Pneumonia causes

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-In-Chief: Priyamvada Singh, M.D. [2]; Alejandro Lemor, M.D. [3]

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Overview

Pneumonia can result from a variety of causes, including infection with bacteria, viruses, fungi, parasites, and chemical or physical injury to the lungs. The etiology will depend upon various factors such as age, immune status, geographical area, and comorbidities.

Causes

Microbiological Etiology

Typical Bacteria	Atypical Bacteria	Viruses
Streptococcus pneumoniae Haemophilus influenzae Staphylococcus aureus Escherichia coli Klebsiella pneumoniae Pseudomonas aeruginosa	Mycoplasma pneumoniae Chlamydophila pneumoniae Legionella pneumophila	Influenza Parainfluenza Respiratory syncytial virus (RSV) Metapneumovirus Adenovirus

Most Common Causative Pathogens Based on the Patient Location

Outpatient	Inpatient (non-ICU)	Inpatient (ICU)
Streptococcus pneumoniae Mycoplasma pneumoniae Haemophilus influenzae Chlamydophila pneumoniae Influenza A and B, adenovirus, respiratory syncytial virus, parainfluenza	Streptococcus pneumoniae Mycoplasma pneumoniae Haemophilus influenzae Legionella Aspiration Influenza A and B, adenovirus, respiratory syncytial virus, parainfluenza Yersinia enterocolitica	Streptococcus pneumoniae Staphylococcus aureus Legionella Gram-negative bacilli Haemophilus influenzae Acinetobacter baumannii

Causes by Pathogen

Click here for bacterial pneumonia

Click here for viral pneumonia

Click here for fungal pneumonia

Causes by Age

• Newborn infants, children, and adults are at risk for different spectrums of disease causing microorganisms.
• In addition, adults with chronic illnesses, who live in certain parts of the world, who reside in nursing homes, who have recently been treated with antibiotics, or who are alcoholics are at risk for unique infections.

Infants

Source of Infection

• Aerosol
• Aspiration of amniotic fluid
• Blood-borne infection across the placenta

Newborn

• Most common cause is Streptococcus agalactiae (Group B Streptococcus)
• GBS causes at least 50% of cases of CAP in the first week of life.
• Other bacterial causes in the newborn period include Listeria monocytogenes and tuberculosis
• Viral causes like herpes simplex virus (most common) adenovirus, mumps, and enterovirus

Children

• For the most part, children older than one month are at risk for the same microorganisms as adults.
• However, children less than five years are much less likely to have pneumonia caused by mycoplasma pneumoniae, chlamydophila pneumoniae, or legionella pneumophila.
• In contrast, older children and teenagers are more likely to acquire mycoplasma pneumoniae and chlamydophila pneumoniae than adults.
• A unique cause of CAP in this group is chlamydia trachomatis, which is acquired during birth but does not cause pneumonia until 2-4 weeks later.
• Common viruses include respiratory syncytial virus (RSV), metapneumovirus, adenovirus, parainfluenza, influenza, and rhinovirus.
• RSV in particular is a common source of illness and hospitalization.
• Fungi and parasites are not typically encountered in otherwise healthy infants, though maternally-derived syphilis can be a cause of CAP in this age group.

Adults

Viruses

• Viruses cause 20% of CAP cases.
• Common viruses are influenza, parainfluenza, respiratory syncytial virus, metapneumovirus, and adenovirus.
• Less common viruses include chicken pox, SARS, avian flu, and hantavirus.

Atypical Organisms

• Mycoplasma pneumoniae, chlamydophila pneumoniae, and legionella pneumophila are often grouped as atypical pneumonia. Community acquired pneumonia caused by these agents present insidiously, with a non-productive cough and prominent extra-pulmonary complaints, such as myalgias and diarrhea (lack the typical pneumonia symptoms of fever, cough, and sputum).
• Mycoplasma pneumonia is often called is "walking pneumonia." It is transmitted via respiratory droplets and is common among healthy individuals in close contact with one another, such as dormitories or military barracks.
• Atypical organisms are more difficult to grow, respond to different antibiotics, and were discovered more recently than the typical bacteria discovered in the early twentieth century.

Streptococcus Pneumoniae

• Streptococcus pneumoniae is the most common cause of community acquired pneumonia.
• Aspiration pneumonia is most commonly caused by anaerobic organisms.
• Prior to the development of antibiotics and vaccination, it was a leading cause of death.
• Traditionally, it was highly sensitive to penicillin, but during the 1970s resistance to multiple antibiotics began to develop.
• Current strains of "drug resistant Streptococcus pneumoniae" or DRSP are common, accounting for twenty percent of all streptococcus pneumoniae infections.
• Adults with risk factors for DRSP including being older than 65, having exposure to children in day care, alcoholism, other severe underlying disease, or recent treatment with antibiotics should initially be treated with antibiotics effective against DRSP.

Hemophilus Influenzae

• Another common bacterial cause of CAP.
• First discovered in 1892, it was initially believed to be the cause of influenza because it commonly causes CAP in people who have suffered recent lung damage from viral pneumonia.

Enteric Gram Negative Bacteria

• Involve colonic bacteria E.coli and K.pneumonia
• Adults with risk factors for infection, which include living in a nursing home, serious heart and lung disease, and recent antibiotic use should initially be treated with antibiotics effective against Enteric Gram negative bacteria.

Pseudomonas Aeruginosa

• Uncommon cause of CAP, but it is a particularly difficult bacteria to treat.
• Individuals who are malnourished, have bronchiectasis, are on corticosteroids, or have recently had strong antibiotics for a week or more, should initially be treated with antibiotics effective against Pseudomonas aeruginosa.

Special Situations

• Coccidioides are common in southwestern US.
• Anaerobic infection is common in alcoholics. Pneumococcal pneumonia remains the most common cause of CAP in alcoholics too.
• Psittacosis (due to Chlamydophila psittaci) should be considered in the patient with exposure to birds or bird droppings.
• Anaerobes are common in patients with poor dental hygiene, and a suspected large volume of aspiration.
• Streptococcus pneumonia, H.influenza, moraxella catarrhalis, and legionella species are the common causes of community acquired pneumonia in chronic obstructive pulmonary disorders and smokers.
• S. pneumonia, gram negative bacilli, H.influenza, staphylococcus aureus, anaerobes, and chlamydiae pneumonia are more common in nursing home residents.
• S. pneumonia, H.influenza, and mycobacterium tuberculosis are common pathogens in early stages of HIV, whereas, P.jiroveci, histoplasma, and cryptococcus are commonly seen in late stages HIV.
• In patients with structural lung disease such as bronchiectasis and cystic fibrosis, pseudomonas aeruginosa, Burkholderia cepacia (pseudomonas), and staphylococcus aureus are the common pathogens involved.

Aspiration Pneumonia Causes

• Incompetent swallowing mechanism, such as in neurological disease (a common cause being strokes) or while a person is intoxicated.
• Iatrogenic causes such as general anaesthesia for an operation. Patients are therefore instructed to be nil per os (NPO) for at least four hours before surgery.
• Whether aspiration pneumonia represents a true bacterial infection or a chemical inflammatory process remains the subject of significant controversy.