Legionnaires’ Disease 
Annual Summary of Reportable Diseases

Legionnaires’ Disease is caused by the bacteria Legionella and is characterized by severe pneumonia, as well as other symptoms. Legionella bacteria are found naturally in freshwater environments, such as lakes and streams, but become a health concern when they grow and spread in human-made water systems. Infection can occur when people breath in small droplets containing the bacteria. In 2022, 109 confirmed cases of Legionnaires’ disease (1.9 per 100,000 population) were reported. This is a 16% decrease from the 130 cases reported in 2021 and an 8% decrease from the median number of cases reported annually from 2017 to 2021 (median, 118; range, 94 to 152). 

Of the 109 confirmed cases, 107 (98%) were hospitalized, with a median duration of hospitalization of 6 days (range, 1 to 49 days). Of those hospitalized, 30 (28%) were admitted to an intensive care unit, and 17 (16%) required mechanical ventilation. Eight (7%) cases died. Seventy (64%) cases were male. Older adults were more often affected, with 90 (83%) cases occurring among individuals ≥50 years (overall median age, 64 years; range, 22 to 92 years). Of the 109 cases, 94 (86%) were white (of which two were Hispanic), 13 (12%) were Black (of which one was Hispanic), and two (2%) were American Indian. Fifty-two (48%) cases were diagnosed during June through September. Sixty-six (61%) were residents of the Twin Cities metropolitan area and 43 (39%) were residents of Greater Minnesota. Two (2%) confirmed cases were part of an outbreak associated with an apartment building, and one (1%) confirmed case was part of an outbreak associated with a vacation rental (this outbreak had one additional confirmed case who was a non-Minnesota resident). 

Although most cases are diagnosed by Legionella urinary antigen test, culture on selective media is useful for public health purposes because clinical and environmental isolates can be compared by molecular typing in outbreak investigations. MDH requests that clinical laboratories submit Legionella isolates, as well as available lower respiratory tract (sputum, BAL) specimens from confirmed and suspect cases for culture and molecular typing.

• Find up to date information at>> Legionellosis (Legionella): Legionnaires' Disease and Pontiac Fever

In 2021, 130 confirmed cases of Legionnaires’ disease (2.3 per 100,000 population) were reported. This is a 38% increase from the 94 cases reported in 2020 and a 13% increase from the median number of cases reported annually from 2016 to 2020 (median, 115; range, 94 to 152). 

Of the 130 confirmed cases, 123 (95%) were hospitalized, with a median duration of hospitalization of 5 days (range, 1 to 43 days). Of those hospitalized, 36 (29%) were admitted to an intensive care unit, and 23 (19%) required mechanical ventilation. Nine (7%) cases died. Seventy-five (58%) cases were male. Older adults were more often affected, with 105 (81%) cases occurring among individuals ≥50 years (overall median age, 64 years; range, 29 to 90 years). Of the 129 cases for which race and ethnicity were reported, 114 (88%) were white (of which 2 were Hispanic), 11 (9%) were Black, two (2%) were Asian American, one (<1%) was American Indian, and one (<1%) was other race. Sixty-six (51%) cases were diagnosed during June through September. Seventy-nine (61%) were residents of the Twin Cities metropolitan area and 51 (39%) were residents of Greater Minnesota. Three (2%) confirmed cases were part of an outbreak associated with a hotel spa pool (the outbreak had 2 additional confirmed cases who were non-Minnesota residents). 

Although most cases are diagnosed by Legionella urinary antigen test, culture is useful for public health purposes because clinical and environmental isolates can be compared by molecular typing in outbreak investigations. MDH requests that clinical laboratories submit Legionella isolates, as well as available lower respiratory tract (sputum, BAL) specimens from confirmed and suspect cases for culture and molecular typing.

• Find up to date information at>> Legionellosis (Legionella): Legionnaires' Disease and Pontiac Fever

In 2020, 94 confirmed cases of Legionnaires’ disease (1.7 per 100,000 population) were reported. This is a 20% decrease from the 118 cases reported in 2019 and is lower than the 5-year median number of cases reported annually from 2015 to 2019 (median, 115; range, 51 to 153 per year). 

Fifty-nine (63%) cases were male. Older adults were more often affected, with 76 (81%) cases occurring among individuals ≥50 years (overall median age, 61 years; range, 24 to 90 years). Fifty (53%) cases were diagnosed during June through September. Fifty-four (57%) were residents of the metropolitan area and 40 (43%) were residents of Greater Minnesota. No outbreaks were identified during 2020. Due to the COVID-19 pandemic response, data on hospitalizations for Legionnaires’ disease could not be systematically collected. 

Although most cases are diagnosed by Legionella urinary antigen test, culture is useful for public health purposes because clinical and environmental isolates can be compared by molecular typing in outbreak investigations. We request clinical laboratories submit Legionella isolates, as well as available lower respiratory tract (sputum, BAL) specimens from confirmed and suspect cases for culture and molecular typing.

• Find up to date information at>> Legionellosis (Legionella): Legionnaires' Disease and Pontiac Fever

In 2019, 118 confirmed cases of Legionnaires’ disease (2.1 per 100,000 population) were reported. This is a 22% decrease from the 152 cases reported in 2018, which was the highest number of cases ever reported, and a 20% increase over the 98 cases reported in 2017. Prior to 2016, there were never more than 60 cases reported annually. The CDC criteria for confirmation of a case are a clinically compatible illness and at least one of the following: 1) isolation of any Legionella organism from respiratory secretions, lung tissue, pleural fluid, or other normally sterile fluid by culture, or 2) detection of L. pneumophila serogroup 1 antigen in urine using validated reagents, or 3) seroconversion of fourfold or greater rise in specific serum antibody titer to L. pneumophila serogroup 1 using validated reagents. A single antibody titer of any level is not considered diagnostic. Patients positive only by PCR or DFA were classified as suspect cases (as of January 1, 2020, PCR positives will be classified as confirmed cases). In 2019, there were 7 suspect cases.

All 118 had pneumonia, and 116 (98%) were hospitalized, with a median duration of hospitalization of 5 days (range, 1 to 34 days). Of those hospitalized, 41 (35%) were admitted to an intensive care unit, and 22 (19%) required mechanical ventilation. Four (3%) cases died. Seventy (59%) were male. Older adults were more often affected, with 103 (87%) occurring among individuals ≥50 years (overall median age, 63 years; range, 33 to 94). Fifty-nine (50%) cases had onset dates in June through September. Seventyfive (64%) were residents of the metropolitan area and 43 (36%) were residents of Greater Minnesota.

Four cases were associated with an outbreak linked to a hotel spa pool. Two cases were associated with an outbreak at a senior living community that had an outbreak of 5 cases in 2018. One case (and 1 additional suspect PCR-only case) was associated with an outbreak at a hospital campus that had 5 cases associated with it in prior years. Three cases were associated with outbreaks in other states. The remaining 108 cases (92%) were epidemiologically classified as sporadic. Of the 98 sporadic cases for whom information was available, 18 (18%) had traveled out of state, and 5 (5%) had traveled out of the country during the 10 days prior to illness onset.

Although most cases are diagnosed by Legionella urinary antigen test, culture is useful for public health purposes because clinical and environmental isolates can be compared by molecular typing in outbreak investigations. MDH requests that clinical laboratories submit isolates or available lower respiratory tract (sputum, BAL) specimens from confirmed and suspect cases for culture and molecular typing.

• Find up to date information at>> Legionellosis (Legionella): Legionnaires' Disease and Pontiac Fever

In 2018, 152 confirmed cases of Legionnaires’ disease (2.7 per 100,000 population) were reported. This is the highest number of cases ever reported, and a 55% increase over the 98 cases reported in 2017. Prior to 2016, there were never more than 60 cases reported annually. The CDC criteria for confirmation of a case are a clinically compatible illness and at least one of the following: 1) isolation of any Legionella organism from respiratory secretions, lung tissue, pleural fluid, or other normally sterile fluid by culture, or 2) detection of L. pneumophila serogroup 1 antigen in urine using validated reagents, or 3) seroconversion of fourfold or greater rise in specific serum antibody titer to L. pneumophila serogroup 1 using validated reagents. A single antibody titer of any level is not considered diagnostic. Patients positive by PCR only are currently classified as suspect cases; in 2018, there were 10 suspect cases.

All 152 had pneumonia, and 148 (97%) were hospitalized, with a median duration of hospitalization of 5 days (range, 1 to 49 days). Of those hospitalized, 57 (38%) were admitted to an intensive care unit, and 31 (21%) required mechanical ventilation. Eight (5%) cases died. One hundred five (69%) were male. Older adults were more often affected, with 125 (82%) occurring among individuals ≥50 years (overall median age, 64 years; range, 32 to 96). Ninety-six (63%) cases had onset dates in June through September. Ninety-four (62%) were residents of the metropolitan area and 58 (38%) were residents of Greater Minnesota.

Five cases were associated with an outbreak at a senior living community, 1 case was associated with an outbreak linked to a hospital that was detected in early 2019, and 6 cases were associated with outbreaks in other states. The remaining 140 cases (92%) were epidemiologically classified as sporadic. Of the 131 sporadic cases for whom information was available, 16 (12%) had traveled out of state, and 1 (<1%) had traveled out of the country during the 10 days prior to illness onset.

The Infectious Diseases Society of America and the American Thoracic Society, in consensus guidelines on the management of community-acquired pneumonia in adults, recommend urinary antigen assay and culture of respiratory secretions on selective media for detection of Legionella infection. Culture is particularly useful for public health because environmental and clinical isolates can be compared by molecular typing in outbreak investigations. MDH requests that clinical laboratories submit isolates or available lower respiratory tract (sputum, BAL) specimens from confirmed and suspect cases for culture and molecular typing.

• Find up to date information at>> Legionellosis (Legionella): Legionnaires' Disease and Pontiac Fever

• Archive of Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health
  Archive of past summaries (years prior to 2023 are available as PDFs).