FIGURE SA-4 The Convergence Model. At the center of the model is a box representing the convergence of factors leading to the emergence of an infectious disease. The interior of the box is a gradient flowing from white to black; the white outer edges represent what is known about the factors in emergence, and the black center represents the unknown (similar to the theoretical construct of the "black box" with its unknown constituents and means of operation). Interlocking with the center box are the two focal players in a microbial threat to health—the human and the microbe. The microbe-host interaction is influenced by the interlocking domains of the determinants of the emergence of infection: genetic and biological factors; physical environmental factors; ecological factors; and social, political, and economic factors. SOURCE: IOM (2003).
term observations of the effects of climate variation on infectious disease, taking into account the influence of confounding factors. Existing observations of these effects fall into two main categories: (1) climate-associated shifts in the geographical ranges of pathogens and vectors, and (2) studies of infectious disease transmission dynamics spanning relatively short periods of climatic variation.
The following illustrative examples suggest that climate change has contributed to recent shifts in the geographic distribution of certain vector-borne diseases. In each case, additional factors may also contribute to the emergence and spread of these diseases.
• Bluetongue, a midge-borne viral disease of ruminant animals, emerged for the first time in northern Europe in 2006, during the hottest summer on record for that region and following nearly a decade of anomalously warm years. In the summer of 2007, the disease was reported in nine European countries, including the United Kingdom9 and Denmark, during a massive outbreak that affected tens of thousands of farms (Enserink, 2008; IOM, 2008; ProMed Mail, 2007a,b, 2008; see Figure SA-5).
• Ticks that carry viruses known to be associated with encephalitis have been found at increasingly higher latitudes in northern Europe. A recent study in Denmark reveals a marked shift in the distribution of the tick-borne encephalitis virus as predicted by climate change models (IOM, 2008; Skarphedinsson et al., 2005).
• A 2004 outbreak of Vibrio parahaemolyticus gastroenteritis, associated with human consumption of raw oysters taken from Alaskan waters, extended the northernmost documented source of shellfish carrying this pathogen by 1,000 km. Vibrio parahaemolyticus had not been found in oyster beds in this region before 2004 (McLaughlin et al., 2005).
• In South Africa, the spread of wheat stripe rust has accompanied changes in rainfall patterns (Garrett et al., 2006), while needle blight of pine trees caused by Dothistroma septosporum, formerly a concern only in the Southern Hemisphere, is causing massive defoliation and mortality in the forests of British Columbia following climate change-associated increases in summer precipitation (Woods et al., 2005).
• Malaria incidence in the highlands of East Africa has risen since the late 1970s. The specific influence of rising temperatures on disease incidence has been a subject of considerable debate. Recent analyses employing a dynamical model
9It is believed that bluetongue was carried in a cloud of midges blown by warm winds across the English Channel from France, the Low Countries, or Germany, who, at the time, had similar outbreaks. The first case in the United Kingdom was discovered at a farm near Ipswich, Suffolk (BBC News, 2007; McKie and Revill, 2007).
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