Cause: Bacteria in the genus Legionella, commonly L. pneumophila serogroup 1 but also other serogroups or other species such as L. micdadei, L. bozemanii, L. longbeachae and L. dumoffii.

Illness and treatment: There are two clinically and epidemiologically distinct illnesses: Legionnaires' disease with fever, muscle aches, cough, pneumonia; and Pontiac fever, a milder illness without pneumonia. Treatment is with antibiotics.

Sources: The organism is ubiquitous. Hot water systems (showers), air conditioning cooling towers, evaporative condensers, humidifiers, whirlpool spas, respiratory therapy devices, decorative fountains, and potting soil have been implicated epidemiologically in outbreaks.

Additional risks: Illness is more common with age over 65 years, smoking, diabetes, chronic lung disease, or immunosuppression (particularly due to corticosteroids or organ transplant).

Prevention: Maintain cooling towers properly. Do not use tap water in respiratory therapy devices.

Recent Washington trends: Nationwide as well as in Washington legionellosis incidence is on a generally upward trend, though reasons for the increase are unclear; increased awareness and testing may be a factor. For annual legionellosis case counts please see the Communicable Disease Annual Report.

Purpose of Reporting and Surveillance

  • To identify sources of transmission (e.g., contaminated water source) and prevent further transmission from such a source
  • To identify outbreaks and educate potentially exposed persons about signs and symptoms of disease, thereby facilitating early diagnosis and treatment

Legal Reporting Requirements

  • Health care providers and Health care facilities: notifiable to local health jurisdiction within 24 hours
  • Laboratories:  Legionella species notifiable to local health jurisdiction within 24 hours; submission of Legionella isolates required (2 business days). If no isolate available but respiratory specimen available and associated with a positive test (as in the case of a PCR positive), submit respiratory specimen associated with positive result. 
  • Local health jurisdictions: notifiable to the Washington State Department of Health (DOH) Communicable Disease Epidemiology (CDE) within 7 days of case investigation completion or summary information required within 21 days