Disease Changes: Microbial and Vector Adaptation

Abstract

In this and the next four chapters, we examine five change agents which have facilitated the emergence and re-emergence of infectious human diseases. Each agent—microbial and genetic adaptation, technology and industry, changes in host populations, environmental and ecological change, and war as a disease amplifier—has underpinned over the centuries both the appearance of new diseases and the waxing and waning of familiar infections. As shown in Figure II.1, the agents are not independent and commonly interact in complex ways to facilitate microbe emergence and re-emergence at different times and in different geographical locations. Accordingly, we also explore these interactions in our account. We begin here with microbial and vector adaptation. Disease microbes are in a continuous state of evolution, responding and adapting to the challenges and opportunities afforded by their hosts and their environments (Morse 1995). New pathogens are evolving, old pathogens are developing enhanced virulence and new clinical expressions, and susceptible pathogens are acquiring resistance to antimicrobial agents. In parallel, the environmental tolerance bands of both old and new pathogens are also changing (Cohen 1998). Not only are disease microbes in a continuous state of evolution. So, too, are the arthropod vectors that transmit many human pathogens. In the second half of the twentieth century, many of these vectors have developed tolerance to an expanding range of insecticides, larvicides, pupicides, and other chemical agents used in their control (World Health Organization 1992c). Against this background, our examination of microbial change and vector adaptation is structured around the three interlinked themes shown in Figure 4.1. We begin in Section 4.2 by examining the issue of natural variation in pathogens and illustrate this with special reference to the emergence and spread of novel subtypes of influenza A virus. We then examine the topic of selective pressure and genetic change in the context of the man-made problems of pathogen resistance to antimicrobials (Section 4.3) and vector resistance to insecticides (Section 4.4). The processes of microbial change and vector adaptation are not intrinsically geographical but they take place within, and are inextricably linked to, specific geographical environments. This gives a strong geographical emphasis to our discussion.