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BACKGROUND: Legionellosis most commonly presents as either Legionnaires’ Disease (LD), a serious, sometimes lethal pneumonia, or Pontiac Fever, a milder influenza-like illness. Contaminated water supply systems have been identified as a point source for many outbreaks, however legionellosis is increasingly being recognized as a significant cause of severe, community-acquired pneumonia. Between June-August 2013, two western New York counties, Erie and Niagara, experienced a dramatic increase in the incidence of legionellosis compared to previous years. Analyses were conducted to identify potential common sources of exposure among cases including the unusual weather patterns that coincided with the sharp increase in cases.
METHODS: Local health department staff interviewed cases of legionellosis using a standardized, state-developed questionnaire to assess potential exposure sources. Data for all cases in Erie and Niagara counties with a specimen collection date between June 1-August 31, 2013, were extracted from New York State Department of Health’s Communicable Disease Electronic Surveillance System and analyzed in SAS 9.3. Case residences were also geocoded in ArcGIS 10 to look for clustering. Daily weather data was extracted from the National Climactic Data Center (NCDC) as well as wunderground.com. Cross-correlations between symptom onset and several weather variables were analyzed using SYSTAT.
RESULTS: In the period between June-August 2013, 95 cases of legionellosis were reported: 72 in Erie and 23 cases in Niagara counties. This represented a 290% and 575% increase, respectively, when compared to the county average of the previous five years. Of the reported cases, 92% met the confirmed case definition for LD (7% of cases were missing a clinical classification). No cases were reported as nosocomial. Analysis of the interview questionnaires failed to identify any exposures commonly associated with legionellosis. Analysis of onset of symptoms to weather indices showed that the highest statistically significant correlation was between sustained cloud cover and incidence of legionellosis with a lag between 3-8 days.
CONCLUSIONS: The large number of community-associated cases in legionellosis in two western NY counties could not be explained through exposures commonly associated with legionellosis. Using weather data from NCDC and as well as wunderground.com, a significant association between legionellosis incidence and sustained cloud cover was identified. Previous studies have assessed the relationship between legionellosis and humidity, temperature, and precipitation, but never cloud cover. These findings suggest that the environment, particularly sustained cloud cover, may play an important role in legionellosis and should be considered, especially when there is a clustering of cases with no other commonalities identified.