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Pathology of Pulmonary Infection

 Dr Sampurna Roy MD       

Pulmonary infection occurs when normal lung or systemic defense mechanisms are impaired.   

Pneumonia and influenza account for a number deaths all over the world and are the fifth leading cause of death, exceeded only by heart disease, cancer, cerebrovascular disease and accidents.

Pneumonia can be broadly define as any infection in the lung.

Acute pneumonias may be caused by pyogenic bacteria  that induce primarily neutrophilic exudates in alveoli, bronchioles and bronchi or by a miscellaneous group of microorganisms that induce predominantly peribronchiolar and interstitial mononuclear inflammation.

The term Pneumonitis refers to the inflammatory conditions that primarily affect the interstitium.  It presents clinically as "atypical" pneumonias.

Chronic pneumonia is caused by -  Nocardia and Actinomyces species and other parasites, fungi and intracellular bacteria .

Bacterial infections occur in two frequently overlapping morphologic patterns (bronchopneumonia and lobar pneumonia ) and can be caused by a variety of gram-positive and gram-negative organisms.

Depending on bacterial virulence and host resistance, the same organism may in one case cause bronchopneumonia and in another case lobar pneumonia and sometimes intermediate involvement.

Nosocomial pneumonias develop in a number of hospitalized patients specially in critically ill patients admitted to intensive care units.

With the use of potent therapies that deliberately or incidentally produce immunosuppression there is a great risk of developing respiratory infection.

Pathogenetic organisms gain access to lung:

(i) through the airways ; (ii) through the bloodstream ; (iii) by traumatic implantation ; (iv) by a direct spread across  the diaphragm from a subphrenic source, probably through the lymphatics.

The most common route is the airways. Airway spread can result from inhalation of the organism as an aerosol on droplet nuclei.

 This is a major mechanism of spread of many viral infections and of tuberculosis.

Often development of infection begins with colonization of the upper respiratory tract by potential pathogens followed by aspiration into the lower respiratory tract.

This is a major mechanism for many of the gram-negative organisms.

Following factors interfere with normal lung defence and cause lung infection :

- Depressed bactericidal and phagocytic function of macrophage caused by starvation, alcohol ingestion, hypoxia, uremia, air pollutant, cigarette smoke and previous viral infection.

- Decrease or loss of cough reflex leading to aspiration caused by drug , anesthesis or coma.

-  Pulmonary Edema -  (a good culture medium).

- Collection of secretion.

- Poor mucociliary elevator function from smoking, infection etc.

- Cystic fibrosis, airway obstruction, chronic bronchitis .


Pulmonary Infection:

Viral Infection:

Viruses are a common cause of airway lesions but those that primarily affect the airways may also severely affect the lung parenchyma.

It is important to note that viral lung infections produce interstitial (rather than alveolar) pneumonia and diffuse alveolar damage.

Influenza (Orthomyxoviruses)

Cytomegalovirus infection



Chlamydia: Chlamydial Infection

Rickettsia: Q Fever(Coxiella burnetii)

Mycoplasma: Mycoplasma pneumonia

Bacterial Infection:

Pneumococcal Pneumonia (Lobar Pneumonia)


Klebsiella pneumoniae

Haemophilus influenza Infection

Legionellosis (Legionnaires'Disease)

Staphylococcal Infection

Streptococcal Infection


Atypical Mycobacterial Infection

Mycobacterium Avium Intracellulare

Mycobacterium Kansasii Infection

Fungal Infection:

Histoplasmosis (Histoplasma Capsulatum)









Infections caused by other organisms:

Pneumocystis Pneumonia


February 2016


Dr Sampurna Roy  MD

Consultant  Histopathologist (Kolkata - India)


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