Managing community-acquired pneumonia during flu season

General internists need to be able to recognize community-acquired pneumonia (CAP) so that diagnostic and therapeutic interventions can be initiated promptly. It is also important to understand the most likely and possible causes of CAP so that appropriate initial antimicrobial therapy can be chosen. Especially during flu season, influenza can present as CAP and should be included in the differential diagnosis.

When managing a patient with CAP, the internist must decide which level of care, diagnostic tests, antimicrobial agents, and follow-up plans are needed. These topics will be reviewed in this article.

TWO TERMS TO REMEMBER

• CAP refers to pneumonia acquired outside a health care facility. It can be either bacterial or viral.
• CABP (community-acquired bacterial pneumonia) refers only to those cases caused by bacterial pathogens.

NUMBERS AND TRENDS

In the United States, CAP is the number-one cause of death from infection and the sixth leading cause of death overall.¹ Each year, it is responsible for about 4.2 million outpatient visits, more than 60,000 deaths, and more than $17 billion in health care expenses.²

Community-acquired bacterial pneumonia: Common, serious

In a population-based US study in 1991, the incidence of CABP requiring hospitalization was 266.8 per 100,000 people.³

Estimates of overall mortality in CABP vary depending on the severity of illness and comorbid conditions. A meta-analysis published in 1996 found the overall mortality rate to be 13.7%, with a range of 5.1% to 36.5% depending on severity.⁴

In hospitalized patients, mortality rates and length of hospital stay appear to be declining over time. Between 1993 and 2005, the age-adjusted mortality rate decreased from 8.9% to 4.1%, and the average length of stay decreased from 7.5 to 5.7 days, with an overall reduction in hospital cost.⁵

CABP is more prevalent in older people than in the general population, and it increases with age from 18.2 cases per 1,000 patient-years in patients 60 to 69 years to 52.3 cases per 1,000 patient-years in those older than 85 years.⁶ Risk factors for pneumonia in the elderly include heart disease, chronic lung disease, immunosuppressive drugs, alcoholism, and increasing age.⁷ Similar to the trend in the general population, the mortality rate in elderly CABP patients appears to be decreasing over time, possibly thanks to rising rates of pneumococcal and influenza vaccination.⁸

Among the general population, risk factors for developing CABP also include smoking, occupational dust exposure, history of childhood pneumonia, unemployment, and single marital status.⁹ The incidence of CABP does not appear to be higher among pregnant women, although it is the most frequent cause of nonobstetric death in this population.¹⁰

The use of proton pump inhibitors may be an emerging risk factor for CABP.¹¹ Also, use of nonsteroidal anti-inflammatory drugs among patients with CABP is associated with a blunted inflammatory response as well as a higher risk of pleuropulmonary complications and a delay in presentation.¹²

Influenza is also common, potentially severe

Influenza is also very common and potentially severe. It can cause a spectrum of disease, from mild upper respiratory tract symptoms to severe viral pneumonia that can be life-threatening and complicated by respiratory failure and the acute respiratory distress syndrome (ARDS).

Influenza infection can also be complicated by subsequent bacterial pneumonia. However, the epidemiology of influenza infection differs from that of CABP in that influenza occurs seasonally.

In the United States, seasonal influenza causes 36,000 deaths and 200,000 hospitalizations annually.^13,14 As with CABP, the risk of death from influenza increases with age: it is 16 times greater in people age 85 and older than in those ages 65 to 69.¹³

During yearly seasonal epidemics, those at the highest risk of hospitalization and death are at the extremes of age. Risk factors for complicated influenza include heart disease, lung disease, diabetes, renal failure, rheumatologic conditions, dementia, and neurologic disease.^15,16 During the 2009 H1N1 influenza pandemic, unexpected severity was seen in previously healthy young adults as well as those with obesity, neurodegenerative disease, pregnancy, and asthma.¹⁷

PATHOGENS: TYPICAL, ATYPICAL, VIRAL

Identifying the etiologic organism in CAP is confounded by limitations in the available diagnostic tests and also by poor-quality specimens that often are contaminated with bacteria that colonize the upper airways. Given these caveats, the primary pathogens responsible for CAP broadly include typical bacterial pathogens, atypical bacterial pathogens, and viruses.

Typical bacterial pathogens include Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, Moraxella catarrhalis, and, less commonly, a variety of aerobic and anaerobic gram-negative rods including Pseudomonas aeruginosa, Acinetobacter species, and Klebsiella pneumoniae.

Atypical bacterial pathogens include Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella species.¹⁸

Viruses implicated in adult CAP include influenza A and B, parainfluenza viruses, respiratory syncytial virus, and adenovirus.¹⁹ More recently, human metapneumovirus has been described as a cause of adult CAP.²⁰

Clues to uncommon microbes

Specific historic features or coexisting conditions that may suggest an uncommon microbiologic diagnosis include²¹:

• Recent travel to the southwestern United States or Southeast Asia
• Ill contacts
• Exposure to birds, bats, rabbits, or farm animals
• Alcoholism
• Chronic obstructive pulmonary disease
• Human immunodeficiency virus infection
• Structural lung disease
• Prolonged cough with whoop or posttussive vomiting
• Aspiration
• Bioterrorism.

In cases in which one or more of these conditions exist, CAP may also be caused by other agents not listed above, including Mycobacterium tuberculosis, oral anaerobes, atypical mycobacteria, Histoplasma capsulatum, Chlamydophila psittaci, Francisella tularensis, Coxiella burnettii, Pneumocystis jiroveci, Cryptococcus, Aspergillus, Coccidioides, Hantavirus, avian influenza, Burkholderia pseudomallei, severe acute respiratory syndrome virus, Bordetella pertussis, Bacillus anthracis, and Yersinia pestis.