Legionnaires’ Disease, 2017

In 2017, 98 confirmed cases of Legionnaires’ disease (1.8 per 100,000 population) were reported. This was the second highest number of cases ever reported. In 2016, there were 115 cases (23 from an outbreak), but from 2011 to 2015, a median of only 51 cases was reported annually. The 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 classified as suspect cases; in 2017, there were 5 suspect cases.

All 98 had pneumonia, and 96 (98%) were hospitalized, with a median duration of hospitalization of 5 days (range, 1 to 56 days). Of those hospitalized, 36 (38%) were admitted to an intensive care unit, and 19 (20%) required mechanical ventilation. Three (3%) cases died. Seventy-three (74%) cases were male. Older adults were more often affected, with 81 (83%) cases occurring among individuals ≥50 years (overall median age, 60 years; range, 26 to 88). Fifty-four (55%) cases had onset dates in June through September. Sixty-four (65%) cases were residents of the metropolitan area and 34 (35%) were residents of Greater Minnesota.

Two cases were associated with an outbreak at a rental accommodation, 2 cases with an outbreak at a senior living community, and 2 cases with an outbreak in another state. The remaining 92 cases (94%) were epidemiologically classified as sporadic. Of the 85 sporadic cases for whom information was available, 15 (18%) had traveled out of state, and 2 (2%) 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.

• For up to date information see>> Legionellosis (Legionella): Legionnaires' Disease and Pontiac Fever
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2017