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Table 1 Heterogeneities in disease risks

From: Mapping populations at risk: improving spatial demographic data for infectious disease modeling and metric derivation

Heterogeneity type Background information and examples
Spatial Understanding relevant spatial heterogeneities underlies our ability to map host risk of pathogen exposure. Predictions of disease importation or emergence are limited by our ability to distinguish disease-specific hotspots from continuous risk surfaces. Spatial variation in risk is defined by the specific biology of each host-pathogen relationship. Epidemiologically relevant spatial heterogeneities can be highly specific to each infection and must be correctly identified within the proper context of the ecology and landscape of each host-pathogen relationship. Spatial heterogeneities that impact risk profiles for exposure to a pathogen include large-scale environmental factors, such as temperature, access to water, and rainfall abundance, which can affect host susceptibility (e.g. within the African meningitis belt [4]), host exposure (e.g. proximity to malaria vector habitats [5]), and pathogen viability (e.g. cholera survival in the environment [6]). Within a population, the transmission events of infections drive the spatial progression of an outbreak after the initial exposure to the pathogen has already taken place. Transmission events are rarely observed and risk profiles must be constructed using proxies for transmission, again highlighting characteristics specific to each host-pathogen relationship. Risk profiles for directly transmitted diseases focus on host contacts between infectious and susceptible individuals. Important components of these contacts are host density, susceptibility, and mobility. Each of these factors can also be defined across spatial scales, from within household contact patterns to settlement-level risk factors. Urban and rural residence can be thought of as a basic (yet dichotomized) spatial heterogeneity that is closely associated with density and landscape, but typically urbanization has not been defined in spatial terms. Similarly, transmission of vector-mediated infections is impacted by spatial heterogeneities at the household and community level determined by host density, prevention measures, vector mobility and vector abundance. Spatial patterns of environmentally mediated infections will also be determined by the host-pathogen relationship.
Temporal Epidemiologically important temporal heterogeneities will also be specific to each infection. For emerging infections, long-term changes in host settlements, habitat loss, and changing levels of interactions between humans and animal species interactions can define the risk of disease emergence over time [7] (e.g. ebola, SARS, monkeypox, HIV, H1N1 and H5N1 influenza). In other situations, seasonal and environmental factors may determine the population level risk of pathogen exposure (e.g. malaria vector habitats, hyperendemic areas of meningitis). Short-term risk of infection, or transmission of a pathogen within a population, is determined by the biology of the relationships between the host, pathogen and vector. These relationships establish the host susceptibility and infectious periods, and therefore the risk of transmission events. Population level susceptibility profiles (natural or derived) vary across temporal scales with respect to prior exposure and preventative measures. Temporal likelihood of transmission will be determined by length of exposure, and changes in abundance and susceptibility of the host and vector. Exposure and contact rates (density, migration) over the course of a day (as in commuter patterns for influenza [8]) are additional examples of temporal heterogeneities in transmission likelihood and risk across temporal scales.
Demographic and Socioeconomic Susceptibility and transmissibility of infectious disease vary across differing demographic and socioeconomic groups due to differences in immunity, mobility, contact patterns and health status. Small-scale variations in socioeconomic and demographic factors can have a large influence on the geographical variation of infections compared to environmental factors. Age represents one of the most significant factors, with risk of morbidity and mortality of many diseases varying substantially across age groups. These include large variations in mortality and morbidity by age for malaria [9] and for clinical attack risk for dengue [10]. Heterogeneities in susceptibility and transmissibility also exist between the sexes, and especially during childbearing age for women, when pregnancy increases the risks of death for both the mother and fetus, and are important for diseases such as congenital rubella syndrome (CRS) [11]. At a population scale, differences in vital rates such as birth rates create heterogeneities in disease risk across space and time, as evidenced by rotavirus in the US [12]. For macro-parasite infections, such as helminths, in addition to environmental risk factors, the population at risk often depends on socioeconomic profiles and access to key infrastructure (housing quality, adequate sanitation and drinking water). For micro-parasite infections with human-to-human transmission, risk is again associated with individual socioeconomic attributes, but also with community/neighborhood attributes. In other words, the concentration of poverty or poor sanitation services increase risk, as evidenced by cholera outbreaks [13]. Finally, in addition to information on poverty status, knowledge of nutritional status is important; malnutrition can increase (i) susceptibility to many infectious diseases, (ii) the period of infectiousness (by reducing immune function and delaying recovery) and (iii) disease associated mortality [14].
  1. Disease morbidity, mortality, and speed of spread vary substantially with demographic profiles, with clear risk groups and vulnerable populations existing. These have important implications for planning and targeting intervention strategies. The risk of pathogen infection to host populations exists at two spatial levels. First, there is a probability of initial exposure of a population to a pathogen, which defines the population risk. Second, there is a probability of transmission of a disease within a population, which defines the individual risk. Within these epidemic and endemic classifications, the implications for interventions vary across disease landscapes dependent upon the host-pathogen relationships.