Abstract
Introduction
Legionnaires’ disease is a form of atypical pneumonia caused by infection from Legionella bacteria. There is a known association between the use of compost-based soil products and Legionnaires’ disease caused by the bacterium Legionella longbeachae. A case-control study was carried out between October 2013 and March 2014 in Canterbury, New Zealand, where reported prevalence of L. longbeachae infection is unusually high, to investigate host risk factors and gardening-related risk factors for the disease.
Methods
Twenty-one laboratory-confirmed notified cases and 69 population controls were interviewed using a structured questionnaire. Logistic regression was used in STATA 13 to calculate univariate odds ratios for variables of interest. Stratified univariate analysis was undertaken to consider the impact of multiple risk factors on disease risk.
Results
Having smoked for 10 or more years (OR 4.00, 95% CI 1.42-11.24) was strongly associated with L. longbeachae infection. Having an indoor garden also increased disease risk (OR 3.81, 1.18-12.27). Use of compost-based products during the three-week reference period (OR 4.16, 1.37-12.64) was associated with L. longbeachae infection. Opening (OR 4.6, 1.64-12.92) and tipping or troweling potting mix or purchased compost (OR 5.00, 1.71-14.5), and touching the face after using these products before washing hands (OR 12.22, 3.16-47.29) were also strongly associated with disease. Having hanging pots or baskets (OR 0.77, 0.27-2.17) and being near dripping hanging pots or baskets (OR 0.38, 0.27-2.17) during the three-week reference period were not associated with disease, as had been observed in a previous case-control study conducted in South Australia from 1996-98. Wearing a mask (OR 1.80, 0.30-10.64) or gloves (OR 0.64, 0.15-2.77) while using potting mix did not appear to be protective against disease.
Discussion
Long-term smoking is an important risk factor for L. longbeachae Legionnaires’ disease, and those with a history of smoking for more than 10 years should be advised to exercise extreme caution when using compost-based products. Important environmental risk factors include use of potting mix or purchased compost, poor hand hygiene while using compost-based products, and other behaviour that transfers these products to the mouth or face. Use of compost-based products indoors may also increase risk. The findings suggest that transmission of bacteria from the environment to humans may occur through aspiration of contaminated soil particles that are transferred to the mouth. Hand-washing and keeping potting mix and compost away from the face appear to be key measures for preventing Legionnaires’ disease.