1997 Professor Pennington: E. coli in Scotland- the relevance of John Snow and William Whewell’s consilience of induction

The following paper from Professor Pennington is based on a lecture he gave for the John Snow Society at the London School of Hygiene and Tropical Medicine in September 1997.The following paper from Professor Pennington is based on a lecture he gave for the John Snow Society at the London School of Hygiene and Tropical Medicine in September 1997.

At the end of a week of myth-making, with the funeral of Diana Princess of Wales only two days before, I felt it appropriate to start the Pump Handle Lecture by looking at John Snow through the eyes of the artist who painted his portrait in 1851, because of its vivid contrast with the well-known photograph by Maull and Polybank taken not long before he died. The idealised image of the former showed no signs of the ravages of hypertension and bad kidneys that can be read into the latter.

To continue with the theme of images, Snow’s 1855 “On the mode of communication of cholera” was revolutionary not only in what he said but in the way that he said it. The significance of the pump handle removal apart, the force of the street map showing the location of cases around Broad Street is overwhelming, not only in its originality in its historical context but for its strength as an investigative and demonstrative tool. Its heuristic power continues. What about Wishaw? What lessons has the 1996 Scottish E. Coli 0157 outbreak taught us? The legal process there is still active with all the constraints that that brings. To protect myself against an inadvertent breach of the sub judice rules I describe the events there through the eyes of the press.

The first cases in the outbreak came to the notice of the local public health department on Friday 22 November 1996. The hidden hand, which determines that the earliest – and therefore the most important – events and activities in an outbreak always occur over a weekend, was clearly acting with full force here! Its evolution over the next fortnight was marked by the headlines: “Town in fear” (25 November, 35 cases), “Food bug outbreak spreading” (26 November, 65 cases), “Traders warned, killer meats still a threat” (29 November). At this point 120 cases had been reported in Lanarkshire, 13 in Forth Valley (30 miles away), 2 in Glasgow, and 1 in Edinburgh, and five attendees at a meal provided for the frail elderly at the Wishaw parish church hall on 17 November had died. By this time it was clear that the butcher’s shop at the centre of the outbreak supplied not only Wishaw, a small run down industrial revolution town in th Lanarkshire badlands, but used its 40 staff to prepare meat and meat products for many outlets across central Scotland.

At this point, the Secretary of State for Scotland asked me to chair an expert group to enquire into the circumstances of the outbreak, learn lessons from it, and make recommendations. Needless to say, this administrative move had no effect on the evolution of the outbreak. Neither did it diminish press coverage. Rather, it intensified it and made it more, rather than less, political, with headlines like “Cutback fears of the food doctors” (30 November, 173 cases; expressing the fear that the categorisation of public health doctors in Scotland as administrators would make them even easier targets for downsizing) and “Raw deal from food police” (1 December, 232 cases; a right-wing attack: “food police already have enough powers … what is needed is a more sophisticated intelligence system to track the when and how of sporadic outbreaks”). The cartoons were also appropriate (figure 1). By mid-December the outbreak was over (figure 2), and the hogmanay my expert group’s interim report with priority recommendations was on the Secretary of State’s desk. It was published in mid-January and accepted in toto by the government. How was it received? To quote my actual words “there’s been an enormous amount of jumping up and down and shouting about what was being done and what wasn’t being done … but as soon as it was suggested that butchers should tighten up their act, especially with the physical separation of raw and cooked meats, we got some moderately hostile publicity” (figure 3). Events moved on, particularly on the policial scene – a row about the alleged rewriting of an official report on conditions in abattoirs received full media coverage. Without a doubt this publicity was good for the expert group (figure 4). Our final report was published on 8 April. It has 32 wide-ranging “plough to plate” recommendations.

At this point I addressed the title of my lecture, which was about the philosophical principles underpinning our work. I epitomised a particular and critical view of epidemiology by quoting Macaulay’s 1837 analysis of Bacon’s scientific method: a plain man finds his stomach out of order. He’s never heard of Lord Bacon’s name but he proceeds in the strictest conformity with the rules laid down in the second book of the Novum Organum, and satisfies himself that minced pies have done the mischief. “I ate minced pies on Monday and Wednesday, and I was kept awake by indigestion all night,” “I did not eat any on Tuesday and Friday, and I was quite well.” “I ate very sparingly of them on Sunday, and was very slightly indisposed in the evening. But on Christmas day I dined on them, and was so ill that I was in great danger.” “It cannot have been the brandy, which I took with them, for I have drunk brandy daily for years without being the worse for it.” Our invalid then proceeds to what is termed by Bacon the Vindemiatio, and pronounces that mince pies do not agree with him.

After raising the issue that much of epidemiology is obliged to rest on such old-fashioned inductive scientific principles – with all the problems attendant on that approach – I went on to ride a personal hobby-horse, which is to emphasise the power of another scientific principle first enunciated by the Victorian polymath, William Whewell. He called it the Consilience of Inductions. Whewell proposed that where “inductions from classes of facts altogether different have jumped together … [this] impresses us with a conviction that the truth of our hypothesis is certain. No accident could give rise to such an extraordinary coincidence. No false supposition could, after being adjusted to one class of phenomena, exactly represent a different class, where the agreement was unforeseen and uncontemplated. That rules springing from remote and unconnected quarters should thus leap to the same point, can only arise from that being the point where truth resides”. The relevance for epidemiology of the Consilience of Inductions is that through its application conclusions cannot just be further supported, but enormously strengthened. This happens when supplementary, non-epidemiological evidence is found that independently supports an epidemiological conclusion. For E. coli 0157, evidence of this kind is produced when strain typing is done by pulse-field gel electrophoresis – a variant of the new classical genotyping approach which compares the bar-code like patterns obtained by cutting the DNA in a gel. During the 1996 outbreak this method of clonal analysis allowed a clear and unequivocal distinction to be made between strains linked epidemiologically to the outbreak and the others that were being isolated in central Scotland at the same time. All the outbreak strains (>200) that were isolated had identical gel profiles.

Did William Whewell and John Snow ever become acquainted? Maybe. They were both members of the Royal Medico-Chirurgical Society in London in the 1850s. Whether they met or not, they lived parallel lives with many similarities.

BornLancaster 1794York 1831
SiblingsOldest of 7Oldest of 9
PatronLocal headmaster (scholarship to Cambridge 1812)Uncle (apprentiship Newcastle 1829)
Position at deathMaster of Trinity College CambridgeAnaesthetist in London
Cause of deathFell from a horse 1866Stroke 1858

Was Whewell interested in public health? Despite his polymathic tendencies (he wrote books on mechanics, gothic architecture, moral philosophy, the history and philosophy of science, translated Plato, was ordained, preached regularly, wrote poetry and science fiction, and at one time was a Professor of Mineralogy) probably not. Nevertheless, the relevant anecdote about him, Cambridge, toilet paper, and Queen Victoria still amuses. Queen Victoria, standing on a bridge over the Cam: “What are all those pieces of paper floating down the river?” The Master of Trinity (Whewell): “Those Ma’am, are notices that bathing is forbidden”.

I felt obliged to finish my lecture with a Scottish connection because I come forth of there. Two came readily to hand. The Aberdeen University Library copy of John Snow’s account of the 1854 cholera outbreak and the Broad Street pump is not on the open shelves because it bears John Snow’s signature. It was presented by Snow to Sir John Forbes. Forbes trained in Aberdeen and Edinburgh, eventually becoming a physician to the Royal household. He was a pioneer stethoscopist. I speculated that this background and these connections may have helped and influenced Snow. I concluded by commenting on John Snow’s explanation of the different patterns of cholera seasonality in Scotland and England – that it could be due to the English preference for beer and the Scots for spirits – by paying tribute to the continuing importance of the symbolism of the pump and the removal of its handle, and I reinforced this by physically removing the handle of a pump placed before me by the Pump Handle President.