Abstract
Abstract
Legionellosis is a generic term for diseases caused by Legionella bacteria, including the most serious, Legionnaires' disease (LD), and the less serious condition of Pontiac fever. Legionellosis is diagnosed worldwide, and since New Zealand’s (NZ) first case was reported in 1979, its incidence has increased. The organism is widespread in the environment, although the source of infection for sporadic LD cases is often not identified.
The overall goal of this thesis is to better understand and provide new evidence on the epidemiology of legionellosis globally and in NZ with a focus on the environmental determinants of disease such as meteorological conditions and air pollution, the needs of affected populations, the sources of exposure, and opportunities to improve prevention and control measures.
Globally, Legionella is under-reported, so systematic review methods were used to synthesise the empirical literature on the contribution of microbiologically confirmed legionellosis to community acquired pneumonia (CAP) and seroprevalence studies of Legionella infection. The results of these reviews set the scene for the thesis by revealing that globally the proportion of Legionella as the causative agent for CAP was 4·6% (95% CI: 4·4–4·7) giving a mean annual rate of 2·8/100 000 population (95% CI: 2·7–2·9) and the seroprevalence for Legionella of 13·7% respectively.
An analysis of LD national surveillance data from 1979 to 2020 showed an increasing rate during that period, driven in part by improvements in laboratory testing methods, notably the shift to molecular techniques. The annual incidence rate for laboratory-identified cases was 2·9/100 000 population and 2·1/100 000 population for notified cases (1979–2020). Between 1979 and 1999 the common Legionella strains identified for clinical laboratory-identified cases were L. pneumophila (39·3%) compared to L. longbeachae (17·2%) and other species (17·6%). From 2000 there was a marked shift in causative organisms with L. longbeachae becoming dominant (57·2% of cases 2010–2020) ahead of L. pneumophila (37·4% of cases).
Two studies assessed environmental risk factors for sporadic Legionella infections unique to NZ. The first study investigated a possible link between aerosolised liquefaction-affected soil and a noticeable increase in LD cases after major earthquakes in 2010 and 2011 in Christchurch which also coincided with the introduction of Legionella polymerase chain reaction (PCR) testing by the Canterbury DHB. It found no effect since no legionellae were isolated from any environmental samples tested and Legionella was shown not to survive in the seeded silt. The second study, a time series analysis, compared two contrasting cities with different meteorological conditions and LD epidemiology. In Christchurch, the maximum temperature recorded one day before the reported disease onset date (RR: 1·03, 95% CI: 1·00, 1·07) and SO2 concentration 6 days prior to the onset date (RR: 1·27, 95% CI: 1·10, 1·45) were positively associated with LD occurrence. The pattern was different in Auckland, where the level of particulate matter 9–10 days prior to the onset date was positively associated with an increase in LD risk (RR: 1·02, 95% CI: 1·00, 1·04). Atmospheric pressure 12 days before (RR: 0·95, 95% CI: 0·90–1·00) and wind speed 13 days before (RR: 0·94, 95% CI: 0·89–0·99) the onset date was negatively associated with LD risk. For both cities radiation was higher for days where L. longbeachae cases were recorded, noting that Tmax was also higher. No association was found between onset date and humidity for either city.
Collectively, findings from these studies support the need for a greater focus on LD prevention and control supported by improved surveillance (to quickly identify new cases and establish epidemiologic links between cases), investigation, and research. In particular, there is a need to reduce contamination of cooling towers and exposure of vulnerable people to commercial products such as potting mix, which are the two dominant sources of human disease in New Zealand.