Current Suspected Overdose Deaths in Delaware for 2024: Get Help Now!

Find school water testing results and additional resources

Attention Medicaid Participants: Eligibility Renewals Restarted April 1, 2023 logo

Delaware Health Alert Network #256

September 22, 2011 9:14am

Health Alert

The Division of Public Health (DPH) alerts the medical community of an increased incidence of Legionnaires’ disease (LD) in Delaware possibly associated with heavy rainfall and flooding in the mid-Atlantic region before, during and after Hurricane Irene.

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. [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 andculture of respiratory secretions on selective media are the preferred diagnostic tests for Legionnaires’s 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.

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.


You are receiving this message because you are a registered member of the Delaware Health Alert Network. If you are not a member and would like to subscribe, please register at

Categories of Health Alert messages:

  • Health Alert: Conveys the highest level of importance; warrants immediate action or attention.
  • Health Advisory: Provides important information for a specific incident or situation; may not require immediate action.
  • Health Update: Provides updated information regarding an incident or situation; unlikely to require immediate action.
NOTE: This page is for informational purposes only and dated material (e.g. temporary websites) may not be available.