Abstract
Modern medicine is often said to have originated with nineteenth century germ theory, which attributed diseases to bacterial contagions. The success of this theory is often associated with an underlying principle referred to as the “doctrine of specific etiology”. This doctrine refers to specificity at the level of disease causation or etiology. While the importance of this doctrine is frequently emphasized in the philosophical, historical, and medical literature, these sources lack a clear account of the types of specificity that it involves and why exactly they matter. This paper argues that nineteenth century germ theory involves two types of specificity at the level of etiology. One type receives significant attention in the literature, but its influence on modern medicine has been misunderstood. A second type is present in this model, but it has been completely overlooked in the extant literature. My analysis clarifies how these types of specificity led to a novel conception of etiology that continues to figure in medicine today.
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Notes
Technically, signs refer to features observed by a third-party (e.g. heavy breathing, pallor, and fast heart-rate), while symptoms refer to features experienced by a patient that cannot be observed in the same way (e.g. nausea, pain, and fatigue). As my analysis does not rely on this distinction, I follow the common practice of referring to both as “symptoms”.
Experiments with cholera differed from other diseases in the sense that Koch could not identify animal models susceptible to the cholera bacilli. In this case, he relied on “natural experiments” to complete the proof that this bacilli caused this disease (Ross and Woodward 2016, 40).
For example, we make causal claims about past events which we cannot intervene on (yesterday the rain caused flooding) or current events which are beyond our technological capacity for actual intervention (such as, the location of the moon causing changes in the tides).
This involves a counterfactual claim (if X were to be changed, then Y would be produced), which is why this is often called a counterfactual account of causation. In the rest of this paper, when I discuss interventionist control I mean hypothetical causal control in this sense.
Not all conceptions of “miasma” had this feature—others were associated with material substances and even physical contagions, both of which could be targeted with interventions aimed at cleaning and purification. In fact, some notions of “miasma” overlapped with the concept of physical “contagion” (Kinzelbach 2006).
For other problems associated with these “regularity” accounts of causation, see (Hitchcock 2018).
This interventionist interpretation should not be viewed as “anachronistic” as one reviewer suggests. It is entirely possible (and I think, likely) that Koch and others expected causes to provide interventionist control over their effects—and that they developed methods and experiments based on this rationale—even if they were unfamiliar with anything similar to modern interventionist accounts of causation. Relying on a causal criterion that is guided by an interventionist rationale (or any other) does not require articulating exactly what that rationale is. The same point holds for scientists in modern contexts—we often find that their causal criteria are well-interpreted with particular philosophical accounts of causation, even when they are completely unfamiliar with such accounts. In some sense, this should be unsurprising. Scientists are often more interested in establishing causal criteria, showing how they work, and what their merits are, as opposed to clarifying their underlying rationale in terms of philosophical, theoretical, or logical concepts. Relatedly, interventionism aims to capture and clarify the reasoning that is already present in successful scientific work on causation. The interventionist account can be understood as making explicit the connection between causation and control that is already present in this work.
In fact, disease etiology is sometimes depicted as a linear process where upstream causes represent the “etiological” factors and the causal intermediates represent the “pathological” process. However, these terms are sometimes used synonymously and often without much clarity (Wullf and Gotzsche 2000, 55).
What about alternative interventions that also prevent disease such as (1) preventing cattle from grazing in a field contaminated with anthrax spores or (2) vaccinating the cattle with an attenuated form of the bacterium? Do these alternative interventions strain this claim of “monocausality” by identifying alternative causes? Neither of these should be viewed as inconsistent with single-cause specificity, because they both involve targeting the same single causal factor. The reason why preventing cattle from grazing and vaccinating them work is because they target the single bacterial factor responsible for the disease (or the spore that produce this bacterium). In other words, just because different interventions can target the same causal factor does not mean there are multiple causes.
For example, consider (a) scurvy, (b) Huntington’s disease, (c) chicken pox, (d) pemphigus, and (e) giardiasis, respectively. These are all diseases that are viewed as having single causal factors. These causes include: (a) a deficiency of vitamin C, (b) a mutation in the huntingtin gene, (c) the varicella virus, (d) antibodies toward an anchoring protein in the skin (desmosomes), and (e) the parasite Giardia lamblia, respectively.
Consider a related objection to the single-cause specificity standard: in some cases, an individual can harbor the bacterial contagion without acquiring the disease. This is seen in cases of “healthy carriers” and it has been used to deny the validity of a single-cause type view (Stewart 1968). For example, although rats injected with anthrax bacteria invariably acquired the disease, the fact that cattle could remain disease-free after being fed anthrax spores, was used to question this causal link. What this objection often fails to keep in mind is that to say that bacteria have causal control over disease does not imply that they have this control when present in any body location. Disease susceptibility depends on the contagion being in particular (but not just any) bodily locations. Thus, finding locations where bacteria can reside without producing disease does not disprove the causal establishment, so long as there are locations where they do produce disease (and thus, exhibit causal control).
In other words, single-cause specificity and shared-cause specificity are not mutually exclusive. Suppose each case of anthrax has a single cause but that there are different single causes across cases (e.g. five different bacteria are individually sufficient to produce this disease). This is a situation that meets \(S_1\) but not \(S_2\). Alternatively, consider a situation where every single case of anthrax is produced by multiple causes, but these causes are the same across all cases of the disease. This is a situation that meets \(S_2\) but not \(S_1\). Our accepted explanation of anthrax meets both of these standards—we view the disease as caused by a single bacterial species (\(S_1\)), where every disease instance has the same cause (\(S_2\)). A situation that meets neither standard would involve there being multiple causes for each instance of disease (lack of \(S_1\)) where these causes differed across cases (lack of \(S_2\)). Multicausal theories of disease in the eighteenth and nineteenth century often fall into this final category and meet neither type of specificity. This highlights how distinct germ theory is from these earlier views, as it contains both types of specificity (\(S_1\) and \(S_2\)).
As Calne states, “[a]etiology is a fundamental criterion for the delineation of individual diseases” (Calne 1989, 18).
Parkinson’s disease can be caused by (1) single gene variants, (2) single environmental factors (such as the drug MPTP, pesticides, and even viral encephalitis), and (3) combinations of genetic and environmental factors (Nandipati and Litvan 2016).
For an overview of the uses, meanings, and applications of the term “validity” in this context, see Schaffner (2012).
As Hyman states, “I use the term ‘diagnostic validity’ throughout this review…as shorthand to signify definitions that capture families of closely related disorders with similar pathophysiology” (Hyman 2010, 162).
One method used in attempts to uncover the etiologies of psychiatric disorders—and subsequently change their characterization and classification—are genome-wide association studies (GWAS). Researchers claim that “carefully designed GWAS with thorough phenotypic characterization have the potential to redefine disease classification” on the basis of identifying “distinct underlying pathological mechanisms” (Detels et al. 2015, 565). It is further claimed that for “complex diseases that have previously been regarded as distinct clinical entities, GWAS findings may point to common underlying disease processes and a shared pathogenesis” (Detels et al. 2015, 565). The assumption that diseases should meet the shared etiology standard (and notion of shared-cause specificity) is seen in these quotes.
As Calne states, “[d]iseases have been been grouped wherever there are any common features that facilitate discussing them for the purposes of teaching, diagnosis, treatment, or research. But the factors that provide cohesion for each of these disciplines are totally different, so it is not surprising that the classification is so heterogeneous” (Calne 1989, 19).
For example, two patients with tuberculosis can present with completely different symptoms, while a patient with tuberculosis and a patient with asthma can present with similar symptoms.
Pathognomonic signs are an exception to this claim as they are signs that are unique to particular diseases. An example of these signs are koplik spots, which are oral lesions found in cases of measles and no other disease. As pathognomic signs are unique to particular diseases, their identification often allows for an immediate and reliable diagnosis without needing to seek further information. These signs are highly useful for diagnostic purposes, but they are also extremely uncommon. Most diseases do not have pathognomic signs.
As Rosenberg states, “[d]isease begins with perceived and often physically manifest symptoms” (Rosenberg 1992, 310).
Many “physical” medicine diseases are also stuck in this first stage in the sense that their etiologies are not understood (or are poorly understood). Examples of these diseases include systemic lupus erythematosus (SLE), Bell’s palsy, and acrocyanosis.
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Acknowledgements
I would like to thank Ken Schaffner, Maureen O’Malley, audiences at the Medical Humanities Colloquium at the University of California, Irvine, audiences at the Issues in Medical Epistemology Conference in Cologne, Germany, and two anonymous reviewers for helpful feedback on this paper.
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Ross, L.N. The doctrine of specific etiology. Biol Philos 33, 37 (2018). https://doi.org/10.1007/s10539-018-9647-x
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DOI: https://doi.org/10.1007/s10539-018-9647-x