Pneumonia: Infections, Stages, and Types

• Pulmonary infections are infections of the lungs.

Lung Defense Mechanisms

• The normal lung removes microbial organisms via entrapment in the mucous blanket and removal by the mucociliary elevator.
• Alveolar macrophages can kill and degrade organisms, removing them from the air spaces.
• Neutrophils recruited by macrophage factors can phagocytose and kill organisms.
• Complement may enter the alveoli and activate the alternative pathway to produce the opsonin C3b, enhancing phagocytosis.
• Organisms, including those ingested by phagocytes, may reach the draining lymph nodes to start immune responses.
• Secreted IgA can block microorganism attachment to the epithelium of the upper respiratory tract.
• Serum antibodies (IgM, IgG) are found in the lower respiratory tract alveolar lining fluid and enhance complement activation.
• IgG is an opsonin and accumulation of immune T cells is important for controlling infections by viruses and other intracellular microorganisms.
• Microbes are airborne and readily inhaled into the lungs.
• The nasopharyngeal flora is regularly aspirated when sleeping.
• Lung diseases can lower immune defenses.

Pneumonia Classification

• Bacterial pneumonias are classified based on the specific etiologic agent.
• Can be classified by clinical setting.
• Can be classified by the area of the lung that is affected.
• Can be classified by the type of the infection.

Community-Acquired Bacterial Pneumonias

• Community-acquired bacterial pneumonias often follow a viral upper-respiratory tract infection.
• S. pneumoniae (pneumococcus) is the most common cause of community-acquired acute pneumonia.
• Other possible causes: Haemophilus influenzae, Moraxella cararrhalis, Staphylococcus aureus, Legionella pneumophila, Enterobacteriaceae, Pseudomonas spp, Mycoplasma pneumoniae, Chlamydia pneumoniae and Q fever.

Lobar Pneumonia

• Lobar pneumonia involves consolidation of a large portion or an entire lobe.
• Lobar pneumonia has four stages of inflammatory response.
• The initial stage is congestion that presents with vascular engorgement, intraalveolar fluid with some present neutrophils, the lung becoming heavy, boggy and read.
• Red hepatization occurs when neutrophils, fibrin, and red cells fill alveolar spaces making lobes red, solid, and airless and liver-like consistency.
• Gray hepatization marks the progressive disintegration of red cells with fibronosuppurative exudate, discoloring the lobe.
• Resolution is the final stage marked by enzymatic digestion of alveolar exudate that produces semi fluid debris, which is then resorbed.

Bronchopneumonia

• Bronchopneumonia has patchy consolidation of the lungs.
• Foci of this kind of infection are consolidated areas of acute inflammation.
• The consolidation is often multi-lobar and frequently bilateral but may be confined to one lobe.
• The infection tends to be basal because secretions gravitate to the lower lobes.
• A neutrophil-rich exudate fills the bronchi, bronchioles, and adjacent alveolar spaces.
• Elevated, dry, granular, gray-red to yellow, poorly delimited legions may develop.

Clinical Features of Pneumonia

• Onset of high-grade fever, chills, and cough producing mucopurulent sputum.
• Patients occasionally have hemoptysis.
• When pleuritis is present, it is accompanied by pleuritic pain and pleural friction rub.
• Radiography will detect opacities.
• Abscess formation, Empyema, and Bacteremic dissemination are complications.

Community-Acquired Viral Pneumonias

• Usual causes include influenza A and B, respiratory syncytial viruses, human metapneumovirus adenovirus, rhinoviruses, rubeola virus, and varicella virus.
• The agents also cause upper-respiratory tract infections.
• Causes interstitial inflammation and some outpouring of fluid into alveolar spaces.
• Clinically mimic bacterial pneumonia on chest films.
• Predisposes individuals to secondary bacterial infections.
• Often occur in infants, older adults, malnourished patients, alcoholics, and immunosupressed individuals.

Viral Pneumonia Morphology

• Viral pneumonia can be patchy, or it may involve whole lobes bilaterally or unilaterally.
• Affected areas are red-blue, congested, and subcrepitant.
• Confined inflammatory reaction occurs in the walls of the alveoli.
• Septa are widened and edematous.
• Contain a mononuclear inflammatory infiltrate of lymphocytes, macrophages and, occasionally, plasma cells.
• Alveolar spaces are free of cellular exudate.
• Full-blown diffuse alveolar damage occurs with hyaline membranes developing in severe cases.
• Reconstitution of the normal architecture upon resolution.

Viral Pneumonia Clinical Features

• The clinical course varies.
• May go undiagnosed, or manifest as a fulminant, life-threatening infection.
• Fever, headache, and malaise develops, followed by later cough with minimal sputum.
• Ventilation and perfusion mismatch often develops.
• The degree of respiratory distress seems out of proportion to the physical and radiographic findings.

Hospital-Acquired Pneumonias

• Hospital-acquired pneumonias are pulmonary infections acquired during a hospital stay.
• Severe underlying disease, immunosuppression, and prolonged antibiotic regimens increase risks.
• Ventilator-associated pneumonia occur.
• Gram-negative rods and S. aureus are the most common isolates.

Aspiration Pneumonia

• Aspiration pneumonia happens in debilitated individuals who aspirate gastric contents while unconscious or during repeated vomiting.
• Those affected have abnormal gag and swallowing reflexes that increase aspiration.
• Aspiration Pneumonia is partly chemical and bacterial.
• Cultures typically recover more than one organism; aerobes are being more common than anaerobes.
• Frequently necrotizing and runs a fulminant clinical course, and is also a leading cause of death in predisposed individuals.
• Microaspiration occurs in individuals with gastroesophageal reflux.

Lung Abscess

• Lung abscesses are localized areas of suppurative necrosis within the pulmonary parenchyma.
• Results in one or more cavities.
• The causative organism is introduced into the lung by.
• Aspiration of infective material or gastric.
• Necrotizing bacterial pneumonias.
• Bronchial obstruction, Septic embolism, or hematogenous spread.
• Anaerobic bacteria are in almost all lung abscesses, and are the exclusive isolates in one-third to two-thirds of cases.
• Common anaerobes commonly inhabit the oral cavity.
• These include Prevotella, Fusobacterium, Bacteroides, Peptostreptococcus and microaerophilic streptococci.
• Abscesses range from a few millimeters to large 5 to 6cm cavities.
• Localization and number depends on their mode of development.
• Abscess from aspirated of infective material is more common on the right and the side of that aspiration, it is often single.
• Appears in the posterior segment of the upper lobe in the apical segments of the lower lobe on the right side.
• Pneumonia or bronchiectasis are multiple, basal, and diffusely scattered.
• Septic emboli and abscesses arising from hematogenous are multiple and may affect any region of the lungs.
• The abscess can rupture into the airways causing an air-fluid level.
• Pneumothorax or empymea, meningitis or brain abscess may develop.
• A suppurative focus is surrounded by variable amounts of fibrous scarring and mononuclear infiltration.

Lung Abscess Clinical Features

• Prominent cough and sputum
• Hemoptysis
• Spiking fever and malaise Clubbing of the fingers weight loss, and anemia may occur.
• Underlying carcinoma should be considered.
• Secondary amyloidosis may occur in chronic cases.

Tuberculosis

• Tuberculosis is a chronic granulomatous disease caused by Mycobacterium tuberculosis.
• Primarily affects the lungs but may affect any organ or tissue.
• Mycobacteria are acid-fast bacilli.
• M. tuberculosis hominis is responsible for most cases of tuberculosis.
• The reservoir of infection is typically found in individuals with active pulmonary disease.

Tuberculosis Transmission

• Transmission usually occurs by inhalation of airborne organisms, or by being exposed to contaminated secretions.
• Can be transmitted from drinking milk contaminated with Mycobacterium bovis infection.
• Other mycobacteria are much less virulent than M. tuberculosis and rarely cause disease in immunocompetent individuals.
• They cause disease in 10% to 30% of patients with AIDS.
• Cell-mediated immunity is important in TB pathogenesis.
• Pathologic features of tuberculosis stem from destructive tissue sensitivity to the host immune response.
• This includes caseating granulomas and calvitation.

Tuberculosis Pathogenesis

• Effector cells for both protective immunity and sensitivity are the same.
• Tissue sensitivity indicates acquisition of immunity to the organism.
• Events in the natural history of primary pulmonary tuberculosis include inhalation of virulent strains of Mycobacterium and development of immunity/sensitivity to the organism.
• Three weeks after exposure involve events for unchecked bacillary proliferation and bacteremia with seeding of multiple sites,.
• Development of resistance occurs with tuberculin skin testing resulting in activated macrophages.
• Includes Class II MHC, Th1, T-cells, and MTb antigens and culminates with granulomas.
• Initial steps of the infection involve entry and replication in macrophages, proliferation of bacilli in alveolar macrophage, and spread to air spaces with subsequent bacteremia.
• Mostly asymptotic flu like symptoms may occur.
• Development of cell -mediated immunity occurs when CD4+ T cells help the macrophage activation
• T cell-mediated killing by macrophages is also key.

Granulomatous Inflammation

• T-Cell mediated macrophage activation promotes production of IFN-y which activates macrophages and inflammatory peptides,
• Includes TNF, iNOS, antimicrobial peptides.

Primary Tuberculosis

• This form of infection happens with a previously unexposed patient.
• Consequence of primary tuberculosis manifests as scarring.
• Those foci may harbor viable bacilli which may serve a site for disease reactivation.
• Progressive primary tuberculosis occurs in about 5% of those newly infected.
• Develops mainly immunocompromised individuals

Morphology (Primary TB)

• Bacilli implant in distal air spaces of the lower of upper lobe or upper part of the lower lobe.
• Often located close to the pleura.
• Ghon focus refers a point of accumulation in the middle of the area of the lung in the upper portion of the lower lobe.
• Ghon Complex happens when a Ghon focus undergoes necrosis.

Secondary Tuberculosis

• Found in previously sensitized hosts.
• Arises from dormant primary lesions reactivating and spreading in the lung,
• May also result from reinfection
• Only a few individuals with a history of TB develop a second bout, and are localized in the apex of one upper lobe.

Effects of immunosupression

• Manifestations differ with differences in immunosuppression.
• Extent of immunosuppression in the extent factors extrapulmonary tuberculosis.
• Lymph nodes are less involved in the disease
• Cavitation readily develops.
• Consideration should be given to HIV positive patients
• Clinical presentations show secondary tuberculosis
• Active lesions show coalescent tubercles with central caseation and peripheral fibrosis, so only fibrocalcific scars remain
• Tubercle bacilli can be demonstrated in early exudative formation phases.
• Lung parenchyma develops fibrous scars in localized instances as the area heals.
• Empyema from a cavity develops and erodes a brochus.

Forms of clinical spread

• Hematogenously disseminate.
• Forms miliary TB and has a liver-like consistency
• Meningitis TB condition shows symptoms and abnormalities in organ systems such as kidneys and spinal fluid.
• Tuberculosis lymphadenitis is an extra pulmonary type is found in cervical regions.
• Tb patients cough up infected material, potentially infecting their own intestines.

Explore pneumonia, including lung susceptibility to infection and the stages of lobar pneumonia. Learn about pleural reactions, bronchopneumonia characteristics, and common causes of community-acquired pneumonia. Discover the resolution phase and factors for categorizing bacterial pneumonias.