Thursday, 16 September 2010

Maybe We Do Learn From History

People who have read and enjoyed accounts of Dr. John Snow's creative statistical sleuthing that led to the end of a cholera outbreak in London (and to the development of modern epidemiology) may enjoy this news story. (Read below if you are not familiar with Dr. Snow and his "Ghost Map")

It seems that an outbreak of legionaires disease in the small Spanish town of Alcoi was stumping the investigators looking for its source. For those who do not know:
"This microbe lives in fresh water nearly everywhere, and it becomes a problem only when inhaled as a fine spray or aerosol. (Legionella is harmless if you drink it.) Outbreaks are usually traced back to man-made supplies of warm water, such as water cooling systems, fountains, hot tubs, even showers." they investigated all the usual suspects, and came up empty.

At that point, perhaps inspired by the success of Dr. Snow, "In the meantime, an analysis of the patients' locations revealed an unusual geographic shift. The first two cases occurred in the northern part of the city; however, the following eight appeared elsewhere, in the Santa Rosa quarter. So investigators expanded their search to include moving devices that used water, such as street-sweeping machines."

"The perp

At this point, road repaving in the Santa Rosa neighborhood was under way. One of the machines in use was a milling machine that ground up old asphalt. It carried a 528-gallon (2,000-liter) water tank to supply atomizers, which sprayed a mist intended to settle dust. The water in this particular machine did not come from the municipal water supply, but directly from a spring, unchlorinated and untreated.

"The suspicious machine, it turned out, had been at work in the northern part of the city around July 15 and in Santa Rosa starting July 31. On Aug. 21, investigators found the machine and took it out of service, although not in time to prevent the final case of Legionnaire's disease, which appeared two days later. "

You can read the report in the CDC's Journal of Emerging Infectious Disease fpr September, 2010... or at this Live Science summary.

For people who haven't read about Dr. Snow's work in breaking the hold of Cholera, I highly recommend the book below:
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