HAEMOPHILUS INFLUENZAE PNEUMONIA
This gram-negative coccobacillary rod can occur in either a typeable, encapsulated form or a nontypeable, unencapsulated form, and either can cause pneumonia. The nontypeable organisms are also a common cause of bronchitis and a frequent colonizer in patients with chronic obstructive pulmonary disease (COPD). The encapsulated organism can be one of seven types, but type B accounts for 95% of all invasive infections. Opsonizing IgG antibody (directed at the capsular polysaccharide PRP and at membrane antigens) is required to phagocytose the encapsulated organisms, and because encapsulated organisms require a more elaborate host response than unencapsulated organ- isms, they are generally more virulent. However several studies have shown that in adults, particularly those with COPD, infection with unencapsulated bacteria is more common than infection with encapsulated organ- isms, and that opsonizing antibody is needed to control unencapsulated bacteria as well. Patients who develop pneumonia with these bacteria usually have some impairment in host defense, which may include both humoral immunity and local phagocytic dysfunction, but this organism may occur in patients whose only risk is cigarette smoking.
When pneumonia is present, some patients may develop
bacteremia, particularly those with segmental pneumonias rather than those with
bronchopneumonia. It has been estimated that 15% of cases are segmental but
that up to 70% of these patients have bacteremia, although only 25% of
bronchopneumonia cases are bacteremic. The encapsulated type B organism is more
common in patients with segmental pneumonia than in those with
bronchopneumonia. In patients with COPD, bronchopneumonia is more common than
segmental pneumonia.
Patients with segmental pneumonia present with a
sudden onset of fever and pleuritic chest pain along with a sore throat. Those
with bronchopneumonia have a slightly lower fever, tachypnea, and
constitutional symptoms. Multilobar, patchy bronchopneumonia is the most common
radiographic pattern, and pleuralv reaction is also common, being seen in more than 50% of patients with
segmental pneumonia and in approximately 20% with bronchopneumonia.
Complications may include empyema, lung abscess, meningitis, arthritis,
pericarditis, epiglottitis, and otitis media (particularly in children).
Therapy had traditionally been with ampicillin, but
now up to 40% of nontypeable Haemophilus influenzae isolates and up to
50% of type B organisms are resistant because of bacterial production of β-lactamase enzymes. Currently,
effective antibiotics are the third-generation cephalosporins, β-lactam/β-lactamase
inhibitor combinations, newer macrolides (azithromycin is more active than
clarithromycin), and fluoroquinolones. A conjugate vaccine against type B
organisms is available but is not used in adults at risk for pneumonia but
rather in children beginning at 2 months to age to prevent invasive infection such as meningitis.
