Abstract
In early June 2018, an increase in non-travel-related cases of Legionella non-pneumophila Legionnaires’ disease (LD) was observed in Sweden and a national outbreak investigation was started. Outbreak cases were defined as notified confirmed or probable cases of L. non-pneumophila LD, with symptom onset after 1 April 2018. From April to August 2018, 41 cases were reported, 30 of whom were identified as L. longbeachae. We conducted a case–control study with 27 cases and 182 matched controls. Results from the case–control study indicated that gardening and handling commercial bagged soil, especially dusty dry soil, were associated with disease. L. longbeachae was isolated in soils from cases’ homes or gardens, but joint analysis of soil and human specimens did not identify any genetic clonality. Substantial polyclonality was noted between and within soil samples, which made finding a genetic match between soil and human specimens unlikely. Therefore, whole genome sequencing may be of limited use to confirm a specific soil as a vehicle of transmission for L. longbeachae. Handling soil for residential gardening was associated with disease and the isolation of L. longbeachae in different soils provided further evidence for Legionella non-pneumophila infection from soil.
Highlights
Legionellosis is caused by the Gram-negative bacterium Legionella
Statistical analysis As all included cases reported gardening during the exposure period of interest, we focused on identifying particular gardening activities and use of products that could be associated with illness
Cases were more likely to suffer from underlying immunosuppression (mOR = 10.6; 95% confidence interval (CI): 3.5–31.7), respiratory disease, heart conditions, diabetes and other underlying disease
Summary
Legionellosis is caused by the Gram-negative bacterium Legionella. The severity of legionellosis varies from mild febrile illness (Pontiac fever) to a potentially fatal form of pneumonia (Legionnaires’ disease (LD)) [1]. The L. non-pneumophila group contains many species, including L. longbeachae and L. bozemanii, which are infrequently reported causes of LD in Sweden. In early June 2018, the Stockholm County Council Department of Communicable Disease Control and Prevention (Stockholm CDC department) undertook routine source tracing among an increasing number of domestic cases of L. non-pneumophila LD, many of whom reported being active gardeners. An outbreak of L. non-pneumophila, including L. longbeachae, was suspected and investigated by a National Outbreak Investigation Team with the following objectives: (i) describe the magnitude of the outbreak, (ii) identify risk factors associated with infection with L. non-pneumophila, and (iii) ascertain, via environmental and laboratory investigation, any link between cases and a common species or strain of L. non-pneumophila causing the outbreak.
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