Conceptual Basis for Virulence Management

Studies of virulence evolution have attracted attention in part because of the potential for practical applications to health problems. One of these potential applications involves forcing disease organisms to evolve toward low virulence (Ewald 1988,1991a, 1994a). If evolutionary interventions to control virulence are feasible they may be especially cost-effective solutions, because they may control entire classes of diseases through interventions considered valuable for other reasons. In particular, interventions that favor reduced frequency of infection may simultaneously favor evolutionary reductions in the inherent harmfulness of infectious agents (referred to hereafter as “pathogen virulence”).

Evolutionary considerations and the current state of empirical investigation suggest that one class of such interventions, potentially capable of tipping the competitive balance in favor of milder pathogen strains, involves alterations in the mode of pathogen transmission. This chapter assesses the current state of investigation into options for the virulence management of diarrheal diseases, as well as of other categories of disease for which transmission may be relatively independent of host mobility. For each category of disease I discuss interventions that may foster virulence management through the alteration of transmission mode. Finally, I broaden the argument to include virulence management through vaccination strategies. A thorough evaluation of these ideas requires a long series of hierarchically organized tests, so the current state of evidence represents work in progress, which may require decades to fully develop.

Virulence Management of Diarrheal Diseases

Principles of virulence management seem especially applicable to diarrheal disease for several reasons.