A Conceptual Framework for Analyzing Social-Ecological Models of Emerging Infectious Diseases
Melissa L. Finucane, Jefferson Fox, Sumeet Saksena, and James H. Spencer
Unraveling mechanisms that underlie new and reemerging infectious diseases (EID) requires exploring complex interactions within and among coupled natural and human (CNH) systems. This scientific problem poses one of the most difficult challenges for society today (Wilcox and Colwell 2005). EID are diseases that have recently increased in incidence or in geographic or host range (e.g., tuberculosis, cholera, malaria, dengue fever), and diseases caused by new pathogens and new variants assigned to known pathogens (e.g., HIV, SARS, Nipah virus, and avian influenza) (Morse 2005). Wilcox and Gubler (2005) and Wilcox and Colwell (2005) argue that transformations in ecological systems caused by multifaceted interactions with anthropogenic environmental changes such as urbanization, agricultural transformations, and natural habitat alterations produce feedbacks that affect natural communities and ultimately their pathogens, animal host, and human populations. These altered “host-pathogen” relationships facilitate pathogen spillover into “new” hosts, rapid adaptations by pathogens, more frequent generation of novel pathogen variants that result in new and reemerging infectious diseases, as well as range expansion and increasing epidemic intensity and frequency of existing diseases.
In this chapter we present a conceptual framework for examining the WilcoxGubler-Colwell hypothesis in the context of whether risks, and perceptions of risk, associated with highly pathogenic avian influenza (HPAI) caused by the H5N1 virus, as measured in terms of poultry deaths, can be associated with anthropogenic environmental changes produced by urbanization, agricultural change, and natural habitat alterations. This is a novel way of looking at HPAI and other health risks like it, suggesting these risks are not an accident of time and place, but rather are the product of the modernization and urbanization transitions.
We present the conceptual framework in the context of Vietnam because (1) it has been one of the nations most affected by HPAI, (2) has a rapid rate of development, and (3) has very comprehensive secondary databases relevant to such studies. The emergence of the HPAI was first reported in Vietnam at the end of 2003 (Delquigny et al. 2004). Three major epidemic waves of HPAI have occurred in poultry, resulting in 45 million birds culled between December 2003 and August 2005, leading to a 0.5 % reduction in GDP in 2004. As of 2012, a total of 123 confirmed human cases and 61 deaths were recorded (World Health Organization 2012). The country has attempted to control the infection through massive, repeated vaccination campaigns in combination with other control measures (Gilbert et al. 2008a). Vietnam is a particularly useful country to examine development transitions and their associated environmental transformations because these processes are occurring both exceptionally rapidly and simultaneously as traditional agricultural lands are converted to intensified commercial farming or reshaped into urban settlements to meet the needs of the growing population attracted to cities for emerging job opportunities (Douglass et al. 2002; Spencer 2007). If development transitions do pose new challenges to governance, and in particular environmental health challenges, then one would expect to see more of these types of problems in transitional agricultural or peri-urban areas as distinct from both predominately urban and rural areas.
The Wilcox-Gubler-Colwell hypothesis of disease emergence was influenced by complexity theory, which argues that as complex adaptive systems (CAS) CNH systems exhibit far-from-equilibrium non-linear behavior often manifested as “surprise” as with the case of abrupt and unexpected epidemiological phenomena including the emergence of entirely new diseases. Parallel to this, in biological science a re-envisioning has occurred in which nature is no longer seen as consisting of balanced ecological systems made up of relatively linear processes. Rather, natural systems are now seen as hierarchical, self-organized, non-equilibrial and non-linear systems in which emerging diseases can themselves be seen as “emerging properties” of these CAS (Levin 1999). The traditional conception of the ecosystem, a fundamental paradigm in the ecological sciences, has thus been overturned (O'Neill 2001).
As such, ecosystems, including “social-ecological systems” are now understood as characteristically producing emergent phenomena like the unexpected appearance of new pathogens, inherently unpredictable by conventional approaches and theory (e.g., epidemiological models). Moreover, nearly all emerging diseases are vector borne or zoonotic (i.e., maintained in natural host-pathogen cycles that “spill over” to humans) (Woolhouse and Gowtage-Sequeria 2005). They, or their immediate progenitors, exist as part of naturally co-evolved host-parasite complexes embedded within ecosystems, whose dynamics normally include non-linear, cross-scale behavior (Horwitz and Wilcox 2005). Based largely on complexity theory, social ecological systems and resilience (SESR) theory was developed to account for the non-linear dynamic behavior of CNH systems that result from intervention in “managed” ecosystems that unwittingly lead to unexpected and sometimes catastrophic outcomes, including disease re-emergence.
Holling (1973) introduced the concept of resilience into the ecological literature as a way of understanding nonlinear dynamics, such as the processes by which ecosystems maintain themselves in the face of change. In a resilient forest ecosystem, for example, four phases of change repeat themselves again and again. The first two phases, exploitation (the establishment of pioneering species) and conservation (the consolidation of nutrients and biomass), lead to an old growth forest or a climax community. But this climax community invites environmental disturbances such as fire or disease, and is more susceptible to disturbances than non-climax forests. When surprise or change occurs, the accumulated capital is suddenly released producing other kinds of opportunity, termed creative destruction. Release, a very rapid stage, is followed by reorganization in which, for example, nutrients released from the trees by fire will be fixed in other parts of the ecosystem as the renewal of the forest starts again (Berkes et al. 2003). Holling suggested that human societies also reproduce and reinvent in the process of cyclic transformations; he writes: “The bewildering, entrancing, unpredictable nature of nature and people, the richness, diversity and changeability of life come from the evolutionary dance generated by cycles of growth, collapse, reorganization, renewal and re-establishment” (Holling 2003: xv).
-  In this chapter HPAI refers to HPAI caused by the H5N1 strain