All roads lead to infection: A mechanistic model to assess transmission routes of brucellosis in a wild ungulate population

Abstract Wildlife and domestic populations frequently share diseases with a potential for cross‐species transmission, posing significant threats to animal and human health, economy and biodiversity conservation. While quantifying disease transmission in natural populations is critical to understand the dynamics of infectious diseases, transmission processes remain difficult to assess as they are difficult to observe directly. Brucellosis, caused by Brucella melitensis and Brucella abortus, is one of the most important worldwide zoonotic diseases and affects both domestic and wild ruminants. Here, we investigated the transmission of B. melitensis in an Alpine ibex ( Capra ibex ) population, the first known wild‐living reservoir for this pathogen. We aimed to quantify disease transmission within the population by disentangling the contributions of four alternative transmission routes: abortion products, infectious parturition products, venereal transmission and vertical transmission. We modelled host‐disease dynamics developing a spatially explicit, sex‐ and age‐structured individual‐based model that reproduces host demography and pathogen transmission by contacts among susceptible and infectious individuals. By fitting our model on 13 years of host‐population and disease monitoring, we estimated the relative importance of the four transmission routes. We quantified the proportion of transmissions attributed to each route and estimated the effective reproduction number ( R eff ), distinguishing age‐ and sex‐specific contribution. We identified abortion products as the primary driver of Brucella transmission, accounting for 57%–85% of all transmissions. All other routes were of secondary importance, although the contribution of venereal transmission remained uncertain (from 4% to 25%). Females played a central role in disease maintenance, with an effective reproduction number well above one (2.1), while males' contribution was quantitatively negligible with a value below one (0.4). Our results provide key insights to implement targeted management operations. Since most transmissions occur through abortion products, targeting recently infected females could reduce the effective reproduction number below one, promoting pathogen extinction. Furthermore, because abortions are expected to peak in late spring, control measures aiming to limit transmission from ibex to other species, including livestock, should be prioritized during this high‐risk period. Read the free Plain Language Summary for this article on the Journal blog.