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Water cultures more sensitive than swab cultures to detect Legionella
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Water cultures are significantly more sensitive than swab cultures in detecting Legionella pneumophila, according to a comparison of the two recommended methods that was mostly conducted in the wake of a hospital outbreak of Legionnaires’ disease in Pittsburgh.
Fatal in about 10% of cases, Legionnaires’ disease is caused by inhaling water droplets that are contaminated with Legionella bacteria. According to the CDC, new cases quadrupled from 2000 to 2014 in the United States. Recent outbreaks have occurred around Flint, Michigan, and at California’s Disneyland Park, raising the disease’s profile.
Since its discovery in 1976, Legionella has been isolated from an increasing number of potential sources, ranging from water births to street cleaning trucks. But water distribution systems in large buildings remain the major source of infection.
Cornelius J. Clancy , MD, chief of the infectious diseases section in the VA Pittsburgh Health Care System, and colleagues conducted a study during and after a Legionnaires’ disease outbreak in three tertiary care VA hospitals in Pittsburgh that sickened 22 patients in 2011 and 2012, killing six. They collected swab and 1-liter first-draw water samples from the same hospital water fixtures, ending up with 2,147 matched pairs for 4,294 total Legionella cultures, with all but 84 of them taken after the outbreak.
According to the results, which were published in Infection Control & Hospital Epidemiology, 89 pairs of samples were positive for Legionella using at least one method. Water cultures detected Legionella 80 times for a sensitivity of 89.9%, compared with 27 positive swab cultures for a sensitivity of 30.3%. Clancy and colleagues said swab-only cultures would have missed 69.7% of the positive samples compared with 10.1% if only water cultures were used.
In 42 pairs of samples taken during the outbreak, the sensitivity of water cultures to detect Legionella was 83% compared with 48% for swab cultures. The difference was even greater after the outbreak — 93.3% vs. 21.7%.
According to Clancy and colleagues, guidelines differ on when to check hospital water systems for Legionella, recommending either routine surveillance or targeted surveillance after a health care-related case. There also are conflicting recommendations about whether to use water or swab cultures to investigate cases.
Clancy and colleagues recommended using water cultures alone in Legionella surveillance and said they have ceased using swab cultures in the VA Pittsburgh Health Care System. In addition to their greater sensitivity, water cultures are relatively easy to collect and do not require taking water fixtures apart, they said.
“This study marks the largest investigation of matched water and swab cultures for Legionella surveillance reported to date,” they wrote. “Our data demonstrate that water cultures were markedly superior to concurrently collected swab cultures in detecting Legionella overall, and L. pneumophila serogroup 1 and serogroups 2-14 in particular. The superiority of water cultures was apparent during an outbreak and in its aftermath.” – by Gerard Gallagher
Disclosures: The authors report no relevant financial disclosures.
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