Key Point

Moraxella catarrhalis is a much more important organism in COPD patients than previously thought. It causes about 10% of COPD exacerbations, making it responsible for two to four million such episodes in the US annually.

BUFFALO—If ever a bacterium was underestimated, it is Moraxella catarrhalis. The organism was written off as a pathogen in COPD patients long ago, leading to the assumption that it harmlessly colonizes the airways of these patients. But Timothy F. Murphy, MD, and coworkers have shown that this assumption could not be more wrong.¹

“Because our study was prospective and longitudinal, we were able to estimate that Moraxella causes about 10% of—two to four million—COPD exacerbations in the US annually,” Dr. Murphy, Chief of Infectious Diseases at the Buffalo Veterans Affairs Medical Center in New York, told Pulmonary Reviews. “After Haemophilus influenzae, Moraxella was the second most common cause of COPD exacerbations in the study,” he added.

The investigators followed 104 COPD patients monthly for almost seven years; the patients made 3,009 clinic visits during that time. Of these visits, 560 took place during exacerbations and 2,727 yielded sputum samples for culture and molecular typing.

Two hundred ten (7.7%) of the cultures grew M catarrhalis. Molecular typing identified 120 cases of acquisition and clearance of the bacterium in 50 patients; 57 (47.5%) of the acquisitions were associated with COPD exacerbations. “Therefore, 57 of 560 (10.2%) total exacerbations recorded in the study clinic occurred simultaneously with the acquisition of a new strain of M catarrhalis,” the investigators observed.

COPD patients who were colonized with M catarrhalis carried the organism for a median of 40.4 days versus a median of 31 days among patients infected by M catarrhalis who then had an exacerbation. However, both intervals were much shorter than those reported for H influenzae, suggesting that lung clearance of M catarrhalis is efficient.

Reacquisition of an M catarrhalis strain after its clearance from the lungs was rare because of the development of immunity. Tests of the paired serum and sputum samples that were available for 106 of the 120 cases of M catarrhalis acquisition indicated systemic or mucosal strain–specific antibody production in 72% of cases.

A sputum immunoglobulin A (IgA) response was more likely in patients who were colonized than in those with M catarrhalis–related exacerbations. “Although the proportion of patients who developed serum IgG was not different between exacerbation and colonization, exacerbation resulted in a significantly greater intensity of serum IgG response compared with colonization,” reported the investigators.

The study findings have two treatment implications, Dr. Murphy said. First, when antibiotics are prescribed for COPD exacerbations, an agent that is active against M catarrhalis should be selected. And second, the observed antibody responses suggest that it may be possible to develop a vaccine that protects against all M catarrhalis strains.

—Timothy Begany

Reference
1. Murphy TF, Brauer AL, Grant BJB, Sethi S. Moraxella catarrhalis in chronic obstructive pulmonary disease: burden of disease and immune response. Am J Respir Crit Care Med. 2005;172:195-199.

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