Luke Woodford

The World Health Organization has classified five species of Candida as pathogens of critical concern on its recent ’Fungal Pathogen Priority List’. Candida albicans is the most common species aetiologically associated with human and animal mucosa; however, we know almost nothing about the environmental survival and the transfer pathways of C. albicans in natural environments. Here, we have isolated C. albicans from the faeces of newly arrived migratory geese in two agricultural fields in Scotland (the United Kingdom). All 14 confirmed isolates were characterised in terms of their pathogenicity (in a Galleria larval infection model) and resistance to four classes of antifungal drugs. Multilocus sequence typing (MLST) was used to assess the relatedness of these isolates with C. albicans isolated from the faeces of sheep grazing in the same field. Finally, mesocosms were used to test whether C. albicans in geese faeces could persist and remain viable when incorporated into agricultural soils. All C. albicans isolates were virulent, and exhibited resistance to at least one of the four classes of antifungal drugs against which they were screened. Maximum likelihood phylogenetic analysis of MLST sequences revealed that C. albicans isolated from goose and sheep faeces were relatively conserved, although they did cluster independently on separate clades. C. albicans remained viable in common agricultural soils, and after 60 days, the concentrations of all three representative C. albicans isolates had only declined by one log. The unrestricted movement of migratory birds provides a significant opportunity for the widespread dissemination of pathogens. Although C. albicans is not generally considered to be zoonotic, its introduction into new environments and subsequent persistence in agricultural systems has the potential to facilitate transmission between animal species or humans. Therefore, it is critical that more emphasis be put on monitoring animal migration and the potential for cross‐boundary movement of pathogens, particularly with climate change widening the geographic range for pathogen persistence.

Agricultural environments are becoming increasingly contaminated with plastic pollution. Plastics in the environment can also provide a unique habitat for microbial biofilm, termed the ‘plastisphere’, which can also support the persistence of human pathogens such as Salmonella. Human enteric Salmonella enterica serovar Typhimurium can enter agricultural environments via flooding or from irrigation with contaminated water. Using soil mesocosms we quantified the ability of S. Typhimurium to persist on microplastic beads in two agriculturally relevant soils, under ambient and repeat flood scenarios. S. Typhimurium persisted in the plastisphere for 35 days in both podzol and loamy soils; while during multiple flood events was able to survive in the plastisphere for up to 21 days. S. Typhimurium could dissociate from the plastisphere during flooding events and migrate through soil in leachate, and importantly could colonise new plastic particles in the soil, suggesting that plastic pollution in agricultural soils can aid S. Typhimurium persistence and facilitate further dissemination within the environment. The potential for increased survival of enteric human pathogens in agricultural and food production environments due to plastic contamination poses a significant public health risk, particularly in potato or root vegetable systems where there is the potential for direct contact with crops.

Varroa destructor is an ectoparasitic mite of honeybees which vectors a range of pathogenic viruses, the most notable being Deformed wing virus (DWV). Mites parasitise bees during pupal development and male honeybees, drones, have a longer development cycle than female workers (24 versus 21 days), allow for more progeny mites to develop per foundress (1.6–2.5 compared to 0.7–1.45). How this longer exposure time influences evolution of the transmitted virus population is unknown. Using uniquely tagged viruses recovered from cDNA we investigated the replication, competition and morbidity of DWV genotypes in drones. Assays examining virus replication and morbidity revealed drones are highly susceptible to both predominant genotypes of DWV. In virus passage studies using an equimolar inocula of major DNA genotypes and their recombinants, the recombinant form dominated but did not reach 100% of the virus population within 10 passages. Using an in-silico model of the virus–mite–bee system we examined bottlenecks during virus acquisition by the mite and subsequent injection of viruses into the host, which may play a significant role in shaping virus diversity. This study furthers our understanding of the variables influencing DWV diversity changes and provides insight into areas of future research in the mite–virus–bee system.

1. Deformed wing virus (DWV) is a pathogenic virus of honey bees transmitted by the ectoparasitic mite Varroa destructor. Annual overwintering colony losses, accounting for similar to 25% of all colonies, are associated with high levels of Varroa-DWV infestation. Effective miticide treatments are available to control Varroa. However, the absence of coordinated treatment means environmental transmission of mites continues unchecked. We aimed to determine whether rational, coordinated treatment is beneficial, and characterized the DWV population as an indicator of colony health. 2. This study uses coordinated treatment of Varroa in a geographically isolated environment (Isle of Arran, Scotland) over 3 years. The study area contained 50-84 colonies managed by similar to 20 amateur beekeepers. Sampling and virus analysis to assess strain diversity and viral loads were conducted before and after treatments, and changes in population diversity were quantified by sequence analysis. 3. Over the 3 years analysis of the virus population revealed that the dominant DWV variant shifted from Type A to Type B in all apiaries, regardless of mite levels or proximity to other colonies. During this period the number of managed colonies increased by 47% (57-84 colonies), but despite this, we estimate total mite numbers decreased by 58%. 4. Synthesis and applications. In this study, the beekeepers in Arran significantly improved the number of colonies they managed, without importing any bees onto the island, indicating that an improved focus on management techniques, through the combination of a coordinated miticide programme and an improved understanding of bee diseases, could yield positive results for bee health and sustainability.

Varroa destructor is an ectoparasitic mite associated with significant losses of honeybee colonies globally. The mite vectors a range of pathogenic viruses, the most important of which is the Deformed wing virus (DWV). In the absence of Varroa, DWV exists as a low-level, highly diverse virus population. However, when transmitted by Varroa, certain variants become highly elevated, and may become near-clonal and cause symptomatic infections. Mite transmission between colonies can occur when parasitised workers drift from or rob adjacent hives. These activities can result in elevated mite levels, but the resulting change in the DWV population, the primary determinant of winter colony losses, has not been determined. In reciprocal studies, we investigated the influence of the removal of mites, or their acquisition, on the DWV population. When mites were removed from heavily infested colonies, there was a striking and rapid reduction in virus load. Conversely, siting Varroa-naïve colonies in a mite-infested apiary resulted in the acquisition of mites and concomitant changes in the virus population. We observed both near-clonal and highly divergent virus populations regardless of titre, suggesting changes were stochastic and colony-specific. Our findings have implications for the outcome of strategies in areas with total or patchy implementation of Varroa control plans.