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Delaware Health Alert Network #189

July 7, 2007 8:11 am

Health Advisory

Delaware Division of Public Health (DPH) would like to alert the medical community of an increased incidence of Legionnaires’ disease (LD) in Delaware.

On average, DPH confirms approximately 11 cases of Legionellosis each year. So far during 2009, 6 cases have been confirmed. Five of these cases, all from New Castle County, have reported onsets of illness between June 20 and June 28, 2009. No common sources of exposure have been identified. All cases have been hospitalized with varying spectrum of illness. One death has occurred from a case reported in May 2009.


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.[1] 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.[2]


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.


  • Urinary antigen assay and culture of respiratory secretions on selective media are the preferred diagnostic tests for Legionnaires' disease
  • Sensitivity and specificity of diagnostic tests

Test Sensitivity (%) Specificity (%)
Culture 80 100
Urine antigen 70 100
Paired serology* 70-80 >90
Direct fluorescent antibody stain 25-75% 95
* Note: A single antibody titer of any level is not diagnostic of legionellosis.


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.

Additional Information

[1] Epidemiology and Infection (2007); Increased Rainfall is Associated with Increased Risk for Legionellosis, 135:811-817.
[2] 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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