Chronic bee paralysis virus: The epidemiology, evolution and mitigation of an emerging threat to honey bees

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In the UK, a myriad of pollinating insects contribute significantly to the yield of many agricultural crops and the reproduction of wild plants. Managed Western honey bees (Apis mellifera) offer mobile pollination services to compliment wild pollinators, providing a large proportion of this ecosystem service. Both honey bees and wild pollinators have been in severe decline for the past 30 years in the face of multiple interacting pressures that include land-use intensification, agrochemical exposure and the impact of parasites - which are often shared between pollinator species. These pressures are frequently considered in isolation and rarely together.Emerging infectious diseases, either newly appearing or rapidly increasing in incidence/geographic range in a population, have a history of causing large impacts on honey bee populations. Here we propose to investigate a rapidly emerging disease in honey bees, known as chronic paralysis, which causes severe symptoms in individual honey bees and often leads to colony loss. UK bee farmers are reporting severe repeated disease outbreaks and the causative RNA virus (Chronic bee paralysis virus [CBPV]), is increasing in prevalence in several countries. The purpose of this project is to advance our understanding of the mechanisms behind the recent emergence of chronic paralysis in honey bees, and work with professional bee farmers to develop new mitigation strategies. We will consider re-emergence as the result of CBPV strain transmissibility, environmental stressors, management practices or co-infection and propose four interlinked objectives, each with clear testable hypotheses, to tackle the problem and deliver management solutions. First, we will monitor the dynamics of disease spread in individual colonies and apiaries to understand where/when the virus becomes associated with the colony. We will gather new case-control data on stressors associated with chronic paralysis to include the presence of pathogens in adult bees; pesticide exposure in adult bees; weather; local land use; and apiary management. We will use a combination of structural equation and microsimulation modelling to elucidate the pre-requisite stressors for disease to develop, with the aim of highlighting early disease detection protocols and developing apiary management strategies to mitigate disease impact. Second, we will assess whether the recent emergence of chronic paralysis is the result of shifts in the transmissibility or virulence of modern CBPV strains. We will describe temporal and spatial sequence variation at the genomic level, by sequencing representative historic and recent samples. We will recover infectious historic strains by developing a reverse genetic system for CBPV, and use these reagents to compare the risk posed by past, current and future strains - the latter by generating virus reassortants - where RNAs are exchanged between virus genotypes. Finally, we will assess the risk posed by CBPV to other non-Apis pollinators. Third, we will investigate co-stressors of chronic paralysis using controlled in vivo challenge tests to include known (lack of foraging due to poor weather and overcrowding) and newly characterised (the gut parasite N. ceranae) co-stressors. We will assess the impact of these factors on transmission and virulence for different genotypic strains.Fourth, we will work closely with the Bee Farmers' Association (BFA) - who represent professional bee farmers from across the UK - to translate research outcomes into improved professional practices when managing chronic paralysis. We have identified a series of potential knowledge exchange opportunities to explore as data arises from our experiments. This programme of work will provide a step-change in our understanding of chronic paralysis disease development, virus evolution and pollinator co-stressors, as well as generating the necessary biological data and modelling tools to develop informed management practices.