PNEUMOCOCCAL PNEUMONIA
Streptococcus pneumoniae is the most common pathogen for community-acquired pneumonia (CAP) in all patient populations, including those without a cause recognized by routine diagnostic testing. The organism is a gram-positive, lancet-shaped diplococcus, of which there are 84 different serotypes, each with a distinct antigenic polysaccharide capsule. Eighty-five percent of all infections are caused by one of 23 serotypes, which are now included in a vaccine. Infection is most common in the winter and early spring, which may relate to the finding that up to 70% of patients have a preceding viral illness. Patients at risk include elderly individuals; people with asplenia, multiple myeloma, congestive heart failure, or alcoholism; after influenza; and patients with chronic lung disease. Individuals with HIV infection have pneumococcal pneumonia with bacteremia more commonly than those in healthy populations of the same age.
PATHOGENESIS AND PATHOLOGY
The organism spreads from person to person, and
asymptomatic colonization of the oropharynx usually precedes the development of
parenchymal lung invasion. Pneumonia develops when colonizing organisms are
aspirated into a lung that is unable to contain the aspirated inoculum, often
because of host defense impairment or because of acquisition of a particularly
virulent strain, such as serotypes I and III. Virulence factors exist in the
pneumococcus that facilitate its invasion in the lung; these include
pneumococcal surface proteins A and C, which promote binding to airway
epithelium and interfere with host defense against the bacteria, and
pneumolysin, which can promote tissue invasion and interfere with ciliary
beating.
The initial response to pneumococcal lung infection is
extensive edema formation, which fills the lung and spreads the infection. At
this phase, the lung looks grossly purple and is filled with frothy fluid when sectioned.
In the next few hours, fibrin and neutrophils enter the alveolar space, and
gradually over the next 24 to 48 hours, the bacteria move intracellularly as
they are phagocytosed. The lung then becomes firmer and of a liverlike
consistency, but with capillary congestion, and there are foci of hemorrhage
that lead to a red color and a phase of “red hepatization.” As the blood clears
over the next 2 or more days, a phase of “gray hepatization” follows.
Generally, the lung returns to its normal appearance in 5 to 10 days, but in
some instances, fibroblasts enter the lobe, and organization and fibrosis may
occur. In most patients, the inflammation initially extends to the pleura and
leads to a parapneumonic effusion, but some patients may develop infection of the
pleural space, or empyema.
CLINICAL FEATURES
A previously healthy individual who develops
pneumococcal pneumonia has symptoms of “typical pneumonia” with a sudden onset
of high fever, shaking chills, pleuritic chest pain, leukocytosis with a left
shift, and purulent (or even blood-tinged, “rusty” colored) sputum. Elderly
patients with immune impairment, often caused by the presence of comorbid
illness, may not have these classic symptoms and may only have malaise,
dyspnea, confusion, and failure to thrive.
The classic radiographic pattern is a lobar
consolidation, but bronchopneumonia may also occur. Bacteremia is present in up
to 20% of hospitalized patients with this infection, but its presence probably
does not lead to increased mortality, although it may be associated with
delayed clinical resolution. Extrapulmonary complications, which may lead to
a failure to respond to therapy, include meningitis, empyema (which is
distinguished from a complicated or uncomplicated parapneumonic effusion by
sampling of pleural fluid), arthritis, endocarditis, and brain abscess. In the
absence of any of these complications, patients usually show clinical
improvement within 24 to 48 hours of the initiation of adequate antibiotic
therapy.
The diagnosis of pneumococcal pneumonia can be
confirmed by positive blood culture results, but other diagnostic tests include
sputum for Gram stain and culture and urinary antigen testing. The value of
sputum Gram stain for establishing the diagnosis and for guiding therapy is
controversial because the test is not always sensitive or specific, many
patients cannot produce a good specimen for evaluation, and the yield of Gram
stain is reduced if the patient has been on antibiotic therapy before sampling.
When a sputum culture is obtained, it should be interpreted in conjunction with
the findings of the Gram stain. Urinary antigen testing for pneumococcus is also
commercially available.
Current recommendations are to treat for a minimum of
5 days, provided that the patient is afebrile for 48 to 72 hours and other
clinical signs of pneumonia have resolved. Pneumococcal bacteremia may delay
the clinical response but does not by itself necessitate prolonged therapy.
In recent years, some investigators have measured serum levels of
procalcitonin, an acute phase reactant synthesized by the liver in response to
bacterial infection, and used serial levels to guide the duration of therapy.
Penicillin is the drug of choice, but penicillin resistance has become
increasingly common since the mid-1990s, with some level of resistance seen in
more than 40% of these organisms in the United States and Europe. Many of these
organisms are also resistant to other common antibiotics (macrolides,
trimethoprim-sulfamethoxazole, selected cephalosporins, and even the
quinolones). The clinical impact of resistance on outcomes such as mortality is uncertain but may lead to an increased
risk of death.
PREVENTION
Pneumococcal capsular polysaccharide vaccine may
prevent pneumococcal pneumonia and is recommended for those at risk, including
those older than age 65 years, those residing in a nursing home or institution,
those with splenic dysfunction (splenectomy, sickle cell disease), anyone with
a chronic medical illness (e.g., heart or lung disease, diabetes), and those
who are immunosuppressed (corticosteroid therapy, chemo-therapy). Adults should
receive the 23-valent capsular polysaccharide vaccine (PPV), although children
are given the 7-valent conjugate vaccine (PCV), which is more immunogenic.
The benefits of the PPV have been confirmed in
immunocompetent patients older than age 65 years, and effectiveness has been
estimated to be 75%, although it ranges from 65% to 84% in patients with
chronic diseases, including diabetes mellitus, coronary artery disease, congestive heart failure, chronic
pulmonary disease, and anatomic asplenia. Its effectiveness has not been as
well established in immune-deficient populations such as those with sickle cell
disease, chronic renal failure, immunoglobulin deficiency, Hodgkin disease,
lymphoma, leukemia, and multiple myeloma. A single revaccination is recommended
in patients 65 years old or older who initially received the vaccine more than
5 years earlier and were younger than 65 years of age when first vaccinated. If
the initial vaccination was given at age 65 years or older, repeat is only
indicated (after 5 years) if the patient has anatomic or functional asplenia or
has one of the immune-compromising conditions listed above. Although the PCV is
recommended for healthy children and has not yet been shown to be effective in
adults, it has had benefit for adults who live with vaccinated children,
demonstrating a “herd immunity” effect. Recently, some children who have
received the 7–valent PCV have developed infection with strains not included in
the vaccine, leading to a higher frequency of severe necrotizing pneumonia, especially with serotype 3.
