Pathogen–host–environment interplay and disease emergence

Engering, Anneke; Hogerwerf, Lenny; Slingenbergh, Jan

The third major disease emergence category concerns the emergence of a disease complex in a novel area. Emergence may entail an expanding range, so-called 'geographic expanders' or 'geographic jumps', pathogens that access novel areas and host resources through saltation dispersal ('virgin-soil' outbreaks). The first applies in particular to pathogens confronted with changing landscapes, as may be due to climate or land-use changes. Geographic jumps when successful may lead to rapid and widespread dissemination and major epidemics. The introduction of the mosquito-borne West Nile virus in the USA in 1999 forms an example. 88 An example of a food-borne geographic jump forms the introduction of African swine fever in the Caucasus region and southern Russian Federation where pathogen introduction could be traced back to food scraps on board of a ship from Southern Africa, fed to pigs in Georgia, early 2007. 89 Other recent examples include the emergence of Chikungunya virus in Italy, and Bluetongue virus in Northern Europe. 90, 91 Dynamics of emergence in a novel geographic area Geographic range expanders encroach the landscape directly beyond the actual distribution limit. A gradual expansion of the geographic range may entail adjustment to an only slightly different landscape, at least during the initial phase. Adaptation during range expansion is mainly via ecological fitting and may translate into gradual genetic evolution. 31, 92 Eventually, all invasions come to a halt, with consolidation in the form of a novel geographic limit. 93 Unlike range expansion, a long distance jump is usually governed by chance, as only few pathogens, vectors or infected hosts find access to the novel landscape. 94 Hence, it is difficult to predict when and from where to where long distance jumps will occur. The initial establishment forms a major bottleneck presenting numerous compatibility issues concerning host and landscape. In addition, when the numbers of infected individuals are still low, extinction may easily occur by chance, even if the R 0 of the pathogen in the new landscape is greater than 1. Once initial establishment is successful, an abundance of susceptible hosts may become within reach, facilitating rapid spread. Geographic jumps may be accompanied by a transient increase in virulence during the spread phase when the number of naive hosts within reach builds up rapidly ('i' in Figure 1 ). The eventual net result of an invasion always entails a range expansion and may or may not be accompanied by more profound ecological dynamics and allopatric speciation of the pathogen.