Ecological sources of zoonotic diseases.

Although of zoonotic origin, pathogens or infections posing a global threat to human health such as human immunodeficiency virus, severe acute respiratory syndrome or emerging influenza type A viruses may actually have little in common with known, established zoonotic agents, as these new agents merely underwent a transient zoonotic stage before adapting to humans. Evolution towards person-to-person transmission depends on the biological features of the pathogen, but may well be triggered or facilitated by external factors such as changes in human exposure. Disease emergence may thus be depicted as an evolutionary response to changes in the environment, including anthropogenic factors such as new agricultural practices, urbanisation, or globalisation, as well as climate change. Here the authors argue that in the case of zoonotic diseases emerging in livestock, change in agricultural practices has become the dominant factor determining the conditions in which zoonotic pathogens evolve, spread, and eventually enter the human population. Livestock pathogens are subjected to pressures resulting from the production, processing and retail environment which together alter host contact rate, population size and/or microbial traffic flows in the food chain. This process is illustrated by two study cases: a) livestock development in the 'Eurasian ruminant street' (the area extending from central Asia to the eastern Mediterranean basin) and the adjacent Arabian peninsula b) poultry production in Southeast Asia. In both scenarios, environmental factors relating to demography, land pressure and imbalances in production intensification have led to an unstable epidemiological situation, as evidenced by the highly pathogenic avian influenza upsurge early in 2004, when the main outbreaks were located in areas which had both large scale, peri-urban commercial holdings and a high density of smallholder poultry units.