Over the past 5 years, DPH has confirmed and average of 15 cases of Legionellosis each year. So far during 2011, 11 cases have been confirmed. Six of these cases (2 from New Castle County, 1 from Kent County, 3 from Sussex County) have reported onsets of illness between August 19 and September 14, 2011. No common sources of exposure have been identified---one possible common denominator is heavy rainfall and flooding. All cases have been hospitalized with varying spectrum of illness. No associated deaths have been reported.
Legionellosis is a reportable disease in Delaware, therefore laboratories and healthcare providers are required to report any diagnosed case to DPH, Bureau of Epidemiology (1-888-295-5156 or 1-302-744-4541). All cases of LD are investigated to include date of onset, method of diagnosis, underlying medical conditions, smoking history, occupation, travel history, recent medical or dental visits (to rule-out hospital-acquired infection), and any known water exposures.
LD is a common cause of severe pneumonia requiring hospitalization. As estimated 8,000 – 18,000 cases occur in the United States each year. While outbreaks of Legionellosis have been linked to sources such as hot tub displays and air conditioning cooling towers, the majority (80-90%) of cases is sporadic and no specific water source is identified.
During 2003, the Mid-Atlantic region experienced a sharp rise in LD coinciding with a period of record-breaking rainfall.  DPH confirmed 32 sporadic cases during this time period. No evidence of a common source or outbreak was identified in Delaware. Wet weather and humidity were associated with the increase. 
Legionellosis is associated with two clinically and epidemiologically distinct illnesses: Legionnaires’ disease, which is characterized by fever, myalgia, cough, and clinical or radiographic pneumonia; and Pontiac fever, a milder illness without pneumonia.
Legionellosis is a respiratory disease caused by the bacterium Legionella. Legionella are widely distributed in the environment, particularly in warm, stagnant bodies of water. Human infection appears to occur through inhalation of bacteria when contaminated water or soil is aerosolized; person-to-person transmission has never been documented. Legionellosis occurs year-round, but is more commonly reported in summer months. The incubation period is 2-10 days and the clinical spectrum of disease is quite varied, from asymptomatic infection to mild, self-limited illness, to severe pneumonia and death. Anyone can develop Legionellosis, but those at most risk are the middle-aged and elderly, smokers, those with chronic lung disease or those who are immune-compromised.
|Test||Sensitivity (%)||Specificity (%)|
|Direct fluorescent antibody stain||25-75%||95|
* Note: A single antibody titer of any level is not diagnostic of legionellosis.
 Epidemiology and Infection (2007); Increased Rainfall is Associated with Increased Risk for Legionellosis, 135:811-817.
 Journal of Infectious Disease (2005); It’s Not the Heat, It’s the Humidity: Wet Weather Increases Legionellosis Risk in the Greater Philadelphia Metropolitan Area, 192:2066-2073.
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