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BACKGROUND: In Michigan, both presentations of legionellosis, Pontiac Fever (PF) and Legionnaire’s Disease (LD), are reportable through the Michigan Disease Surveillance System (MDSS), a web-based electronic database. Legionella pneumophila serogroup-1 is responsible for nearly 80% of reported cases. The objectives of this study were to evaluate Michigan’s legionellosis surveillance system and to determine if diagnostic methods influenced case distribution.
METHODS: Descriptive and quantitative analyses were conducted using suspect and confirmed legionellosis cases reported to the MDSS from 2004-2013. Michigan’s legionellosis surveillance system was evaluated according to the 2001 MMWR surveillance system evaluation guidelines focusing on data quality, timeliness, and sensitivity. A survey of local health departments (LHD) was also used to evaluate LHD perceptions of data quality, acceptability, and usefulness. The average time between each step in the reporting system was calculated and compared with state reporting requirements. Data from 2013 hospitalized LD cases were compared with 2013 Michigan hospital discharge data for LD cases to estimate the system’s sensitivity. Laboratory and infection control personnel of key hospitals in Michigan were surveyed for their diagnostic techniques and procedures.
RESULTS: During the study timeframe, 1756 cases were reported. Incidences of 1.44 and 0.07 per 100,000 were calculated for LD and PF, respectively. Annual legionellosis incidence increased from 1.23 to 2.75 per 100,000 between 2004 and 2013. The median time from diagnosis to reporting was 2 days, with a median of 14 days between case entry and completion. 13.8% of key variables had unknown or missing values, and completeness improved by more than 20% over 10 years. Hospital discharge data recorded 284 LD cases in 2013, while the MDSS recorded 246 cases. With 189 records matching on birthdate and zip code, the sensitivity of reporting was 67.5%. Overall, 87.8% of cases were confirmed via Urinary Antigen Test (UAT) of which 30.5% were confirmed by additional testing. Cultures were performed on 10.5% of the cases.
CONCLUSIONS: Overall, the surveillance system showed moderate sensitivity, and reporting times in excess of the 24-hour state requirements. However, case completion rates were high. With few cases diagnosed by culture, linking cases to an environmental source, and therefore investigating outbreaks, is challenging. The high proportion of cases confirmed by UAT alone could lead to 20% of cases going undiagnosed. Further research is needed to develop standardized molecular diagnostic testing methods, e.g. PCR, that are comprehensive to allow for linking cases with environmental sources, but also rapid and non-invasive.