Legionnaires’ Disease, 2019

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
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2019