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Legionellosis surveillance highlights disease severity, diagnostic gaps
Recent CDC surveillance data suggest nearly half of the patients with legionellosis in the United States require intensive care and that current diagnostic testing may not be detecting many infections.
Passive surveillance data collected through the National Notifiable Diseases Surveillance System (NNDSS) from 2000 to 2011 showed a 249% increase in crude legionellosis incidence, but little data concerning clinical course and diagnostic methods were available through this method. To fill this gap, the CDC increased legionellosis surveillance through its Active Bacterial Core surveillance (ABCs) program in 2011. Researchers examined incidence, demographics and testing data from the first 3 years of active legionellosis surveillance and compared results with those collected through passive surveillance. Cases of ABCs-detected legionellosis were confirmed through isolation of Legionella from a respiratory culture, detection of Legionella antigen in urine or seroconversion to Lp1.
There were 1,426 cases of legionellosis identified by ABCs from 2011 to 2013, an incidence of 1.3 cases per 100,000 population. This rate was similar to the 1.1 cases per 100,000 population determined by 2012 data collected through NNDSS. Both surveillance systems detected an increased rate of infection among blacks, although 17% of cases reported to NNDSS were missing race categorization. Infection rates increased with age, according to ABCs data, and were highest among catchment regions in the Northeast or Mid-Atlantic states.
Common underlying conditions among legionellosis patients included current smoking, diabetes and chronic obstructive pulmonary disease. Ninety-eight percent of cases were hospitalized, 44% were admitted to an ICU, 27% required mechanical ventilation, and 9% died. Urine antigen testing diagnosed 91% of cases detected by ABCs. Respiratory specimen cultures were performed for 23% of cases, 42% of which were positive for Legionella.
This additional surveillance data is likely to be an underestimate due to the reliance on urine antigen testing which can only detect Lp1 species, the researchers wrote. As a result, fewer patients are being tested by culture, and non-Lp1 cases of legionellosis are likely going undetected.
“Until better diagnostics are developed, obtaining respiratory cultures from persons suspected to have legionellosis continues to be important for diagnosing disease and detecting the source of infection in outbreaks,” the researchers wrote. “The underlying reasons for geographic and racial differences in legionellosis incidence need further exploration, which will be done through additional analyses in ABCs.” – by Dave Muoio
Disclosure: The researchers report no relevant financial disclosures.