Community-Acquired Pneumonia

The most common bacterial causes are

Pneumonias caused by chlamydia and mycoplasma are often clinically indistinguishable from other pneumonias.

C. pneumoniae accounts for 2 to 5% of community-acquired pneumonia and is the 2nd most common cause of lung infections in healthy people aged 5 to 35 years. C. pneumoniae is commonly responsible for outbreaks of respiratory infection within families, in college dormitories, and in military training camps. It causes a relatively benign form of pneumonia that infrequently requires hospitalization. Chlamydia psittaci Community-Acquired Pneumonia Community-acquired pneumonia is defined as pneumonia that is acquired outside the hospital. The most commonly identified pathogens are Streptococcus pneumoniae, Haemophilus influenzae... read more pneumonia (psittacosis) is rare and occurs in patients who own or are often exposed to psittacine birds (ie, parrots, parakeets, macaws).

Since the year 2000, the incidence of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) skin infections has increased markedly. This pathogen can rarely cause severe, cavitating pneumonia and tends to affect young adults.

S. pneumoniae and MRSA can cause necrotizing pneumonia.

P. aeruginosa is an especially common cause of pneumonia in patients with cystic fibrosis Cystic Fibrosis Cystic fibrosis is an inherited disease of the exocrine glands affecting primarily the gastrointestinal and respiratory systems. It leads to chronic lung disease, exocrine pancreatic insufficiency... read more , neutropenia Neutropenia Neutropenia is a reduction in the blood neutrophil count. If it is severe, the risk and severity of bacterial and fungal infections increase. Focal symptoms of infection may be muted, but fever... read more , advanced acquired immunodeficiency syndrome Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection results from 1 of 2 similar retroviruses (HIV-1 and HIV-2) that destroy CD4+ lymphocytes and impair cell-mediated immunity, increasing risk of certain... read more (AIDS), and/or bronchiectasis Bronchiectasis Bronchiectasis is dilation and destruction of larger bronchi caused by chronic infection and inflammation. Common causes are cystic fibrosis, immune defects, and recurrent infections, though... read more . Another risk factor for P. aeruginosa pneumonia is hospitalization with receipt of IV antibiotics within the previous 3 months

A host of other organisms causes lung infection in immunocompetent patients.

Q fever, tularemia, anthrax, and plague are uncommon bacterial syndromes in which pneumonia may be a prominent feature. Tularemia Tularemia Tularemia is a febrile disease caused by the gram-negative bacterium Francisella tularensis; it may resemble typhoid fever. Symptoms are a primary local ulcerative lesion, regional lymphadenopathy... read more , anthrax Anthrax Anthrax is caused by the gram-positive Bacillus anthracis, which are toxin-producing, encapsulated, facultative anaerobic organisms. Anthrax, an often fatal disease of animals, is transmitted... read more , and plague Plague and Other Yersinia Infections Plague is caused by the gram-negative bacterium Yersinia pestis. Symptoms are either severe pneumonia or large, tender lymphadenopathy with high fever, often progressing to septicemia... read more should raise the suspicion of bioterrorism Biological Agents as Weapons Biological warfare (BW) is the use of microbiological agents for hostile purposes. Such use is contrary to international law and has rarely taken place during formal warfare in modern history... read more .

Bacterial superinfection can make distinguishing viral from bacterial infection difficult.

Common viral causes include

Epstein-Barr virus and coxsackievirus are common viruses that rarely cause pneumonia. Seasonal influenza Influenza Influenza is a viral respiratory infection causing fever, coryza, cough, headache, and malaise. Mortality is possible during seasonal epidemics, particularly among high-risk patients (eg, those... read more can rarely cause a direct viral pneumonia but often predisposes to the development of a serious secondary bacterial pneumonia. Varicella virus and hantavirus Hantavirus Infection Hantaviridae are a family of enveloped single-stranded RNA viruses, which consists of at least 4 serotypes with 9 viruses causing 2 major, sometimes overlapping, clinical syndromes: Hemorrhagic... read more cause lung infection as part of adult chickenpox and hantavirus pulmonary syndrome. Coronaviruses Coronaviruses and Acute Respiratory Syndromes (MERS and SARS) Coronaviruses are enveloped RNA viruses that cause respiratory illnesses of varying severity from the common cold to fatal pneumonia. Numerous coronaviruses, first discovered in domestic poultry... read more cause severe acute respiratory syndrome Severe Acute Respiratory Syndrome (SARS) Coronaviruses are enveloped RNA viruses that cause respiratory illnesses of varying severity from the common cold to fatal pneumonia. Numerous coronaviruses, first discovered in domestic poultry... read more (SARS), the Middle East respiratory syndrome Middle East Respiratory Syndrome (MERS) Coronaviruses are enveloped RNA viruses that cause respiratory illnesses of varying severity from the common cold to fatal pneumonia. Numerous coronaviruses, first discovered in domestic poultry... read more (MERS), and COVID-19 COVID-19 COVID-19 is a respiratory illness caused by the novel coronavirus SARS-CoV-2. Infection may be asymptomatic or have symptoms ranging from mild upper respiratory symptoms to acute respiratory... read more .

Common fungal pathogens include Histoplasma capsulatum (histoplasmosis Histoplasmosis Histoplasmosis is a pulmonary and hematogenous disease caused by Histoplasma capsulatum; it is often chronic and usually follows an asymptomatic primary infection. Symptoms are those... read more ) and Coccidioides immitis (coccidioidomycosis Coccidioidomycosis Coccidioidomycosis is caused by the fungi Coccidioides immitis and C. posadasii; it usually occurs as an acute, benign, asymptomatic or self-limited respiratory infection. The... read more ). Less common fungal pathogens include Blastomyces dermatitidis (blastomycosis Blastomycosis Blastomycosis is a pulmonary disease caused by inhaling spores of the dimorphic fungus Blastomyces dermatitidis. Occasionally, the fungi spread hematogenously, causing extrapulmonary... read more ) and Paracoccidioides braziliensis (paracoccidioidomycosis Paracoccidioidomycosis Paracoccidioidomycosis is progressive mycosis of the lungs, skin, mucous membranes, lymph nodes, and internal organs caused by Paracoccidioides brasiliensis. Symptoms are skin ulcers... read more ). Pneumocystis jirovecii commonly causes pneumonia in patients who have human immunodeficiency virus (HIV) infection or are immunosuppressed (see Pneumonia in Immunocompromised Patients Pneumonia in Immunocompromised Patients Pneumonia in immunocompromised patients is often caused by unusual pathogens but may also be caused by the same pathogens that cause community-acquired pneumonia. Symptoms and signs depend on... read more ).

Parasites causing lung infection in developed countries include Toxocara canis or T. catis (toxocariasis Toxocariasis Toxocariasis is human infection with nematode ascarid larvae that ordinarily infect animals. Symptoms are fever, anorexia, hepatosplenomegaly, rash, pneumonitis, asthma, or visual impairment... read more ), Dirofilaria immitis (dirofilariasis Dirofilariasis Dirofilariasis is a filarial nematode infection with Dirofilaria immitis, the dog heartworm, or other Dirofilaria species, which are transmitted to humans by infected mosquitoes... read more ), and Paragonimus westermani (paragonimiasis Paragonimiasis Paragonimiasis is infection with the lung fluke Paragonimus westermani and related species. Humans are infected by eating raw, pickled, or poorly cooked freshwater crustaceans. Most infections... read more ).

Pulmonary tuberculosis Tuberculosis (TB) Tuberculosis is a chronic, progressive mycobacterial infection, often with an asymptomatic latent period following initial infection. Tuberculosis most commonly affects the lungs. Symptoms include... read more and nontuberculous mycobacterial infections Nontuberculous Mycobacterial Infections There are over 170 recognized species of mycobacteria, mostly environmental. Environmental exposure to many of these organisms is common, but most exposures do not cause infection and many infections... read more are discussed elsewhere.

In children, the most common causes of pneumonia depend on age:

Pneumonia in neonates Neonatal Pneumonia Neonatal pneumonia is lung infection in a neonate. Onset may be within hours of birth and part of a generalized sepsis syndrome or after 7 days and confined to the lungs. Signs may be limited... read more is discussed elsewhere.

Diagnosis of etiology can be difficult. A thorough history of exposures, travel, pets, hobbies, and other exposures is essential to raise suspicion of less common organisms. For example, exposure to farm animals may suggest Q fever, and a recent hotel or cruise ship stay may suggest Legionella infection.

Identification of the pathogen can be useful to direct therapy and verify bacterial susceptibilities to antibiotics. However, because of the limitations of current diagnostic tests and the success of empiric antibiotic treatment, experts recommend limiting attempts at microbiologic identification (eg, cultures, specific antigen testing) unless patients are at high risk or have complications (eg, severe pneumonia, immunocompromise, asplenia, failure to respond to empiric therapy). In general, the milder the pneumonia, the less such diagnostic testing is required. Critically ill patients require the most intensive testing, as do patients in whom a antibiotic-resistant or unusual organism is suspected (eg, Mycobacterium tuberculosis, P. jirovecii) and patients whose condition is deteriorating or who are not responding to treatment within 72 hours.

Chest x-ray findings generally cannot distinguish one type of infection from another, although the following findings are suggestive:

Blood cultures, which are often obtained in patients hospitalized for pneumonia, can identify causative bacterial pathogens if bacteremia is present. About 12% of all patients hospitalized with pneumonia have bacteremia; S. pneumoniae accounts for two thirds of these cases.

Sputum testing can include Gram stain and culture for identification of the pathogen, but the value of these tests is uncertain because specimens often are contaminated with oral flora and overall diagnostic yield is low. Regardless, identification of a bacterial pathogen in sputum cultures allows for susceptibility testing. Obtaining sputum samples also allows for testing for viral pathogens via direct fluorescence antibody testing or polymerase chain reaction (PCR), but caution needs to be exercised in interpretation because 15% of healthy adults carry a respiratory virus or potential bacterial pathogen. In patients whose condition is deteriorating and in those unresponsive to broad-spectrum antibiotics, sputum should be tested with mycobacterial and fungal stains and cultures.

Sputum samples can be obtained noninvasively by simple expectoration or after hypertonic saline nebulization (induced sputum) for patients unable to produce sputum. Alternatively, patients can undergo bronchoscopy or endotracheal suctioning, either of which can be easily done through an endotracheal tube in mechanically ventilated patients. Otherwise, bronchoscopic sampling is usually done only for patients with other risk factors (eg, immunocompromise, failure of empiric therapy).

Urine testing for Legionella antigen and pneumococcal antigen is now widely available. These tests are simple and rapid and have higher sensitivity and specificity than sputum Gram stain and culture for these pathogens. Patients at risk of Legionella pneumonia (eg, severe illness, failure of outpatient antibiotic treatment, presence of pleural effusion, active alcohol abuse, recent travel) should undergo testing for urinary Legionella antigen, which remains present long after treatment is initiated, but the test detects only L. pneumophila serogroup 1 (70% of cases).

The pneumococcal antigen test is recommended for patients who are severely ill; have had unsuccessful outpatient antibiotic treatment; or who have pleural effusion, active alcohol abuse, severe liver disease, or asplenia. This test is especially useful if adequate sputum samples or blood cultures were not obtained before initiation of antibiotic therapy. A positive test can be used to tailor antibiotic therapy, though it does not provide antimicrobial susceptibility.

COVID-19 test Diagnosis COVID-19 is a respiratory illness caused by the novel coronavirus SARS-CoV-2. Infection may be asymptomatic or have symptoms ranging from mild upper respiratory symptoms to acute respiratory... read more using reverse transcriptase-polymerase chain reaction (RT-PCR) testing of respiratory secretions (nasopharyngeal specimen is preferred) is recommended in patients presenting with pneumonia during the current pandemic.

Serum procalcitonin can help distinguish bacterial infection from other causes of infection or inflammation. However, use of the serum procalcitonin level as a criterion for whether to initiate antibiotic therapy in community-acquired pneumonia is not recommended. It can be used, along with clinical judgement, to guide early discontinuation of antibiotics in lower respiratory tract infections.

Risk stratification via risk prediction rules may be used to estimate mortality risk and thus help guide decisions regarding hospitalization. These rules have been used to identify patients who can be safely treated as outpatients and those who require hospitalization because of high risk of complications (see table Risk Stratification for Community-Acquired Pneumonia Risk Stratification for Community-Acquired Pneumonia (the Pneumonia Severity Index) ). However, these rules should supplement, not replace, clinical judgment because many unrepresented factors, such as likelihood of adherence, ability to care for self, ability to maintain oral intake, should also influence triage decisions.

Intensive care unit (ICU) admission is required for patients who

Other criteria, especially if ≥ 3 are present, that should lead to consideration of ICU admission include

The Pneumonia Severity Index (PSI) is the most studied and validated prediction rule. However, because the PSI is complex and requires several laboratory assessments, simpler rules such as CURB-65 are usually recommended for clinical use. Use of these prediction rules has led to a reduction in unnecessary hospitalizations for patients who have milder illness.

In CURB-65, 1 point is allotted for each of the following risk factors:

Scores can be used as follows:

In clinical settings where access to a BUN measurement is not readily available, a CRB-65 score can be used instead. Use of CRB-65 scores is similar to that for CURB-65, with 0 points: appropriate for outpatient therapy; 1-2 points: consider hospitalization; ≥ 3 points: consider ICU admission.

Antibiotic therapy is the mainstay of treatment for community-acquired pneumonia. Appropriate treatment involves starting empiric antibiotics as soon as possible, preferably ≤4 hours after presentation. Because pathogen identification is difficult and takes time, the empiric antibiotic regimen is selected based on likely pathogens and severity of illness. Consensus guidelines have been developed by many professional organizations; one widely used set is detailed in the table Community-Acquired Pneumonia in Adults Community-Acquired Pneumonia in Adults (see also Infectious Diseases Society of America Clinical Guideline on Community-Acquired Pneumonia). Guidelines should be adapted to local susceptibility patterns, drug formularies, and individual patient circumstances. If a pathogen is subsequently identified, the results of antibiotic susceptibility testing can help guide any changes in antibiotic therapy. Omadacycline and lefamulin can be considered, especially in situations in which the usually recommended choices are not appropriate.

For children, treatment depends on age, previous vaccinations, and whether treatment is outpatient or inpatient.

For children treated as outpatients, treatments are dictated by age:

For children treated as inpatients, antibiotic therapy tends to be more broad-spectrum and depends on the child's previous vaccinations:

Full details are described in the Clinical Practice Guidelines by the Pediatric Infectious Diseases Society and the Infectious Diseases Society of America.

With empiric treatment, 90% of patients with bacterial pneumonia improve. Improvement is manifested by decreased cough and dyspnea, defervescence, relief of chest pain, and decline in white blood cell count. Failure to improve should trigger suspicion of

When usual therapy has failed, consultation with a pulmonary and/or infectious disease specialist is indicated.

Antiviral therapy may be indicated for select viral pneumonias. Ribavirin is not used routinely for respiratory syncytial virus pneumonia in children or adults but may be used occasionally in high-risk children age < 24 months.

For influenza, oseltamivir 75 mg orally twice a day or zanamivir 10 mg inhaled twice a day started within 48 hours of symptom onset and given for 5 days reduce the duration and severity of symptoms in patients who develop influenza infection. Alternatively, baloxavir started within 48 hours of symptom onset can be given in a single dose of 40 mg for patients 40 to 79 kg; 80 mg is used for patients ≥ 80 kg. In patients hospitalized with confirmed influenza infection, observational studies suggest benefit even 48 hours after symptom onset.

Acyclovir 10 mg/kg IV every 8 hours for adults or 250 to 500 mg/m² body surface area IV every 8 hours for children is recommended for varicella lung infections.

Though pure viral pneumonia does occur, superimposed bacterial infections are common and require antibiotics directed against S. pneumoniae, H. influenzae, and S. aureus.

Follow-up x-rays are generally not recommended in patients whose pneumonia resolves clinically as expected. Resolution of radiographic abnormalities can lag behind clinical resolution by several weeks. Chest x-ray should be considered in patients with pneumonia symptoms that do not resolve or that worsen over time.

Supportive care includes fluids, antipyretics (as needed for high grade fever), analgesics, and, for patients with hypoxemia, oxygen. Prophylaxis against thromboembolic disease and early mobilization improve outcomes for patients hospitalized with pneumonia. Cessation counseling Smoking Cessation Most people who smoke want to quit and have tried doing so with limited success. Effective interventions include cessation counseling and pharmacologic treatment, such as varenicline, bupropion... read more should also be done for smokers.