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ABSTRACT Between 2013 and 2018, the novel A/Anhui/1/2013 (AH/13)-lineage H7N9 virus caused at least five waves of outbreaks in humans, totaling 1,567 confirmed human cases in China. Surveillance data indicated a disproportionate distribution of poultry infected with this AH/13-lineage virus, and laboratory experiments demonstrated that this virus can efficiently spread among chickens but not among Pekin ducks. The underlying mechanism of this selective transmission remains unclear. In this study, we demonstrated the absence of Neu5Gc expression in chickens across all respiratory and gastrointestinal tissues. However, Neu5Gc expression varied among different duck species and even within the tissues of the same species. The AH/13-lineage viruses exclusively bind to acetylneuraminic acid (Neu5Ac), in contrast to wild waterbird H7 viruses that bind both Neu5Ac and N-glycolylneuraminic acid (Neu5Gc). The level of Neu5Gc expression influences H7 virus replication and facilitates adaptive mutations in these viruses. In summary, our findings highlight the critical role of Neu5Gc in affecting the host range and interspecies transmission dynamics of H7 viruses among avian species.IMPORTANCEMigratory waterfowl, gulls, and shorebirds are natural reservoirs for influenza A viruses (IAVs) that can occasionally spill over to domestic poultry, and ultimately humans. This study showed wild-type H7 IAVs from waterbirds initially bind to glycan receptors terminated with N-acetylneuraminic acid (Neu5Ac) or N-glycolylneuraminic acid (Neu5Gc). However, after enzootic transmission in chickens, the viruses exclusively bind to Neu5Ac. The absence of Neu5Gc expression in gallinaceous poultry, particularly chickens, exerts selective pressure, shaping IAV populations, and promoting the acquisition of adaptive amino acid substitutions in the hemagglutinin protein. This results in the loss of Neu5Gc binding and an increase in virus transmissibility in gallinaceous poultry, particularly chickens. Consequently, the transmission capability of these poultry-adapted H7 IAVs in wild water birds decreases. Timely intervention, such as stamping out, may help reduce virus adaptation to domestic chicken populations and lower the risk of enzootic outbreaks, including those caused by IAVs exhibiting high pathogenicity. 

This paper describes the development of the disease ecology tradition of health and medical geography including some key themes and innovations. It first grounds disease ecology in the history of ecology from the natural sciences and the human ecology traditions within the social sciences. These ecological studies of disease developed in response to limitations in the biomedical approach to studying health and disease that developed after germ theory. While the biomedical approach, which mostly focused on human biology, led to groundbreaking advances in medicine for many decades, it had its limits. Disease ecology applications have modern roots in the decades before and after World War II through colonial and tropical medicine as well as work conducted in an array of other sites, including Nazi Germany, the Soviet Union, and the United States when there were large efforts to create infectious diseases maps and conduct ecological analyses of diseases. Hundreds of disease ecology studies have been implemented on diverse disease systems since World War II. The field progressively broadened in scope, especially during the 1990s and beyond, with several innovations including the application of political ecology approaches to the study of health and disease. Two other recent innovations are summarized through case studies: disease ecology approaches in health intervention research and applications of theory and methods from landscape genetics. The first case study highlights the ecological and geographic heterogeneity associated with the health impacts of drinking‐water tubewell interventions in rural Bangladesh. The paper also considers ‘landscape genetics’ approaches via a case study about influenza that uses modern genetic and spatial tools along with an ecological approach; it describes how the evolution of the virus is related to human‐environment‐animal interactions. The paper concludes by outlining promising future directions for disease ecology, emphasizing the field's ongoing incorporation of new theories and methods. 

While mammals can be infected by influenza A virus either sporadically or with well adapted lineages, aquatic birds are the natural reservoir of the pathogen. So far most of the knowledge on influenza virus dynamics was however gained on mammalian models. In this study, we infected turkeys using a low pathogenic avian influenza virus and determined the infection dynamics with a target-cell limited model. Results showed that turkeys had a different set of infection characteristics, compared with humans and ponies. The viral clearance rates were similar between turkeys and ponies but higher than that in humans. The cell death rates and cell to cell transmission rates were similar between turkeys and humans but higher than those in ponies. Overall, this study indicated the variations of within-host dynamics of influenza infection in avian, humans, and other mammalian systems.