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Reporting Obligations

Individuals who have or may have legionellosis shall be reported to the local Health Unit.

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Aetiologic Agent

Legionellae species are fastidious aerobic bacilli that stain gram-negative after recovery on artificial media. At least 20 of the more than 60 species have been implicated in human disease, but Legionella pneumophila (L. pneumophila) is most commonly associated with disease in humans.

Clinical Presentation

There are two distinct illnesses:

  • Legionnaires’ Disease – characterized by anorexia, malaise, myalgia, headache, productive cough, temperature > 39 degrees Celsius, chills, nausea, diarrhea; and
  • Pontiac Fever – A milder form of the illness without pneumonia. It is characterized by anorexia, malaise, myalgia, headache, productive cough, temperature > 37.5 degrees Celsius.

Modes of Transmission

Legionella are pathogens most commonly associated with water-droplet transmission to humans through inhalation of aerosolized contaminated water.

Incubation Period

For Legionnaires’ disease it is 2–10 days, most often 5–6 days, but can be up to 19 days.

For Pontiac fever it is 5–72 hours, most often 24–48 hours.

Period of Communicability

Person-to-person transmission has not been documented.

Risk Factors/Susceptibility

Illness occurs most frequently with increasing age (most cases are at least 50 years of age). Persons who smoke, have diabetes, lung, or renal disease, malignancy and compromised immunity are at most risk. Generally, more men than women contract Legionnaires’ disease. The disease is rare in persons under 20 years of age. Outbreaks are often identified among institutionalized patients/residents.

Diagnosis and Laboratory Testing

Consider the diagnosis of legionellosis infection in any cluster of respiratory illness with pneumonia, or individual presenting with a respiratory illness and pneumonia.

Any of the following will constitute a confirmed case of Legionellosis:

  • Positive Legionella spp. Culture
  • A significant (i.e. fourfold or greater) rise in Legionella spp total antibody titre between acute and convalescent sera
  • A positive Legionella urinary antigen test.

Note: Seroconversion requires up to eight weeks for antibody levels to peak. A four fold increase in antibody levels requires two samples taken 3–6 weeks apart. Urinary antigen testing is the most rapid and sensitive test however only detects infection with L. pneumophila serogroup 1. Cases with positive urine antigen are recommended to have confirmatory cultures.

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Treatment and Case Management

Treatment is under the direction of the attending health care provider. Public Health staff will investigate the case to determine the source of infection.

Patient Information

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Additional Resources

Centers for Disease Control and Prevention. “What clinicians need to know about Legionnaires’ Disease”.


Ministry of Health and Long-Term Care, Infectious Diseases Protocol, 2022.