Cells that count: philosophy of a diagnostic test for bovine mastitis.

Cells that count: networks of a diagnostic test for bovine mastitis.

Social Epistemology, 2014, in press. 

Can be accessed http://www.tandfonline.com.proxy.library.uu.nl/doi/full/10.1080/02691728.2013.818730#.Uuoa3D3ox8E

Abstract

Somatic cell count (SCC) is a diagnostic test of milk for mastitis in cows. Its specificity and sensitivity are less than 1.0, making test results uncertain. I discuss epistemological problems of the test such as underdetermination, undercalibration and underdiscrimination, in the solution of which biomedical and economic factors may play a role.

Diagnostics of the SCC should be considered as an epistemological network, functioning in a network in which farmers, veterinarians, epidemiologists and milk industry shift their position following biomedical, technological and economic changes, which is described as an actor-network to understand how human and non-human agents in mastitis may be associated.

Introduction

The aim of the paper is to investigate and discuss the epistemology of a diagnostic test and discuss the role of such a diagnostic test in the framing of disease. The background of this investigation is a study I am performing of the history and philosophy of bovine mastitis, a disease or syndrome of lactating cows that has considerable economic consequences but was hardly a problem at all before milk became a modern commercial product at the end of the 19^th century. I use this study of the history of mastitis as an effort to frame the disease in terms of Rosenberg’s definition of framing disease: ‘In some ways disease does not exist until we have agreed that it does, by perceiving, naming, and responding to it’ (Rosenberg  1992). The way mastitis has been framed, and is still reframed continuously, is in my view driven by both biomedical and social (i.e. non-epistemological) factors, such as technology and commercial interests.

The perceiving and naming of a disease is strongly related to its diagnosis and I have argued before that the diagnosis is part of a system of biomedical knowledge of the disease in which it is interconnected to epidemiology, pathogenesis (i.e. the etiology and pathophysiology) and therapy (Nederbragt 2000). This interconnectedness and mutual dependence of the pieces of knowledge from the different biomedical disciplines result in a more or less stable knowledge of the disease as a biomedical entity. Framing a disease reaches beyond the biomedical; it is a process that must be traced back in the past and the frame can only be understood with knowledge of its history. For a diagnosis to point to a disease in this frame at least some agreement must exist of what the disease is; so both disease and its diagnosis must co-evolve. When we assume non-epistemological influences in the framing of the disease we should also assume such influences in the framing of the diagnostic test of the disease. This will be a main theme in this paper.

The framing of a diagnostic test may be seen as an effort to standardize the test, notwithstanding the uncertainties connected to the outcome of the test. I shall discuss these uncertainties in an epistemological approach, making use of more or less traditional concepts. The uncertainty of tests is a common theme in the discipline of epidemiology. Notwithstanding these uncertainties, diagnosis is, according to Rosenberg (2002), ‘central to the definition and management of the social phenomenon that we call disease’. Apparently, it is necessary that the diagnostic test as an epistemological network moves, via the network of the subdisciplines that constitute the disease as a biomedical entity, to the social phenomenon that we call a disease.

I shall proceed as follows. First I will give a description of the most important aspects of bovine mastitis. I will then concentrate on the somatic cell count as a test for subclinical mastitis in milk and discuss three epistemological aspects of this test, ­i.e. underdetermination, undercalibration and underdiscrimination. These three form key factors in establishing the diagnostic test as an epistemic network. In the final paragraph I will sketch the biomedical-statistical network in which diagnosis participates and I will argue that for understanding a patient and her disease we have to weigh the evidence of the diagnostic test against the context of the patient; to be able to do this we have to leave the biomedical-statistical network and enter a social network. The significance of this approach is that we better understand the patient when we make associations between patient, diagnosis, disease and patient-environment. I shall argue that the application of some of the approaches of the actor-network theory may help us to understand how we can associate these actors and that this may clarify the incompatibility of knowledge of disease and knowledge of the patient that I discussed before (Nederbragt 2000).

Nederbragt, H. 2000. The biomedical disciplines and the structure of biomedical and clinical

Rosenberg, C.E. 1992. Framing disease. Illness, society and history. In Framing Disease. Studies in Cultural History, edited by C.E.Rosenberg and J.Golden, pp. xiii –xxvi, New Brunswick: Rutgers University Press.

For the complete paper, see the website of the journal Social Epistemology.

Those who have no access to this journal may send me an e-mail for a copy.

Counting leukocytes to provide a wholesome milk-supply Part 3

Since in milk “good” streptococci cannot be distinguished from “bad” cocci to characterize the quality of the milk as “good” or “bad” for consumption, maybe it is possible to evaluate the quality of the milk by looking at the leukocytes in the milk.  After all, given the assumption that the consumption of milk with mastitis streptococci and sore throats and gastrintestinal disturbances in man are causally related, the determination of inflammation markers in milk may offer an alternative solution for such an evaluation. This raises two problems. First, it has to be established that increased numbers of leukocytes indeed reflect increased numbers of cocci; second, that a threshold value can be determined on which milk can be judged as safe or harmful. This subject constitutes the last part of Harris’ paper. (1)

Harris starts to discuss the use of the word “pus cell” that was common usage in his time. “All milk contains leukocytes” he quotes W.G.Savage, a British public health expert. “When does a leukocyte become a pus cell, and what distinguishes one from the other?” The presence of leukocytes in milk up to a certain point is physiological, but beyond that point is pathological. The literature shows large variations in leukocyte number, among cows and among quarters of one cow. Some authors report proportional relationships between streptococci and leukocytes, others cannot find them.

Next, Harris discusses the attempts to develop practical methods of leukocyte counting. These methods are mostly based on staining cells in known volumes of milk and counting them with a microscope and a hemocytometer; the staining procedures are complicated and time consuming, but Harris consideres them as good.  An alternative method is the use of the tube developed by Trommsdorff, that is filled with 5 ml of milk, centrifuged, and the volume of the sediment read of as a measure of cell number, bur Harris is not very positive about it.

(from a recent catalogue of Gerber Instruments)

One of the authors he mentions had stated that milk containing 500,000 cells per c.c. togther with the presence of fibrin (an inflammation marker, BN) is to be regarded as suspicious, whereas a cell number of one million per c.c. and associated with fibrin “is conclusive of the presence of pus, i.e., evidence of mastitis.

In the next blog I will discuss Harris’ conclusions.

1. N.M.Harris, ‘The relative importance of streptococci and leukocytes in milk’, The Journal of Infectious Diseases, 4 (1907) 50-62.