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Epidemiologic Approaches to Investigating Pathogen Spillover and Spread Across Diverse Systems


Rapid ecological changes have led to an increase in pathogen spillover risk between different host species, including between animals and humans. Spillover occurs when pathogens are transmitted to a new species which had previously not encountered the disease. The process of spillover and subsequent spread of a pathogen in a new host population can include three to four phases, which are often circular: 1) maintenance of disease in a reservoir host, 2) initial spillover into a novel host due to high-risk interactions, and 3) rapid pathogen spread causing an epidemic in the new, immunologically naïve host population, which can potentially be followed by 4) maintenance of disease in the new host. Disease dynamics in these phases of emergence are explored in three unique systems with relevance to both animal and human health.

In chapter 1, animal-human interfaces with risk of pathogen spillover were characterized along wildlife “supply chains” in Africa and Asia, to guide prevention efforts that will preempt spillover events. Observational surveys of sites along the wildlife supply chain were conducted by the PREDICT Consortium to characterize the settings in which wild animals are sourced, traded, and sold. Questionnaires were also administered to hunters and supply chain workers to assess their exposure to zoonotic disease and any spillover prevention measures implemented. Findings from this study inform community education efforts regarding zoonotic pathogen transmission, wildlife trade policies, and biosecurity and PPE guidelines.

In chapter 2, the efficacy of behavior changes to mitigate the expansion of an emerging epidemic are evaluated in the months immediately following a spillover event. The progenitor virus of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was likely introduced into humans by a wildlife host, then adapted to spread human-to-human, causing a global pandemic in the span of a few months. The efficacy of social distancing was evaluated at the state level during the first two months of the pandemic in the United States by examining the relationship between daily SARS-COV-2 case incidence and human community mobility. Lag times between decreases in mobility and case counts were measured, and social distancing was found to be most effective when put into place early in an epidemic. These findings inform management of emerging infectious disease outbreaks by identifying areas where social distancing was the most effective in reducing disease transmission (e.g., indoor public spaces such as workplaces and transit station), and the expected time frame between behavioral changes and measurable changes in disease incidence.

In chapter 3, factors contributing to the maintenance of disease were investigated in the decades following spillover from reservoir host to novel host species. Endangered Peninsular bighorn sheep (Ovis canadensis nelsoni) have suffered population declines due to infectious diseases introduced from domestic sheep (the original reservoir host), which now circulate within bighorn sheep herds in the absence of continued spillover. Demographic and geographic risk factors for pathogen exposure in individual bighorn sheep were examined, and the impact that pathogen exposure has on adult survival and lamb recruitment was measured at the herd level. These results will inform targeted management and conservation of bighorn sheep as they face the compounding challenges of disease, habitat loss, and climate change.

The factors contributing to pathogen spillover and spread are highly complex, necessitating the study of these pathways in many diverse systems. The research presented here provides insights into the maintenance, spillover, and control of pathogens across select host species and ecological systems. The animal-human interfaces identified at live animal markets will help us identify targets for surveillance of pathogens with pandemic potential in the pre-emergence setting, and guide local education and mitigation measures to prevent spillover. Implementing rapid behavior changes such as social distancing can slow the spread of a newly introduced pathogen in the absence of other control measures, such as vaccination. The impact of introduced pathogens on bighorn sheep survival and reproduction may be compounded by increasing temperature and decreasing precipitation, which can be expected to worsen due to climate change. These interwoven threats necessitate the longitudinal monitoring of bighorn sheep survival and systematic, range-wide surveillance of disease prevalence and food/water resources to guide conservation strategies. These findings can be extrapolated to other systems, so we are better prepared to identify, prevent, and respond to future emerging pathogens.

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