Examples of weighting evidence
a) Theoretical example
This image shows an idealised scenario, in which two patients from two different countries had both visited country C, but stayed in three different hotels. When the patients returned to their respective countries, they had samples taken for diagnosis and typing of clinical isolates.
b) High assurance linking exposure to cases - published example
In 1999, an outbreak of Legionnaires' disease affected 188 visitors to a flower show in the Netherlands. To identify the source of the outbreak, an environmental investigation, a case-control study among visitors and a serologic cohort study among exhibitors were carried out to measure exposure to possible sources. Two whirlpool spas in halls 3 and 4 of the exhibition and a sprinkler in hall 8 were culture positive for Legionella pneumophila. One of three genotypes found in both whirlpool spas was identical to the isolates from 28 of 29 culture-positive patients. Persons who paused at the whirlpool spa in hall 3 were found to be at increased risk of becoming ill.
There were matching environmental and clinical isolates linking exposure to the whirlpool spa to the cases. The analytical epidemiological study and findings from the environmental investigation added to the weight of the evidence implicating the whirlpool spa 3 as the source of the outbreak .
c) Low assurance linking exposure to cases - published example
In 2008, seven persons went for a weekend to a cottage complex which had a sauna, spa and swimming pool. Six of these persons developed Pontiac fever within 24 hours of arrival. Legionella pneumophila serogroup 1 was confirmed in three of the six cases. Hyperchlorination of the spa pool, shortly before the taking of water samples, limited the ability to isolate L. pneumophila from the spa-pool balance tank. Environmental investigations revealed that, a day prior to the weekend, the spa pool's automatic chlorine-dosing pump had not been in working order. Additionally, record keeping of maintenance at the spa was inadequate.
Although matching isolates were not available, the strength of evidence linking the whirlpool spa to the outbreak was improved by strong epidemiological links from the descriptive epidemiology and environmental investigation .