DISEASE RISK ASSESSMENT OF INVASIVE ALIEN SPECIES

Increased global trade and travel have led to a rise in the number of invasive alien species (IAS), i.e. species introduced by humans in geographic areas where are not naturally found, worldwide. Despite the recognized role of wildlife, as well as of wildlife translocations, in the emergence and re-emergence of infections of public health significance, IAS remain mainly studied for their environmental impacts, and their disease risk towards humans and animals is still largely neglected by health professionals. The main aim of this thesis is therefore to cover this gap by setting the ground for a new “invasion epidemiology” field, and this has been done through two main steps: the review and analysis of both the mechanisms underlying IAS disease risk and the information available in literature on IAS pathogens, and the development of a standardized qualitative disease risk assessment method, applicable to different geographic contexts, to assess the risk of mammal IAS to impact on human and animal health. First, I reviewed the existing biological and ecological literature on IAS to identify the main mechanisms by which animal IAS may affect disease risk in their area of release. IAS resulted to potentially affect disease risk both directly, by acting as hosts of infectious agents, thus possibly leading to the introduction of new pathogens, and/or the amplification of endemic ones, or indirectly, by altering the ecosystem equilibrium, through competitive and trophic interactions with native host species or the modification of local habitats. This literature review highlighted how IAS may have important health implications, which should be better acknowledged by people working in the human and animal health field, and how the mechanisms underlying the sanitary outcome of a biological invasion, and in particular indirect ones, are extremely complex, being the product of multiple factors. Acknowledging the important limitations of our current ability to predict possible health impacts driven by indirect mechanisms, I decided to address the issue of IAS disease risk by focusing specifically on IAS possible role as infectious agents’ host. As information on IAS pathogens is not systematized, preventing from knowing the amount and quality of available data to inform possible disease risk assessment procedures, I systematically reviewed the literature on the infectious agents of the main mammal species of European Union concern. Current knowledge on the pathogens harbored by mammal IAS was evaluated through different statistical approaches: the identification of the main factors associated with research intensity and the observed pathogen species richness, the estimation of the true helminth species richness, and a meta-analysis of prevalence of the pathogens of public and animal health significance. Results highlighted the existence of strong information gaps and biases in the way research on mammal IAS pathogens is carried out, the current underestimation of the amount of pathogens harbored by these species and high levels of uncertainty in the pooled prevalence of pathogens of public and animal health significance. However, the review confirmed that mammal IAS harbor pathogens of human and animal health significance, and therefore, the need to identify high-risk species. Considering that the existing knowledge gaps would have resulted in strong limitations in informing a risk assessment procedure, I developed a qualitative disease risk assessment methodology informed by expert opinion. This tool is specifically aimed at assessing IAS disease risk towards humans, domestic animal populations, and/or wildlife populations and allows to obtain a list of the pathogens of animal and human health significance that mammal IAS could transmit to a population of interest (directly or through the communities of local hosts), each with the related level of risk and uncertainty. Key features of the tool are its flexibility, being applicable to different contexts and for different purposes, and the high resolution of the mechanisms under assessment, which make possible for risk managers identifying the most critical pathogens and mechanisms involved in disease risk, allowing them to direct targeted actions and surveillance plans. Finally, the need to combine multiple likelihood estimates deriving from several pathways in an overall risk estimate led me to tackle a methodological aspect of qualitative risk assessment procedures, and I proposed a standardized method applicable in such cases, to reduce the subjectivity that relies in the different ways multiple estimates are currently combined.