematogeneous seeding of the lung by pyogenic organisms all may lead to lung abscess formation.

When all these causes are excluded there are still cases in which no resonable basis for the abscess formation can be identified. These are referred to as primary cryptogenic lung abscesses.

Clinical course

The manifestations of pulmonary abscesses are much like those of bronchiectasis and are charaterized principally by cough, fever and copious amounts of foul-smelling purulent or sangioneous sputum. Fever, Chest pain, and weight loss are common. Clubbing of the fingers and toes may appear within a few weeks after onset of an abscess. Diagnosis of this condition can be only suspected from the clinical findings and must be confirmed by roentgenography and bronchoscopy. Whenever and abscess is discovered, it is important to rule out an underlying carcinoma because this is present in 10 to 15% of cases.

The course of abscesses is variable. With antimicrobial therapy, most resolve with no major sequelae. Complications include extension of the infection into the pleural cavity, hemorrhage, the development of brain abscesses or meningitis from septic emboli and (rarely) reactive secondary amyloidosis (type AA)