Romola Davenport (Cambridge Group for the History of Population and Social Structure)

Trade-off theory and the surprising efficacy of early public health interventions: the case of faecal-oral diseases in England c.1840-1930

The ‘trade-off’ theory of pathogen evolution predicts that pathogen life histories represent a (complex) compromise between virulence and transmission (Acevedo et al., 2019; Anderson & May, 1982; Ewald, 1983). The evolutionary trade-offs involved vary depending on transmission mode(s) among other factors, and result in a very wide range of virulence from benign to extremely lethal. Here I consider the history of infectious disease mortality in England from the perspective of trade-off theory, with particular reference to faecal-oral diseases in the 19th century.
Faecal-oral diseases are caused by ingestion of infected faeces via contaminated water, food or via flies or touch. The importance of these transmission pathways varies for different faecal-oral pathogens, and virulence is associated with water-borne transmission (Ewald, 1991). In 19th century England the most lethal faecal-oral diseases were ‘Asiatic’ cholera, typhoid and dysentery, with case-fatality rates of perhaps 20 %. Diarrhoeal diseases on the other hand killed mainly very young children and the elderly and are assumed to have had very low case-fatality rates. As water supplies gradually improved after c. 1850, the most lethal diseases underwent major declines. Infant diarrhoeal mortality on the other hand did not decline substantially until the 1910s. I suggest that the precocious declines of the most lethal faecal-oral diseases reflected the relative ease with which waterborne transmission of these diseases was disrupted by early and relatively crude attempts to source clean water. Infant diarrhoeal mortality on the other hand failed to fall even after decades of very costly improvements in water treatment and sewerage, and the eventual disappearance of summer diarrhoeal epidemics remains poorly understood. This paradox mirrors current conundrums in WaSH (Water, Sanitation and Hygiene) programmes globally: while huge gains have been made in controlling typhoid and cholera, infant diarrhoeal disease has proven more intractable. This suggests that public health interventions may often face a problem of diminishing returns, because early interventions often fortuitously target ‘low-hanging fruit’, that is, diseases that are both relatively lethal, and relatively easy to control.