Legionnaires’ Disease, 2016

During 2016, 115 confirmed cases of Legionnaires’ disease (2.1 per 100,000 population) were reported. This was the highest number of cases ever reported in Minnesota and represented a 125% increase over the median number of cases (51) reported over the previous 5 years (range, 31 to 58 cases). 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 2016, there were 8 suspect cases.

All 115 had pneumonia, and 105 (91%) were hospitalized, with a median duration of hospitalization of 6 days (range, 1 to 39 days). Of those hospitalized, 40 (38%) were admitted to an intensive care unit and 23 (22%) required mechanical ventilation. Six (5%) cases died. Seventy-two (63%) cases were male. Older adults were more often affected, with 94 (82%) cases occurring among individuals >50 years (overall median age, 60 years; range, 23 to 97 years). Eightyone (70%) cases had onset dates in June through September. Seventy-two (63%) cases were residents of the metropolitan area and 43 (37%) were residents of Greater Minnesota.

Twenty-three (20%) of the 115 cases, plus one resident of another state, were associated with a community outbreak traced to Legionella-contaminated aerosols from a business cooling tower. One case was linked to an outbreak associated with an improperly maintained hotel spa; the other illnesses in that outbreak fit the profile for Pontiac fever, a milder form of legionellosis than Legionnaires’ disease. One case was linked to a cluster of 4 cases diagnosed over 5 years among workers at a manufacturing facility where water mist exposures occurred. Two cases were linked to hotel-associated outbreaks in other states. The remaining 88 cases (77%) were epidemiologically classified as sporadic. Of the 75 sporadic cases for whom information was available, 13 (17%) had traveled out of state, and 4 (5%) 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 legionellosis. Culture is particularly useful for public health because environmental and clinical isolates can be compared by molecular typing in outbreak investigations.

• 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, 2016