The “father” of epidemiology is a nineteenth century doctor, John Snow. He had more than one disciplinary child, since he is also considered the “father” of anesthesiology, having popularized the use of chloroform in obstetrics by using it on Queen Victoria in the 1850s. That distinction aside, Snow is famous for his pioneering studies showing cholera was a waterborne disease. I’ve been thinking about this in relation to bird flu. Here’s the connection.
In Snow’s Victorian London, the predominant scientific theory on cholera’s etiology was from miasmas, a general term for noxious elements in the atmosphere, usually produced by decaying organic matter and waste. The miasma theory as explanation had a lot going for it. It explained, for example, why disease epidemics, especially cholera and typhoid, seemed to concentrate in the poorest parts of town, where sanitation was worst. In a city like London, drowning in its own excrement, the smells and conditions of the poorest parts of town don’t even seem credible today, exceeding the worst barrios anywhere. There was no domestic sewer system to carry away the wastes of millions of residents resulting in overflowing cesspools, cellars and eventually the river Thames, running through the middle of the city. The miasma theory connected smells with disease, and bad smells were the rule, not the exception, in Victorian London. It was not surprising the city would be the locus of devastating cholera epidemics in the 19th century. While there were a variety of theories as to how foul air could cause disease (general debilitating effects, specific gases, electrical influences, etc.), the principal view of the mode of transmission was through the air, not the water.
Steven Johnson has a new book (The Ghost Map) on Snow and his effect on how we view the city. It is a pleasant read (if you find reading about fecal contamination and sudden death pleasant, in which case maybe you should consider epidemiology as a career), although not revelatory for those who know their public health history. But I found this observation about the miasma theory especially poignant:
So often what is lacking in many of these explanations and prescriptions is some measure of humility, some sense that the theory being put forward is still unproven. It’s not just that the authorities of the day were wrong about miasma; it’s the tenacious, unquestioning way thy went about being wrong. An investigator looking for holes in the theory could find them everywhere, in even in the writings of the miasmatists themselves. (Steven Johnson, The Ghost Map, p. 125)
Snow and others saw the same thing, but interpreted it differently. The scavengers, sewer hunters and “night soil men,” those who made a living combing the city’s waste to eke out a living on the margins, were not unusually sick. On the contrary, they appeared to be unusually robust. More significantly, only some of the people who lived under the exact same atmospheric noxiousness came down with the disease. A vast public health campaign to eliminate cesspools (and thus divert the waste into the river) had not solved the problem, which continued as before or worse. In defense of the miasmatists, the true source of the disease was essentially invisible while the truly noxious sanitary conditions of the hardest hit population was all too sensible and visible. The non-uniform pattern of attack had a ready explanation: individual susceptibility. This could be framed in many ways, depending on one’s social outlook. It might have reflected a moral failing or secret sinfulness of the victim, or a moral failing in a society that allowed poor people to live in such squalor. We have our modern versions of this and we are helped by handwaving but plausible arguments from immunology and genetics.
The techniques that Snow used to such good effect, statistical correlations between mortality and risk factors, also seemed to confirm the truth of the miasma theory. Deaths were greater at the lower elevations (which were also closer to the River and contaminated water sources). The same day that Snow convinced local officials to shut down a contaminated well on Broad Street that had led to an explosive outbreak around Golden Square, the General Board of Health issued detailed instructions for an investigation of the cause of the outbreak which involved collecting information, almost all of which was germane only to the miasma theory. The idea of compiling and analyzing statistical data was the same as Snow’s. But the questions asked dictated the possible outcomes.
What’s the connection with bird flu? If you use your word processor to do a Global Search and Replace of “almost all human cases of bird flu so far have been from close contact with poultry” for “miasma theory” you’d have somethingsimilar to what we read today. I have said here before that I still consider a poultry source a reasonable epidemiological position, since the causative agent, influenza A/H5N1 is widespread in poultry and exposure is prevalent. But humility is also called for. There is an abundance of human cases for which a good history of poultry exposure is difficult to ascertain and establish. On the other hand, exposure is so widespread, many wonder why there isn’t more disease in identically exposed individuals. The investigations naturally (and not unreasonably) ask questions pitched strongly toward poultry exposures. As in Victorian London’s unsanitary conditions, poultry exposures are common in the areas of the world where this virus is endemic, so investigations naturally reveal a connection (the example of the General Board of Health cholera investigation in 1855 comes to mind). Again the idea of individual susceptibility is invoked as the explanation, although in a much more sophisticated form buttressed by modern genetics and immunology, but with no more hard evidence. Generalized explanations about moral weakness had special cogency for the Victorian sensibility. Any explanation that uses the language of genetics, molecular biology and immunology has special cogency for us. We believe there are good and rational reasons for this cogency, but then the Victorians also believed the same about their spontaneous and transparently obvious (to them) moral reasoning. Again, it is a question of a little humility.
If it’s not poultry, then what is it? This isn’t an either/or question. It can be poultry and other things. The things that still seem to be out there, albeit at the margins for most epidemiologists who work on flu, is gastrointestinal exposure (food and water) and another vector or natural reservoir besides birds. Another infectious risk factor might also be involved, i.e., the risk of human illness is greater when there is exposure in combination with another agent or pre-existing infection. There’s not a lot of evidence for any of this at the moment. There is also some counter evidence for the poultry hypothesis, evidence that is convienetly ignored. Someday maybe it will be explained.
But the example of John Snow and waterborne cholera should remind us humility is also a scientific virtue.