Nosocomial Legionnaires' disease remains a significant problem with many unresolved
questions regarding transmission of legionella organisms to patients. We performed
a case-control and environmental study to identify risk factors and modes of transmission
of Legionella infection during an outbreak of nosocomial Legionnaires' disease in
a military medical center.
During the calendar year 1989, 14 cases of nosocomial Legionnaires' disease were identified
by active surveillance following the discovery of 2 culture-proven cases among organ
transplant recipients. Four control patients were matched to each case by age, sex,
and date of admission. Cases and controls were compared with respect to past medical
history and hospital exposure variables. Odds ratios (ORs) and 95% confidence intervals
(CIs) were calculated for matched variables. Environmental culturing of air and water
supplies in and around the medical center was also performed.
The case-control study revealed the following significant risk factors for the acquisition
of nosocomial Legionnaires' disease: immunosuppressive therapy (OR = 32.7, CI = 4.5
to 302.6), nasogastric tube use (OR = 18.4, CI = 2.6 to 166.2), bedbathing (OR = 10.7,
CI = 2.2 to 59.0), and antibiotic therapy (OR = 14.6, CI = 2.9 to 84.4). Shower use
(OR = 0.1, CI = 0 to 0.4) appeared to be a negative risk factor. Water cultures revealed
Legionella pneumophila serogroup 1, monoclonal antibody subtype Philadelphia (identical
to all patient isolates) in the ground-water supply to the hospital, 1 hot-water tank,
and 15% of 85 potable water sites tested. Air sampling of cooling towers, hospital
air intakes, and medical air and oxygen supplies were negative for Legionella organisms.
This study confirms the importance of potable water in transmitting nosocomial Legionnaires'
disease and suggests that the organism gains access to the hospital via external water
supplies. The risk factors identified in this case-control study provide evidence
that Legionnaires' disease may act as a superinfection in a nosocomial setting and
is likely acquired by aspiration, similar to other nosocomial pneumonias.