Legionnaire's Disease
I have always enjoyed reading mysteries, the complete Sherlock Holmes was the first hardcover book I bought. I equally enjoy doing puzzles: crossword, acrostic, Sudoku, and jig-saw. These two interests seem to come together whenever I read about public health outbreaks. Two recent studies on Legionnaire's Disease help to illustrate how epidemiologists work to to discover the missing pieces from the facts they need to solve an outbreak.
When cases of Legionnaire's disease cropped up last year in Alcoi, Spain, molecular analysis linked the cases to one another, and to a series of cases that occurred earlier in the decade. However, the cases occurred in two different parts of the city, and at two different periods of time.
Legionnaire's disease is associated with exposure to atomized water that is contaminated by the bacteria causing the disease. Usual sources are stationary. The CDC guidelines have investigators ask specifically about exposure to these potential sources: showers, decorative fountains, humidifiers, hot tubs, wet saunas, cooling towers, respiratory therapy equipment, and evaporative condensors. During the earlier outbreak, investigators had tested the usual locations. With the new outbreak, they knew they had to find a water source that fit the time and location parameters.
The likely suspect turned out to be an asphalt milling machine - a device that scrapes asphalt from a street so that the new paving remains at the same level. Water is sprayed during the process to control dust. This particular device was filled from a spring, not from the city water supply, which likely allowed for the original contamination. The machine was removed from service, cleaned, and renovated to use dripping water rather than spraying water. No further cases turned up. Read all about it in Emerging Infectious Diseases.
Sporadic cases of Legionnaire's Disease often occur with no source ever being determined. In England and Wales, studies found that professional drivers were five times more likely than normal to contract the disease. This was not an ongoing outbreak that could be molecularly analyzed, but did provide investigators a population to survey. Drivers who had contracted Legionnaire's were contacted and asked to participate in a survey as a case group. The same survey was given to individuals of the same age, gender, and geographic location to provide a control group.
One of the interesting results came from a question about using screenwash - either prediluted or concentrate. Whereas 50 of 51 members of the control group used it, only 34 of 49 cases used it. The case group also drove more with their windows down, and drove through industrial locations (another possible source of exposure). Public health officials conjecture that the windscreen washer reservoir is in a warm location (the engine compartment), the water is atomized by the windscreen sprayer, and the drivers are exposed to the contaminated spray through the open window.
This case control study cannot prove causation; but it seems to be well-done and has led to a public health effort to have drivers use screenwash in their windscreen wiper system. After a few years, we might well see a follow-up study to compare the rate of Legionnaire's Disease compared to the rate of use of screenwash by British drivers. This study was written up in the European Journal of Epidemiology.
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