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Delaware Health Alert Network #308

July 3, 2013 12:18 pm

Health Alert

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


Legionnaires’ disease is a cause of severe pneumonia requiring hospitalization. An estimated 8,000 – 18,000 cases occur in the United States each year. While outbreaks of Legionellosis have been linked to common sources such as hot tub use and air conditioning cooling towers, the majority (80-90%) of cases are sporadic and no specific water source is identified.

Currently, several states in the Mid-Atlantic and North East are experiencing an increase in the number of Legionnaires’ disease cases --- particularly during the month of June. DPH is currently collaborating with the Centers for Disease Control and Prevention (CDC) and regional states to characterize the increase in cases.

On average, DPH confirms approximately 17 cases of Legionellosis each year. So far during 2013, 9 cases have been confirmed. All cases are from New Castle County and have reported onsets of illness between May 18, 2013 and June 14, 2013. No common sources of exposure have been identified. All cases have been hospitalized with varying degrees of illness from the infection. No deaths have been reported.

Ten years ago, in 2003, the Mid-Atlantic region experienced a sharp rise in Legionnaires’ disease 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 but wet weather and humidity were associated with the increase.[2]


Legionnaires’ disease is a respiratory disease caused by bacteria of the genus Legionella. “Legionellosis” refers to all diseases caused by or attributed to Legionella organisms including Legionnaires’ disease, Pontiac fever and focal non-pulmonary infections. Legionellae are widely distributed in the environment, particularly in warm, stagnant bodies of water.

Human infection occurs through inhalation of bacteria when contaminated water is aerosolized; person-to-person transmission has not 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 immunocompromised.


  • Urinary antigen assay and culture of respiratory secretions on selective media are the preferred diagnostic tests for Legionnaires’ disease.
  • Urine antigen testing detects only Legionella pneumophilia serotype 1 and not other serotypes of L. pneumophilia or other species of Legionella.

Sensitivity and specificity of diagnostic tests

Test Sensitivity (%) Specificity (%)
Culture 20-80 100
Urine antigen 70-100 100
Paired serology* 80-90 >99
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, Office of Infectious Disease Epidemiology (1-888-295-5156 or 1-302-744-1033). All cases of Legionnaires’ disease 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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