Community-acquired pneumonia

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Community-acquired pneumonia (CAP) refers to pneumonia (any of several lung diseases) contracted by a person with little contact with the healthcare system. The chief difference between hospital-acquired pneumonia (HAP) and CAP is that patients with HAP live in long-term care facilities or have recently visited a hospital. CAP is common, affecting people of all ages, and its symptoms occur as a result of oxygen-absorbing areas of the lung (alveoli) filling with fluid. This inhibits lung function, causing dyspnea, fever, chest pains and cough.

CAP, the most common type of pneumonia, is a leading cause of illness and death worldwide. Its causes include bacteria, viruses, fungi and parasites.[1] CAP is diagnosed by assessing symptoms, making a physical examination and on x-ray. Other tests, such as sputum examination, supplement chest x-rays. Patients with CAP sometimes require hospitalization, and it is treated primarily with antibiotics, antipyretics and cough medicine.[2] Some forms of CAP can be prevented by vaccination and by abstaining from tobacco products.[3]

Signs and symptoms[edit]

Common symptoms[edit]

Illustration of pneumonia symptoms on a human body
  • Shortness of breath (dyspnea)
  • Coughing which produces greenish or yellow sputum
  • A high fever, accompanied by sweating, chills and shivering
  • Sharp, stabbing chest pains
  • Rapid, shallow, often-painful breathing

Less-common symptoms[edit]

In the elderly[edit]

  • New or worsening confusion
  • Hypothermia
  • Poor coordination, leading to falls

In infants[edit]

  • Unusual sleepiness
  • Yellowing of the skin (jaundice)
  • Difficulty feeding[4]


Over 100 microorganisms can cause CAP, with most cases caused by Streptococcus pneumoniae. Certain groups of people are more susceptible to CAP-causing pathogens; for example, infants, adults with chronic conditions (such as chronic obstructive pulmonary disease, senior citizens, alcoholics and others with compromised immune systems are more likely to develop CAP from Haemophilus influenzae or Pneumocystis carinii.[5] A definitive cause is identified in only half the cases.


Infants can acquire lung infections before birth by breathing infected amniotic fluid or through a blood-borne infection which crossed the placenta. Infants can also inhale contaminated fluid from the vagina at birth. The most prevalent pathogen causing CAP in newborns is Streptococcus agalactiae, also known as group-B streptococcus (GBS). GBS causes more than half of CAP in the first week after birth.[6] Other bacterial causes of neonatal CAP include Listeria monocytogenes and a variety of mycobacteria. CAP-causing viruses may also be transferred from mother to child; herpes simplex virus (the most common) is life-threatening, and adenoviridae, mumps and enterovirus can also cause pneumonia. Another cause of CAP in this group is Chlamydia trachomatis, acquired at birth but not causing pneumonia until two to four weeks later; it usually presents with no fever and a characteristic, staccato cough.

CAP in older infants reflects increased exposure to microorganisms, with common bacterial causes including Streptococcus pneumoniae, Escherichia coli, Klebsiella pneumoniae, Moraxella catarrhalis and Staphylococcus aureus. Maternally-derived syphilis is also a cause of CAP in this age group. Viruses include human respiratory syncytial virus (RSV), human metapneumovirus, adenovirus, human parainfluenza viruses, influenza and rhinovirus, and RSV is a common source of illness and hospitalization in infants.[7] CAP caused by fungi or parasites is not usually seen in otherwise-healthy infants.


Although children older than one month tend to be at risk for the same microorganisms as adults, children under five 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.[8]


A full spectrum of microorganisms is responsible for CAP in adults, and patients with certain risk factors are more susceptible to infections of certain groups of microorganisms. Identifying people at risk for infection by these organisms aids in appropriate treatment. Many less-common organisms can cause CAP in adults, and are identified from specific risk factors or treatment failure for common causes.

Risk factors[edit]

Some patients have an underlying problem which increases their risk of infection. Some risk factors are:

  • Obstruction: When part of the airway (bronchus) leading to the alveoli is obstructed, the lung cannot eliminate fluid; this can lead to pneumonia. One cause of obstruction, especially in young children, is inhalation of a foreign object such as a marble or toy. The object lodges in a small airway, and pneumonia develops in the obstructed area of the lung. Another cause of obstruction is lung cancer, which can block the flow of air.
  • Lung disease: Patients with underlying lung disease are more likely to develop pneumonia. Diseases such as emphysema and habits such as smoking result in more-frequent and more-severe bouts of pneumonia. In children, recurrent pneumonia may indicate cystic fibrosis or pulmonary sequestration.
  • Immune problems: Immune-deficient patients, such as those with HIV/AIDS, are more likely to develop pneumonia. Other immune problems range from severe childhood immune deficiencies, such as Wiskott–Aldrich syndrome, to the less-severe common variable immunodeficiency.[9]


CAP's symptoms are the result of lung infection by microorganisms and the immune system's response to the infection. Mechanisms of infection are different for viruses and other microorganisms.


Viruses cause 20 percent of CAP cases. The most common viruses are influenza, parainfluenza, human respiratory syncytial virus, human metapneumovirus and adenovirus. Less-common viruses which may cause serious illness include chickenpox, SARS, avian flu and hantavirus.[10]

Typically, a virus enters the lungs through the inhalation of water droplets and invades the cells lining the airways and the alveoli. This leads to cell death; the cells are killed by the virus or they self-destruct. Further lung damage occurs when the immune system responds to the infection. White blood cells, particularly lymphocytes, activate chemicals known as cytokines which cause fluid to leak into the alveoli. The combination of cell destruction and fluid-filled alveoli interrupts the transportation of oxygen into the bloodstream. In addition to their effects on the lungs, many viruses affect other organs. Viral infections weaken the immune system, making the body more susceptible to bacterial infection (including bacterial pneumonia).

Bacteria and fungi[edit]

Although most cases of bacterial pneumonia are caused by Streptococcus pneumoniae, infections by atypical bacteria such as Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila can also cause CAP. Enteric gram-negative bacteria, such as Escherichia coli and Klebsiella pneumoniae, are a group of bacteria that typically live in the large intestine; contamination of food and water by these bacteria can result in outbreaks of pneumonia. Pseudomonas aeruginosa, an uncommon cause of CAP, is a difficult bacteria to treat.

Bacteria and fungi typically enter the lungs through the inhalation of water droplets, although they can reach the lung through the bloodstream if an infection is present and often live in the respiratory tract. In the alveoli, bacteria and fungi travel into the spaces between cells and adjacent alveoli through connecting pores. The immune system responds by releasing neutrophil granulocytes, white blood cells responsible for attacking microorganisms, into the lungs. The neutrophils engulf and kill the microorganisms, releasing cytokines which activate the entire immune system. This response causes fever, chills and fatigue, common symptoms of CAP. The neutrophils, bacteria and fluids leaked from surrounding blood vessels fill the alveoli, impairing oxygen transport. Bacteria may travel from the lung to the bloodstream, causing septic shock (very low blood pressure which damages the brain, kidney, and heart).


A variety of parasites can affect the lungs, generally entering the body through the skin or by being swallowed. They then travel to the lungs through the blood, where the combination of cell destruction and immune response disrupts oxygen transport.


Patients with symptoms of CAP require evaluation. Physical examination by a health provider may reveal fever, an increased respiratory rate (tachypnea), low blood pressure (hypotension), a fast heart rate (tachycardia) and changes in the amount of oxygen in the blood. Palpating the chest as it expands and tapping the chest wall (percussion) to identify dull, non-resonant areas can identify stiffness and fluid, signs of CAP.

Listening to the lungs with a stethoscope (auscultation) can also reveal signs associated with CAP. A lack of normal breath sounds or the presence of crackles can indicate fluid consolidation. Increased vibration of the chest when speaking, known as tactile fremitus, and increased volume of whispered speech during auscultation can also indicate fluid.[11]

When signs are discovered, chest X-rays, examination of the blood and sputum for infectious microorganisms and blood tests are commonly used to diagnose CAP. Diagnostic tools depend on the severity of illness, local practices and concern about complications of the infection. All patients with CAP should have their blood oxygen monitored with pulse oximetry. In some cases, arterial blood gas analysis may be required to determine the amount of oxygen in the blood. A complete blood count (CBC) may reveal extra white blood cells, indicating infection.

Chest X-rays and X-ray computed tomography (CT) can reveal areas of opacity (seen as white), indicating consolidation. CAP does not always appear on x-rays, because the disease is in its initial stages or involves a part of the lung an x-ray does not see well. In some cases, chest CT can reveal pneumonia not seen on x-rays. X-rays can often mislead, as Heart failure or other types of lung damage can mimic CAP on x-rays.[12]

Several tests can identify the cause of CAP. Blood cultures can isolate bacteria or fungi in the bloodstream. Sputum Gram staining and culture can also reveal the causative microorganism. In severe cases, bronchoscopy can collect fluid for culture. Special tests can be performed if an uncommon microorganism is suspected, such as urinalysis for Legionella antigen in Legionnaires' disease.


Chest X-rays of a CAP patient before (left) and after treatment

CAP is treated by administering an antibiotic that kills the offending microorganism, and by managing any complications of the infection. If the causative microorganism is unidentified, the laboratory tests different antibiotics to identify which is most effective. Often, however, no microorganism is ever identified. Also, laboratory testing can take several days, which delays organism identification.

In any cases, health professionals must consider a person's risk factors for different organisms when choosing the initial antibiotics, using empiric therapy. Additional consideration must also be given to the setting in which the individual is treated. Most people are fully cured after taking oral pills, while others must be hospitalized for intravenous antibiotics and, possibly, intensive care.

In general, all therapies in older children and adults include treatment for atypical bacteria. Typically this is a macrolide antibiotic such as azithromycin or clarithromycin; though a fluoroquinolone such as levofloxacin can substitute. Doxycycline is the antibiotic of choice in the UK for complete coverage of atypical bacteria, due to the increased levels of Clostridium difficile seen in hospital patients being linked to increased use of clarithromycin.


Most newborn infants with CAP are hospitalized and given intravenous ampicillin and gentamicin for at least ten days. This treats Streptococcus agalactiae, Listeria monocytogenes, and Escherichia coli, all common causative agents. In cases of herpes simplex virus, the intravenous medication acyclovir is administered for 21 days.


Treatment of CAP in children depends on both the age of the child and the severity of his/her illness. Children less than five do not typically receive treatment to cover atypical bacteria. If the circumstances do not require hospitalization, a seven day course of amoxicillin is often prescribed. However, with increasing prevalence of DRSP, other agents—such as cefpodoxime—will likely become more popular.[13] Hospitalized children should receive intravenous ampicillin, ceftriaxone, or cefotaxime, and a recent study found that a three days course of antibiotics seems to be sufficient for most cases of mild to moderate CAP in children.[14]


In 2001, the American Thoracic Society—drawing on work by the British and Canadian Thoracic Societies—established guidelines for the management of adults with CAP that divided individuals with CAP into four categories, based on common organisms.[15]

  • Healthy outpatients without risk factors
This group, the largest, is composed of otherwise healthy patients without risk factors for DRSP, enteric Gram negative bacteria, Pseudomonas, or other less common causes of CAP. The primary microoganisms in this group are viruses, atypical bacteria, penicillin sensitive Streptococcus pneumoniae, and Hemophilus influenzae. Recommended management is with a macrolide antibiotic such as azithromycin or clarithromycin for seven[16] to ten days.
  • Outpatients with underlying illness and/or risk factors
This group does not require hospitalization; its members either have underlying health problems (such as emphysema or congestive heart failure) or is at risk for DRSP and/or enteric Gram negative bacteria. Treatment is with a fluoroquinolone active against Streptococcus pneumoniae such as levofloxacin or a beta-lactam antibiotic such as cefpodoxime, cefuroxime, amoxicillin, or amoxicillin/clavulanate plus a macrolide antibiotic such as azithromycin or clarithromycin for seven to ten days.[17]
  • Hospitalized individuals not at risk for Pseudomonas
This group requires hospitalization and administration of intravenous antibiotics. Treatment is with either an intravenous fluoroquinolone active against Streptococcus pneumoniae such as levofloxacin or beta-lactam antibiotic such as cefotaxime, ceftriaxone, ampicillin/sulbactam, or high-dose ampicillin plus an intravenous macrolide antibiotic such as azithromycin or clarithromycin for seven to ten days.
  • Individuals requiring intensive care at risk for Pseudomonas
Individuals being treated in an intensive care unit with risk factors for infection with Pseudomonas aeruginosa require specific antibiotics targeting this difficult to eradicate bacteria. One possible regimen is an intravenous antipseudomonal beta-lactam such as cefepime, imipenem, meropenem, or piperacillin/tazobactam plus an intravenous antipseudomonal fluoroquinolone such as levofloxacin. Another recommended regimen is an intravenous antipseudomonal beta-lactam such as cefepime, imipenem, meropenem, or piperacillin/ tazobactam plus an intravenous aminoglycoside such as gentamicin or tobramycin plus either an intravenous macrolide such azithromycin or an intravenous nonpseudomonal fluoroquinolone such as ciprofloxacin.

For mild to moderate community-acquired pneumonia shorter courses of antibiotics (3–7 days) seem to be sufficient according to a recent meta-analysis.[14]


Some people with CAP require hospitalization and more intensive care than the majority. In general, a discussion between the individual and his or her health care provider determines the need for hospitalization. Clinical prediction rules, such as the pneumonia severity index and CURB-65 have been developed to help guide the decision.[18] Factors that increase the need for hospitalization include:

  • Age greater than 65
  • Underlying chronic illnesses
  • Respiratory rate greater than thirty breaths per minute
  • Systolic blood pressure less than 90 mmHg
  • Heart rate greater than 125 beats per minute
  • Temperature less than 35 or greater than 40 °C
  • Confusion
  • Evidence of infection outside the lung

Laboratory results that increase the need for hospitalization include:

  • Arterial oxygen tension less than 60 mm Hg

Carbon dioxide of greater than 50 mmHg, or pH less than 7.35 while breathing room air

X-ray findings that increase the need for hospitalization include:

  • Involvement of more than one lobe of the lung
  • Presence of a cavity
  • Presence of a pleural effusion.


Individuals treated for CAP outside of the hospital have a mortality rate less than 1%. Fever typically responds in the first two days of therapy and other symptoms resolve in the first week. The x-ray, however, may remain abnormal for at least a month, even when CAP is successfully treated. Among individuals who require hospitalization, the mortality rate averages 12% overall, but is as much as 40% in people who have bloodstream infections or require intensive care.[19] Factors that increase mortality are the same as those that increase the need for hospitalization, and are listed above.

When CAP does not respond as expected, there are several possible causes. A complication of CAP may have occurred or a previously unknown health problem may be playing a role. Both situations are covered in more detail below. Additional causes include inappropriate antibiotics for the causative organism (i.e. DRSP), a previously unsuspected microorganism (such as tuberculosis), or a condition that mimics CAP (such as Wegener's granulomatosis). Additional testing may be performed and may include additional radiologic imaging (such as a computed tomography scan) or a procedure such as a bronchoscopy or lung biopsy.


Despite appropriate antibiotic therapy, severe complications can result from CAP, including:


Sepsis can occur when microorganisms enter the blood stream and the immune system responds. Sepsis most often occurs with bacterial pneumonia; Streptococcus pneumoniae is the most common cause. Individuals with sepsis require hospitalization in an intensive care unit. They often require medications and intravenous fluids to keep their blood pressure from going too low. Sepsis can cause liver, kidney, and heart damage among other things.

Respiratory failure[edit]

Because CAP affects the lungs, often individuals with CAP have difficulty breathing. If enough of the lung is involved, it may not be possible for a person to breathe enough to live without support. Non-invasive machines such as a bilevel positive airway pressure machine may be used. Otherwise, placement of a breathing tube into the mouth may be necessary and a ventilator may be used to help the person breathe.

Pleural effusion and empyema[edit]

Occasionally, microorganisms from the lung cause fluid to form in the space surrounding the lung, called the pleural cavity. If the microorganisms themselves are present, the fluid collection is often called an empyema. If pleural fluid is present in a person with CAP, the fluid should be collected with a needle (thoracentesis) and examined. Depending on the result of the examination, complete drainage of the fluid may be necessary, often with a chest tube. If the fluid is not drained, bacteria can continue to cause illness because antibiotics do not penetrate well into the pleural cavity.


Rarely, microorganisms in the lung form a pocket of fluid and bacteria called an abscess. Abscesses can be seen on an x-ray as a cavity within the lung. Abscesses typically occur in aspiration pneumonia and most often contain a mixture of anaerobic bacteria. Usually antibiotics are able to fully treat abscesses, but sometimes they must be drained by a surgeon or radiologist.


CAP is a common illness in all parts of the world. It is a major cause of death among all age groups. In children, the majority of deaths occur in the newborn period, with over two million worldwide deaths a year. In fact, the WHO estimates that one in three newborn infant deaths are due to pneumonia.[20] Mortality decreases with age until late adulthood; elderly individuals are particularly at risk for CAP and associated mortality.

More cases of CAP occur during winter months than during other times of the year. CAP occurs more commonly in males than females and in blacks than Caucasians. Individuals with underlying illnesses such as Alzheimer's disease, cystic fibrosis, emphysema, tobacco smoking, alcoholism, or immune system problems are at increased risk for pneumonia.[21]


In addition to treating underlying illness which may increase risk for CAP, additional measures can aid in prevention. Smoking cessation is important to treat underlying lung disease, and cigarette smoke interferes with many of the body's natural defenses against CAP.

Vaccination is important in both children and adults. Vaccinations against Haemophilus influenzae and Streptococcus pneumoniae in the first year of life have greatly reduced their role in childhood CAP. A vaccine against Streptococcus pneumoniae is also available for adults, and is recommended for healthy individuals over 65 and all adults with COPD, heart failure, diabetes mellitus, cirrhosis of the liver, alcoholism, cerebrospinal fluid leaks, or who have had a splenectomy. A revaccination may be required after five or ten years.[22]

Influenza vaccines should be given annually to individuals who are vaccinated against Streptococcus pneumoniae. In addition, health care workers, nursing-home residents and pregnant women should receive the vaccine.[23] During an influenza outbreak, drugs such as amantadine, rimantadine, zanamivir and oseltamivir have been shown to prevent influenza.[24]

See also[edit]


  1. ^ "Pneumonia Causes - Mayo Clinic". Retrieved 2015-05-18. 
  2. ^ "Pneumonia Treatments and drugs - Mayo Clinic". Retrieved 2015-05-18. 
  3. ^ "Pneumonia Prevention - Mayo Clinic". Retrieved 2015-05-18. 
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  17. ^ Vardakas KZ, Siempos II, Grammatikos A, Athanassa Z, Korbila IP, Falagas ME (December 2008). "Respiratory fluoroquinolones for the treatment of community-acquired pneumonia: a meta-analysis of randomized controlled trials". CMAJ 179 (12): 1269–77. doi:10.1503/cmaj.080358. PMC 2585120. PMID 19047608. 
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  19. ^ Woodhead MA, Macfarlane JT, McCracken JS, Rose DH, Finch RG (March 1987). "Prospective study of the aetiology and outcome of pneumonia in the community". Lancet 1 (8534): 671–4. doi:10.1016/S0140-6736(87)90430-2. PMID 2882091. 
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  • Mandell LA, Wunderink RG, Anzueto A et al. (March 2007). "Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults". Clinical Infectious Diseases 44 (Suppl 2): S27–72. doi:10.1086/511159. PMID 17278083. 

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