title: Models for leprosy. An appraisal of graphic representations of the "spectrum" concept as models and a suggestion for a catastrophe theory model for leprosy.
abstract: Graphic representations of the spectrum concept of leprosy are examined in some detail as models for this disease. This reveals that this concept is somewhat inadequate and that the spectrum metaphor may itself be inappropriate because, by its very linearity of logic, it may not be able to depict the nonlinear behavior of leprosy properly. The assumptions underlying this concept and their logical consequences, brought out by the graphic representations, include an invariable relation between CMI and BI, identity of one type of leprosy with one specific level of CMI, a fixed sequence of types, and the consequent impossibility of skipping the sequence. However, our experience with leprosy does not bear out these assumptions. Further, development and progress of leprosy from a normal (nonleprous) state cannot be represented in these models. A search for alternative conceptual models therefore appears reasonable and even necessary. The catastrophe theory (a branch of topology in mathematics) describes a number of models for explaining how continuous causes could produce sudden or discontinuous changes. Of the various catastrophe theory models available, the relatively simple "cusp" model appears capable of application to leprosy. This model, as applied here, requires two control factors (identified tentatively as the amount of dead bacilli and the amount of living bacilli or their indicators) and one pattern of behavior, identified as progress towards limited or extensive disease. This model suggests under what conditions leprosy will change from one type to another and whether that will happen gradually or suddenly. It also suggests that for certain values of control factors the disease may manifest in one of two forms of borderline leprosy, and that lesions very similar to start with can progress to quite different states under similar conditions of change. The behavior of leprosy agrees more or less with that suggested by this model. The cusp model thus seems to: a) provide an insight into the behavior of leprosy, enabling us to understand the dynamics of the disease; b) explain some of its intriguing manifestations; c) ask meaningful questions; and d) plan new therapeutic approaches. Although this is a highly speculative and probably too simple a model, this attempt shows that it is possible to view leprosy outside the framework of the concepts of spectrum scale and polar types of leprosy, the conceptual models which dominate all of our current thinking about the disease.
mesh_terms: D006801:Humans; D007111:Immunity, Cellular; D007918:Leprosy / Q000276:immunology*; D008962:Models, Theoretical