Search

This study investigated the relationships between Legionnaires' disease (LD) incidence and weather in Glasgow, UK, by using advanced statistical methods. Using daily meteorological data and 78 LD cases with known exact date of onset, we fitted a series of Poisson log-linear regression models with explanatory variables for air temperature, relative humidity, wind speed and year, and sine-cosine terms for within-year seasonal variation. Our initial model showed an association between LD incidence and 2-day lagged humidity (positive, P = 0·0236) and wind speed (negative, P = 0·033). However, after adjusting for year-by-year and seasonal variation in cases there were no significant associations with weather. We also used normal linear models to assess the importance of short-term, unseasonable weather values. The most significant association was between LD incidence and air temperature residual lagged by 1 day prior to onset (P = 0·0014). The contextual role of unseasonably high air temperatures is worthy of further investigation. Our methods and results have further advanced understanding of the role which weather plays in risk of LD infection.

This review considers the value of the observation that Legionnaires' disease varies geographically. Estimates of disease incidence, derived from case registers and from studies measuring the proportion of cases of pneumonia which are Legionnaires' disease, and of the prevalence of the population with antibody, show that there is geographical variation in disease frequency. Much, but not all, of this variation is artefact due to differences in definitions, diagnostic methods, surveillance systems and data presentation. Some of the variation is attributed to publication bias, e.g. in 10 small studies (< 100 patients) 13.2% of pneumonia patients had Legionnaires' disease but in five large studies (> or = 500 patients) the figure was 3.6%. Research to explain variations has been neglected but a few studies have provided important insights into disease transmission. Future studies should: be based on agreed disease definitions and data collection and analysis methods; analyse subgroups separately; and collect data to develop explanations for geographical variation.

Objectives: To test the hypothesis that many non-outbreak cases of legionnaires' disease are not sporadic and to attempt to pinpoint cases clustering in space and time., Design: Descriptive study of a case series, 1978-86., Setting: 15 health boards in Scotland., Patients: 203 probable cases of non-outbreak, non-travel, community acquired legionnaires' disease in patients resident in Scotland., Main Measures: Date of onset of disease and postcode and health board of residence of cases., Results: Space-time clustering was present and numerous groups of cases were identified, all but two being newly recognised. Nine cases occurred during three months within two postcodes in Edinburgh, and an outbreak was probably missed. In several places cases occurred in one area over a prolonged period--for example, nine cases in postcode districts G11.5 and G12.8 in Glasgow during five years (estimated mean annual incidence of community acquired, non-outbreak, non-travel legionnaires' disease of 146 per million residents v 4.8 per million for Scotland). Statistical analysis showed that the space time clustering of cases in the Glasgow and Edinburgh areas was unusual (p = 0.036, p = 0.068 respectively)., Conclusion: Future surveillance requires greater awareness that clusters can be overlooked; case searching whenever a case is identified; collection of complete information particularly of date of onset of the disease and address or postcode; ongoing analysis for space-time clustering; and an accurate yet workable definition of sporadic cases. Other researchers should re-examine their data on apparently sporadic infection.

Demonstration of geographical variations in disease can yield powerful insight into the disease pathway, particularly for environmentally acquired conditions, but only if the many problems of data interpretation can be solved. This paper presents the framework, methods and principles guiding a study of the geographical epidemiology of Legionnaires' Disease in Scotland. A case-list was constructed and disease incidence rates were calculated by geographical area; these showed variation. Five categories of explanation for the variation were identified: short-term fluctuations of incidence in time masquerading as differences by place; artefact; and differences in host-susceptibility, agent virulence, or environment. The methods used to study these explanations, excepting agent virulence, are described, with an emphasis on the use of previously existing data to test hypotheses. Examples include the use of mortality, census and hospital morbidity data to assess the artefact and host-susceptibility explanations; and the use of ratios of serology tests to disease to examine the differential testing hypothesis. The reasoning and process by which the environmental focus of the study was narrowed and the technique for relating the geographical pattern of disease to the putative source are outlined. This framework allows the researcher to plan for the parallel collection of the data necessary both to demonstrate geographical variation and to point to the likely explanation.

A summer and autumn peak in incidence is a characteristic epidemiological feature of Legionnaires' disease, a fact which may help in diagnosis. Previous reports from Scotland have reported a winter peak. This study examined the seasonal variation in Legionnaires' disease in Scotland in each year and by category of diagnosis. A characteristic summer/autumn peak was seen for travel-related infection. For non-travel infection, there was an autumnal peak between 1978 and 1982 but an early winter peak between 1983 and 1986. Overall, the pattern could be described as a summer/autumn plateau reaching a peak in early winter. This pattern, seen in men and women, was clearest in the Greater Glasgow area. Nosocomial cases were clustered in the months of October, November and December. In non-travel cases the classical seasonal pattern was not observed and clinicians cannot rely upon this epidemiological feature for diagnosis. However, the relative infrequency of the disease between January and May was a consistent and potentially useful feature. Most reports on seasonal variation are based on aggregated data on travel and non-travel cases. The present observations question the hypotheses developed to explain the seasonal pattern and call for further studies.

Objective: To study the source of non-outbreak legionnaires' disease, particularly the role of cooling towers, by comparing the locations of patients' homes in relation to the location of cooling towers., Design: Retrospective, descriptive study of a case series of patients with legionnaires' disease ill between 1978 and 1986 and, for comparison, a case series of patients with lung cancer. A prospectively developed register and interview based survey provided data on the location of cooling towers., Setting: The city of Glasgow., Patients: 134 patients aged 14-84 with legionnaires' disease during 1978-86 and 10,159 patients with lung cancer during the same period., Main Outcome Measures: The locations of patients' homes and cooling towers as defined by postcodes, which provided map grid references accurate to 10 m; numbers of expected and observed cases of legionnaires' disease in census enumeration districts; and distance of enumeration districts from the nearest cooling tower as defined by five distance categories., Results: Most cooling towers were in or near the city centre or close to the River Clyde, as were the places of residence of patients with community acquired, non-travel, non-outbreak legionnaires' disease (n = 107). There was an inverse association between the distance of residence from any cooling tower and the risk of infection, the population living within 0.5 km of any tower having a relative risk of infection over three times that of people living more than 1 km away. There was no such association with respect to travel related legionnaires' disease, and for lung cancer the association was weak (relative risk less than or equal to 1.2 in any distance group)., Conclusion: In Glasgow cooling towers have been a source of infection in two outbreaks of legionnaires' disease and, apparently, a source of non-outbreak infection also. Better maintenance of cooling towers should help prevent non-outbreak cases. This method of inquiry should be applied elsewhere to study the source of this and other environmentally acquired disease.

The main aim of this study was to measure and explain geographic variations in the incidence of Legionnaires' disease in Scotland, particularly to help understand the source of non-outbreak infection. Between 1978 and 1986 the overall mean annual incidence rate was 7.9 per million (range 3.1-20.2), and for non-outbreak, non-travel cases it was 5.6. There were geographical variations by health board, by city and within cities, e.g. the mean annual incidence rate per million for non-travel, non-outbreak disease was 1.2 in Tayside Health Board, 3.7 in Lanarkshire, 5.6 in Lothian and 14.4 in Greater Glasgow. In Greater Glasgow Health Board non-travel cases lived in and around the city centre and in some postcode sectors there, the mean annual incidence rate exceeded 100. Travel-related cases lived in peripheral areas. These variations could not be explained by differences in access to and use of diagnostic services, surveillance, or host susceptibility (as reflected by socioeconomic status and frequency of other respiratory disease). The explanation probably lay in environmental factors, though differences in agent virulence were not excluded. The two main conclusions are, that non-outbreak cases were not truly sporadic, and that the space-time variations in incidence support the hypothesis that cooling towers were an important source of infection for non-travel, non-outbreak cases. If so such infection is potentially preventable.

As laboratory investigation is essential for the diagnosis of Legionnaires' Disease, the lack of immediate access to a suitable laboratory may result in under, or late, diagnosis. Recently, there has been emphasis on culture as the principal means of diagnosis. This survey describes the diagnostic facilities for Legionnaires' Disease in Scottish hospitals, particularly in relation to the role of the acting reference laboratory in Ruchill Hospital, Glasgow. Facilities have clearly been expanding, with six laboratories setting up local services since 1985. In 1990, 16 out of 36 laboratories (44%) in 10 out of 15 health boards offered diagnostic tests for Legionnaires' Disease; of these 14 offered culture. All laboratories used the acting reference laboratory (directly or indirectly) to confirm positive results. Three laboratories stated that they would test all specimens from patients with pneumonia; the others would do so only on request or when indicated by clinical judgement. These findings provide confidence about the completeness of the surveillance data compiled at the acting reference laboratory and make easier the interpretation of information about geographical and secular variations in disease incidence. Further expansion of diagnostic facilities, particularly culture, with continuing use of the acting reference laboratory for more complex tests, conformation of results and quality control, would meet the objectives of early diagnosis and effective surveillance.

This survey assessed the maintenance of evaporative cooling towers in Glasgow, following two Legionnaires' disease outbreaks. Information was obtained from 76 of 81 premises and a maintenance score was calculated for each of 174 towers. The quality of maintenance was extremely varied (range of maintenance scores, 8-30; mean, 22 (S.D., 5.0); median, 23; maximum possible, 33) and some towers were neglected. Breaches of maintenance principles were mainly structural and organizational, e.g. inadequate drift control, rather than failure to use chemicals. Low maintenance scores were associated with no log book, no guidelines, no change in procedures in last 5 years, solitary cooling towers, and towers on industrial premises. Despite intense publicity the standard of cooling tower maintenance in Glasgow remained a concern. Information campaigns directed at those responsible for cooling-tower maintenance are necessary.