PURPOSE: The authors evaluated a geographic and temporal cluster of lower respiratory tract infections due to unencapsulated (serologically nontypeable) Haemophilus influenzae to determine whether this event represented the transmission of a single clone. METHODS AND MATERIALS: H influenzae was recovered from eight patients at a nursing home and from three patients in an adjacent acute care hospital. Serotypes, biotypes, outer membrane protein profiles, and multilocus enzyme genotypes were determined to characterize bacterial isolates. Patient records were retrospectively examined to determine clinical and epidemiologic characteristics. RESULTS: During a 10-day period in September 1991, lower respiratory tract infections caused by H influenzae were diagnosed in four patients residing in a single nursing home unit. Oropharyngeal cultures from four of seven asymptomatic roommates of these patients also grew H influenzae. During the month before and after the nursing home cluster of cases, four other individuals in acute care areas of the hospital had positive sputum cultures for H influenzae. Three of these latter specimens were also available for analysis. All H influenzae isolates were unencapsulated and β-lactamase-negative. Eight of the nine isolates from the nursing home patients (two morphologically distinct colony types of H influenzae were isolated from one case) had a single outer membrane protein profile arbitrarily designated as X and a single multilocus enzyme genotype arbitrarily designated as A. In contrast, none of the isolates from the acute care cases had this profile (P ≤0.02; two-tailed Fisher's exact test). The isolates obtained from two of the patients in acute care areas had an outer membrane protein profile arbitrarily designated as Y and a single multilocus enzyme genotype designated as B. These two patients were contemporaneously hospitalized in adjacent intensive care unit cubicles. The remaining isolates displayed an outer membrane protein profile arbitrarily designated as W. All roommates of the four patients in the nursing home were administered oral rifampicin 600 mg daily for 4 days. H influenzae was not recovered from follow-up oropharyngeal cultures obtained 1 week after the completion of therapy. No β-lactamase-negative H influenzae were identified in this unit during the subsequent 9 months. CONCLUSION: This study furnishes strong evidence for the nosocomial transmission of a clone of unencapsulated H influenzae in a nursing home unit. Epidemiologic data showed temporal and geographic clustering of respiratory tract infections and colonization by H influenzae. Outer membrane protein profiles and multilocus enzyme genotype analysis indicated that seven of eight patients at the nursing home carried a single clone of unencapsulated H influenzae. Laboratory and epidemiologic data also demonstrated the presence, and possible nosocomial transmission, of a second clone of unencapsulated H influenzae in a physically separate area of the hospital. Finally, although a causal relationship is not proven, the outbreak ended following the administration of rifampin prophylaxis of asymptomatic carriers.
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