Tracking a route to prevention
Learning how a disease is transmitted is crucial to stopping its spread.
When cholera was the scourge of London in the mid-1800s, people thought the disease was carried by vapors. (Vapors were hard to ignore, given the stench of human waste in a time before efficient sewers.) An early epidemiologist of sorts, cataloging who died and who didn’t, identified cases that didn’t fit with what one would predict if the disease traveled by air.
The common factor among cholera victims was not living (and breathing) in the most affected neighborhood, but getting water from a particular public well. Cholera is now known to be caused by bacteria, and sanitation in the form of modern plumbing prevents outbreaks in the developed world.
A chance occurrence in 1977 -- an Alaska Airlines flight suffered a three-hour delay on a layover -- provided an impromptu naturalistic setting in which to study influenza transmission. A single passenger had a bad case of the flu. Within days, 38 of the other 53 passengers and crew became ill. A research team interviewed all but one person from the flight after the fact and deconstructed who was where when.
The study, published in the American Journal of Epidemiology in 1979, is widely praised for its meticulousness, but it only muddies the argument over airborne transmission. The ventilation on the plane was turned off during the delay, a fact that has been used to argue for airborne transmission. However, passengers also moved around the cabin and were allowed to deplane and reboard at will.
The only thing the study proves is that it was extremely unpleasant to be on that plane, says Dr. Michael Gardam, director of Infectious Disease Prevention and Control for the Ontario Agency for Health Protection and Promotion in Canada.
If transmission routes determine prevention methods, testing prevention strategies may give clues about transmission.
In addition to fresh air ventilation, UV light has been proposed as a flu-killer in hospitals, in part based on a study performed at the Livermore VA Hospital during the flu epidemic of 1957-1958. One wing of the hospital was equipped with UV radiation. During the height of the outbreak, 39 cases of flu occurred in the non-radiated wings versus zero in the wing with the UV lights.
Gardam agrees that UV radiation kills the flu virus, but doesn’t agree with those who conclude from this observation that flu is airborne. “The challenge is they weren’t controlling for anything else,” he says. “For all I know, two additional healthcare workers came to work in the [non-radiated] building and brought in influenza.”
-- Jill U. Adams
