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Community-acquired pneumonia (CAP) is pneumonia (any of several lung diseases) acquired infectiously from normal social contact (that is, in the community) as opposed to being acquired during hospitalization (hospital-acquired pneumonia). In community-acquired pneumonia, individuals who have not recently been hospitalized develop an infection of the lungs (pneumonia). CAP is a common illness and can affect people of all ages. CAP often causes problems such as difficulty breathing, fever, chest pains, and a cough. CAP occurs because the areas of the lung that absorb oxygen (alveoli) from the atmosphere become filled with fluid and cannot work effectively.
CAP occurs throughout the world and is a leading cause of illness and death. Causes of CAP include bacteria, viruses, fungi, and parasites. CAP can be diagnosed by symptoms and physical examination alone, though x-rays, examination of the sputum, and other tests are often used. Individuals with CAP sometimes require hospitalization. CAP is primarily treated with antibiotics. Some forms of CAP can be prevented by vaccination.
Symptoms of CAP commonly include:
Less common symptoms include:
The manifestations of pneumonia, like those for many conditions, might not be typical in older people. They might instead experience:
Additional symptoms for infants could include:
Over a hundred microorganisms can cause CAP. The most common microorganism types differ among different groups of people. 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. Even when aggressive measures are taken, a definite cause for pneumonia is only identified in half the cases.
Newborn infants can acquire lung infections prior to being born either by breathing infected amniotic fluid or by blood-borne infection across the placenta. Infants can also inhale (aspirate) fluid from the birth canal as they are being born. The most important infection in newborns is caused by Streptococcus agalactiae, also known as Group B Streptococcus or GBS. GBS causes >50% of cases of CAP in the first week of life. Other bacterial causes in the newborn period include Listeria monocytogenes and tuberculosis. Viruses can also be transferred from mother to child; herpes simplex virus is the most common and life-threatening, but adenovirus, mumps, and enterovirus can also cause disease.
CAP in older infants reflects increased exposure to microorganisms. Common bacterial causes include Streptococcus pneumoniae, Escherichia coli, Klebsiella pneumoniae, Moraxella catarrhalis, and Staphylococcus aureus. A unique cause of CAP in this group is Chlamydia trachomatis, which is acquired during birth but does not cause pneumonia until two to four weeks later. Classically it presents with no fever and a characteristic staccato cough. Maternally-derived syphilis can be a cause of CAP in this age group. 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.
For the most part, children older than one month of life 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.
The full spectrum of microorganisms is responsible for CAP in adults. Several important groups of organisms are more common among people with certain risk factors. Identifying people at risk for these organisms is important for appropriate treatment.
Many less common organisms cause CAP. They are typically identified because an individual has special risk factors or after treatment for the common causes has failed. These rarer causes are covered in more detail in their specific pages: bacterial pneumonia, viral pneumonia, fungal pneumonia, and parasitic pneumonia.
Some people have an underlying problem that increases their risk of infection. Some important situations are covered below:
When part of the airway (bronchi) that leads to the alveoli is obstructed, the lung can't clear fluid when it accumulates. This can lead to infection of the fluid, resulting in CAP. One cause of obstruction, especially in young children, is inhalation of a foreign object, such as a marble or toy. The object lodges in the small airways, and pneumonia forms in the obstructed areas of lung. Another cause of obstruction is lung cancer, which can grow into the airways, blocking the flow of air.
People with underlying lung disease are more likely to develop CAP. Diseases such as emphysema or habits such as smoking result in more frequent and more severe bouts of CAP. In children, recurrent episodes of CAP may be the first clue to diseases such as cystic fibrosis or pulmonary sequestration.
People who have immune system problems are more likely to get CAP. People who have AIDS are much more likely to develop CAP. Other immune problems range from severe immune deficiencies of childhood such as Wiskott-Aldrich syndrome to less severe deficiencies such as common variable immunodeficiency.
The symptoms of CAP are the result of both the invasion of the lungs by microorganisms and the immune system's response to the infection. The mechanisms of infection are quite different for viruses and the other microorganisms.
Individuals with symptoms of CAP require further 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/or changes in the amount of oxygen in the blood. Feeling the way the chest expands (palpation) and tapping the chest wall (percussion) to identify dull areas that do not resonate can identify lung areas that are stiff and full of fluid (consolidated).
Listening to the lungs with a stethoscope (auscultation) can reveal several things. A lack of normal breath sounds, or the presence of crackling sounds (rales) can indicate consolidation. Increased vibration of the chest when speaking (tactile fremitus) and increased volume of whispered speech during auscultation of the chest can also reveal consolidation.
X-rays of the chest, examination of the blood and sputum for infectious microorganisms, and blood tests are commonly used to diagnose individuals with suspected CAP based upon symptoms and physical examination. The use of each test depends on the severity of illness, local practices, and the concern for any complications resulting from the infection. All patients with CAP should have the amount of oxygen in their blood monitored with a machine called a pulse oximeter. This helps determine how well the lungs are able to work despite infection. In some cases, analysis of arterial blood gas may be required to accurately determine the amount of oxygen in the blood. Complete blood count (CBC), a blood test, may reveal extra white blood cells, indicating an infection.
Chest x-rays and chest computed tomography (CT) can reveal areas of opacity (seen as white), which represent consolidation. A normal chest x-ray makes CAP less likely. However, sometimes CAP does not appear on x-rays—either because the disease is in its initial stages, or involves a part of the lung an x-ray does not easily see. In some cases, chest CT can reveal a CAP not present on chest x-ray. X-rays can often mislead, as many other diseases can mimic CAP—such as heart problems or other types of lung damage.
Several tests can be performed to identify the cause of an individual's CAP. Blood cultures can be drawn to isolate any bacteria or fungi in the blood stream. Sputum Gram's stain and culture can also reveal the causative microorganism. In more severe cases, a procedure wherein a flexible scope is passed through the mouth into the lungs (bronchoscopy) can be used to collect fluid for culture. Special tests can be performed if an uncommon microorganism is suspected (such as testing the urine for Legionella antigen when Legionnaires' disease is a concern).
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 both cases, health professionals must consider a person's risk factors for different organisms when choosing the initial antibiotics (called empiric therapy). Additional consideration must be given to the setting in which the individual is treated. Most people are fully treated 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 although a fluoroquinolone such as levofloxacin can substitute. Doxycycline is now the antibiotic of choice in the UK for complete coverage of the atypical bacteria. This is due to increased levels of Clostridium difficile seen in hospital patients being linked to the increased use of clarithromycin.
Most newborn infants with CAP are hospitalized and given intravenous ampicillin and gentamicin for at least ten days. This treats the common bacteria Streptococcus agalactiae, Listeria monocytogenes, and Escherichia coli. If herpes simplex virus is the cause, intravenous 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 a child does not need to be hospitalized, amoxicillin for seven days is a common treatment. However, with increasing prevalence of DRSP, other agents—such as cefpodoxime—will likely become more popular. Hospitalized children should receive intravenous ampicillin, ceftriaxone, or cefotaxime. According to a recent meta-analysis a three days course of antibiotics seems to be sufficient for most cases of mild to moderate CAP in children.
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.
For mild to moderate community-acquired pneumonia shorter courses of antibiotics (3–7 days) seem to be sufficient according to a recent meta-analysis.
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. Factors that increase the need for hospitalization include:
Laboratory results that increase the need for hospitalization include:
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:
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. 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.
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.
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. 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.
In addition to treating underlying illness that can increase a person's risk for CAP, several additional measures can help prevent CAP. Smoking cessation is important, not only for treatment of any underlying lung disease, but also because 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 CAP in children. A vaccine against Streptococcus pneumoniae is also available for adults and is currently recommended for all healthy individuals older than 65 and any adults with emphysema, congestive heart failure, diabetes mellitus, cirrhosis of the liver, alcoholism, cerebrospinal fluid leaks, or who do not have a spleen. A repeat vaccination may also be required after five or ten years.
Influenza vaccines should be given yearly to the same individuals as receive vaccination against Streptococcus pneumoniae. In addition, health care workers, nursing home residents, and pregnant women should receive the vaccine. When an influenza outbreak is occurring, medications such as amantadine, rimantadine, zanamivir, and oseltamivir have been shown to prevent cases of influenza.