THE ROLE OF SPILLOVER IN EMERGING DISEASE
Spillover defines the transfer of infections from a reservoir population to sympatric species. Spillover may occur in any direction (i.e. between or within domestic animals, humans and wildlife species) and is considered a significant global cause of disease emergence in wildlife (Daszak et al.
2000). As spillover requires opportunities for transmission between reservoir and sympatric species, expansion of the geographical range of a reservoir, either by natural or human-assisted spread, is one of the significant ways in which new spillover events occur. Changes in the distribution of vectors and increased interaction of sympatric species within existing host ranges may also drive spillover. Although spillover events may occur naturally, they are often increased through human activity, including ‘abnormal’ mixing of host species. Exposure of a naive host population to new pathogens may have significant impacts, including high morbidity and mortality, because a lack of previous exposure may result in poor immunological defences against the new pathogen.The ability for spillover into a new host is in part determined by the host-specificity of the pathogen in question. As Australian marsupials and monotremes are taxonomically distant from the domestic and feral species present in Australia, there may be reduced opportunities for host-specific pathogen spillover compared with other taxonomic groups. However, many pathogens have broad host ranges, enhancing the opportunities for spillover. For example, the mange mite Sarcoptes scabiei and protozoa T. gondii are known to affect almost all classes of mammals.
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