Legionnaires’ Disease, 2018

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.

• 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, 2018