Community-acquired pneumonia (CAP) refers to pneumonia contracted by a person outside of the healthcare system. In contrast, hospital-acquired pneumonia (HAP) is seen in patients who are in a hospital or who have recently been hospitalized in the last 48 hours. Those who live in long-term care facilities or who had pneumonia after 48 hours of hospitalization for another cause are also classified as having CAP (they were previously designated as having HCAP (healthcare associated pneumonia)). CAP is common, affecting people of all ages, and its symptoms occur as a result of oxygen-absorbing areas of the lung (alveoli) becoming colonized by a pathogenic microorganism (such as bacteria, viruses or fungi). The resulting inflammation and tissue damage causes fluid to fill the alveoli, inhibiting lung function and causing the symptoms of the disease. Common symptoms of CAP include dyspnea, fever, chest pains and cough.
10% of those with CAP are hospitalized. The 30 day mortality for those hospitalized with CAP is 2.8% for adults younger than 60 and 26.8% for adults older than 60 with other medical conditions.
CAP, the most common type of pneumonia, is a leading cause of illness and death worldwide. Its causes include bacteria, viruses, fungi and parasites. CAP is diagnosed by assessing symptoms, performing a physical examination, by x-ray or by sputum examination. Some form of chest imaging, usually in the form of a chest x-ray, showing characteristic findings is required for the diagnosis. Most cases can be treated on an outpatient basis, but some patients with CAP require hospitalization. CAP is treated primarily with antibiotics, antivirals or antifungals depending on the confirmed or suspected microorganism pathogen. Some forms of CAP can be prevented by vaccination and by abstaining from tobacco products. Vaccination against influenza, COVID, respiratory syncytial virus and the pneumococcal conjugate vaccine can all prevent CAP.
Signs and symptoms
Common symptoms
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 Coughing up blood (hemoptysis) Headaches, including migraines Loss of appetite Excessive fatigue Bluish skin (cyanosis) Nausea Vomiting Diarrhea Joint pain (arthralgia) Muscle aches (myalgia) Rapid heartbeat Dizziness or lightheadedness
In the elderly New or worsening confusion Hypothermia Poor coordination, which may lead to falls
In infants Unusual sleepiness Yellowing of the skin (jaundice) Difficulty feeding
Complications Major complications of CAP include: Sepsis - A life-threatening reaction to infection. A common cause of sepsis is bacterial pneumonia, frequently the result of infection with streptococcus pneumoniae. Patients with sepsis require intensive care with blood pressure monitoring and support against hypotension. Sepsis can cause liver, kidney and heart damage. Respiratory failure - CAP patients often have dyspnea, which may require support. Non-invasive machines (such as bilevel positive airway pressure), a tracheal tube or a ventilator may be used. Pleural effusion and empyema - Microorganisms from the lung may trigger fluid collection in the pleural cavity, or empyema. Pleural fluid, if present, should be collected with a needle and examined. Depending on the results, complete drainage of the fluid with a chest tube may be necessary to prevent proliferation of the infection. Antibiotics, which do not penetrate the pleural cavity well, are less effective. Abscess - A pocket of fluid and bacteria may appear on X-ray as a cavity in the lung. Abscesses, typical of aspiration pneumonia, usually contain a mixture of anaerobic bacteria. Although antibiotics can usually cure abscesses, sometimes they require drainage by a surgeon or radiologist.
Causes Many different microorganisms can cause CAP. However, the most common cause is Streptococcus pneumoniae. Certain groups of people are more susceptible to CAP-causing pathogens - infants, adults with chronic conditions (such as chronic obstructive pulmonary disease), and senior citizens. Alcoholics and others with compromised immune systems are more likely to develop CAP from Haemophilus influenzae or Pneumocystis jirovecii. A definitive cause is identified in only half the cases.
Neonates and infants It is possible for a fetus to develop a lung infection before birth by aspirating 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. 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 neonatal CAP is Chlamydia trachomatis, which, though acquired at birth, does not cause 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 infants. Viral causes 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. CAP caused by fungi or parasites is not usually seen in otherwise-healthy infants.
Children Although children older than one month tend to be at risk for the same microorganisms as adults, children under five years of age are much less likely to have pneumonia caused by Mycoplasma pneumoniae, Chlamydophila pneumoniae or Legionella pneumophila than older children. In contrast, older children and teenagers are more likely to acquire Mycoplasma pneumoniae and Chlamydophila pneumoniae than adults.
Adults A full spectrum of microorganisms is responsible for CAP in adults, and patients with certain risk factors are more susceptible to infections by 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; these may be determined by identifying specific risk factors, or when treatment for more common causes fails.
Risk factors 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 that increase the risk of developing pneumonia range from severe childhood immune deficiencies, such as Wiskott–Aldrich syndrome, to the less severe common variable immunodeficiency.
Pathophysiology The symptoms of CAP are the result of lung infection by microorganisms and the response of the immune system to the infection. Mechanisms of infection are different for viruses and other microorganisms.
Viruses Up to 20 percent of CAP cases can be attributed to viruses. The most common viral causes 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.
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 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 by inhalation of water droplets, although they can reach the lung through the bloodstream if an infection is present. 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).
Parasites 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.
Diagnosis Patients with symptoms of CAP require evaluation. Diagnosis of pneumonia is made clinically, rather than on the basis of a particular test. Evaluation begins with a physical examination by a health provider, which 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 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 the presence of fluid.