June 23 – Anticipated Increase in Legionellosis Cases Due to Seasonality and Building Reopenings

The following health alert was released by the Pennsylvania Department of Health on June 23:


  • The Pennsylvania Department of Health (PADOH) is alerting health care providers to an anticipated seasonal increase in Legionella infections combined with a potential increase in cases resulting from building reopenings as described in PA-HAN 507.
  • Health care providers should maintain a high index of suspicion in adult patients with pneumonia.
  • The preferred diagnostic tests for Legionnaires’ disease are culture of lower respiratory secretions (e.g., sputum, bronchoalveolar lavage) on selective media AND the Legionella urinary antigen test. Submit Legionella isolates to the PADOH Bureau of Laboratories for serotyping.
  • Report all legionellosis cases to the health department via PA-NEDSS or by calling PADOH (877-PA-HEALTH) or the local health department.


Since 2000, the number of reported legionellosis cases has been increasing in both Pennsylvania and the United States overall. Pennsylvania had exceptionally high case counts in 2017, 2018, and 2019. Although the number of legionellosis cases has been low since COVID-19 arrived Pennsylvania, the expectation is that the number of cases will begin to increase imminently due to a combination of the following.

  • Seasonality: Legionellosis cases follow a seasonal pattern in Pennsylvania. Legionella bacteria grow best in warm, humid, and wet weather, and the majority of Legionella infections occur between May and November each year.
  • Building reopenings: Water systems in buildings with low or no occupancy during the COVID-19 shut down are at risk of being contaminated with Legionella which grows well in stagnant water. Buildings that reopened without thoroughly flushing and disinfecting their water systems or cleaning and maintaining other devices that use water (such as decorative fountains or cooling towers) may expose individuals to the bacteria. Instructions for preparing water systems prior to reopening buildings were described in PA-HAN 507.

Legionellosis can manifest as Legionnaires’ disease or Pontiac fever.

  • Legionnaires’ disease is a severe illness with pneumonia. Symptoms are similar to those for COVID-19 and include cough, shortness of breath, fever, muscle aches, and headaches. Some patients also experience diarrhea, nausea, and confusion. Most patients are hospitalized, and treatment is required. The case-fatality rate is about 10% for community-acquired Legionnaires’ disease and about 25% for healthcare-acquired disease.
  • Pontiac fever is a milder illness, frequently characterized by fever and muscle aches. Patients with Pontiac fever do not develop pneumonia, do not require treatment, and typically recover within a week.

Risk factors for Legionella infection include:

  • Male sex
  • Age ≥50 years
  • Current or past cigarette smoking
  • Underlying conditions such as chronic lung disease, cancer, diabetes, renal disease, or immunocompromising conditions

Legionella infection occurs when a person inhales aerosolized water containing the bacteria. Potential sources include cooling towers, whirlpool spas, showers, faucets, and decorative fountains. Patients may also be infected through aspiration of contaminated drinking water. Legionellosis is not transmitted person-to-person except in extremely rare circumstances. Most Legionella infections are sporadic, but outbreaks can occur.


The preferred diagnostic tests for Legionnaires’ disease are the Legionella urinary antigen test AND culture of lower respiratory secretions.

  • Legionella urinary antigen test: The most commonly used laboratory test for diagnosis of Legionnaires’ disease is the urinary antigen test, which detects a molecule of the Legionella bacterium in urine. The test can remain positive for a few weeks after infection, even with antibiotic treatment. The urinary antigen test detects the most common cause of Legionnaires’ disease, L. pneumophila serogroup 1. However, other species and serogroups of Legionella are pathogenic, so a patient with a negative urinary antigen result could have Legionnaires’ disease caused by other Legionella species and serogroups.
  • Culture of lower respiratory secretions (e.g., sputum, bronchoalveolar lavage) on selective media: Culture can detect Legionella species and serogroups that the urinary antigen test does not, and it allows for comparison of clinical and environmental isolates in the event of an outbreak. When specimens are submitted for culture, laboratories must be informed that Legionella is suspected because it requires the use of specialized media (Buffered Charcoal Yeast Extract [BCYE] agar). Legionella isolates should be forwarded to PADOH Bureau of Laboratories for serotyping.
    • Ship isolates at room temperature. Slanted media is preferred. If plates are sent, please seal with parafilm. Package as Category B sample and ship (Monday through Thursday only) to: PA Department of Health, Bureau of Laboratories, 110 Pickering Way, Exton, PA 19341 (610-280-3464)

Best practice is to obtain both the urinary antigen test and sputum culture concurrently. Sputum should ideally be obtained prior to antibiotic administration, but antibiotic treatment should not be delayed to facilitate this process.

Serologic assays can be nonspecific and are not recommended in most situations.


For patients with Legionnaires’ disease, follow the IDSA-ATS guidelines for treatment. Legionella-directed antibiotics include macrolides and respiratory fluoroquinolones. While it is preferred that diagnostic testing specimens are obtained before antibiotic administration, antibiotic treatment should not be delayed to facilitate this process.

Patients with Pontiac fever should not be prescribed antibiotic treatment. It is a self-limited illness that does not benefit from antibiotics and patients usually recover within 1 week.

Report all legionellosis cases through the health department’s web-based reportable disease surveillance system, PA-NEDSS (https://www.nedss.state.pa.us/nedss/default.aspx), or call the local health department or PADOH (877-PA-HEALTH).

Any questions or concerns regarding these recommendations should be directed to the local health department or PADOH (877-PA-HEALTH).

Further information on legionellosis is available at:

Pennsylvania Department of Health: www.legionellosis.health.pa.gov

Centers for Disease Control and Prevention (CDC): https://www.cdc.gov/legionella/index.html.

CDC’s Legionella information for clinicians: https://www.cdc.gov/legionella/clinicians.html

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