Exploring Yersinia ruckeri (O1 Biotype 2) infection in three early life-stages of rainbow trout.

Enteric redmouth disease (ERM) caused by the enterobacterium Yersinia ruckeri poses a significant threat to salmonid aquaculture globally. Despite decades of experimental infection studies, key knowledge gaps remain regarding the onset of disease susceptibility and mechanisms of immunity during early developmental stages, undermining disease management efforts in all susceptible life-stages. In this study, a series of immersion challenges were conducted, challenging and re-challenging rainbow trout Oncorhynchus mykiss (Walbaum) at 7, 14 and 51 d post-hatch (dph; mean weights = 0.085, 0.1 and 2.0 g respectively) to high concentrations (1.72 × 107-1.1 × 108 CFU) of Y. ruckeri at 15°C. This study indicates the hitherto unknown initial point of susceptibility to infection as the time of first ingestion of exogenous food (14 dph), and shows that individuals surviving primary challenge at 14 dph are significantly more likely to survive re-challenge at 51 dph compared with naive individuals (hazard ratio = 1.446, p = 0.032). Other key findings include large variation in mortality between different development-stages, from 21.1% at 14 dph to 81.2% at 51 dph, and novel age-dependent symptoms not reported previously. Results from this study enhance our understanding of ERM in juvenile rainbow trout and inform the development of improved aquatic animal health management strategies, thereby contributing to the productivity and sustainability of salmonid aquaculture into the future.

The aquaculture disease network model (AquaNet-Mod): A simulation model to evaluate disease spread and controls for the salmonid industry in England and Wales.

Infectious disease causes significant mortality in wild and farmed systems, threatening biodiversity, conservation and animal welfare, as well as food security. To mitigate impacts and inform policy, tools such as mathematical models and computer simulations are valuable for predicting the potential spread and impact of disease. This paper describes the development of the Aquaculture Disease Network Model, AquaNet-Mod, and demonstrates its application to evaluating disease epidemics and the efficacy of control, using a Viral Haemorrhagic Septicaemia (VHS) case study. AquaNet-Mod is a data-driven, stochastic, state-transition model. Disease spread can occur via four different mechanisms, i) live fish movement, ii) river based, iii) short distance mechanical and iv) distance independent mechanical. Sites transit between three disease states: susceptible, clinically infected and subclinically infected. Disease spread can be interrupted by the application of disease mitigation measures and controls such as contact tracing, culling, fallowing and surveillance. Results from a VHS case study highlight the potential for VHS to spread to 96% of sites over a 10 year time horizon if no disease controls are applied. Epidemiological impact is significantly reduced when live fish movement restrictions are placed on the most connected sites and further still, when disease controls, representative of current disease control policy in England and Wales, are applied. The importance of specific disease control measures, particularly contact tracing and disease detection rate, are also highlighted. The merit of this model for evaluation of disease spread and the efficacy of controls, in the context of policy, along with potential for further application and development of the model, for example to include economic parameters, is discussed.

Invasiveness risks of naked goby, Gobiosoma bosc, to North Sea transitional waters.

In recent decades, gobies have dispersed or introduced from the Ponto-Caspian region of eastern Europe in a westerly direction to North American and western European waters. By contrast, the naked goby, Gobiosoma bosc, is the only known gobiid species to have been introduced in an easterly direction from North American to western Europe. The potential invasiveness of G. bosc was assessed using the Aquatic Species Invasiveness Screening Kit (AS-ISK) for rivers and transitional waters for the western and eastern sides of the North Sea. Using globally-derived thresholds, G. bosc was assessed as low-medium invasiveness risk for both sides of the North Sea under current climate conditions. Under future climate conditions, potential invasiveness will increase for both risk assessment areas. Environmental suitability assessment indicated an increase in environmental suitability for G. bosc on the eastern coastline of the North Sea under climate change scenarios and suitability remained unchanged on the western coastline, reflecting the authors' expectations of invasiveness risk.

Understanding drivers of wild oyster population persistence.

Persistence of wild Pacific oyster, Magallana gigas, also known as Crassostrea gigas, has been increasingly reported across Northern European waters in recent years. While reproduction is inhibited by cold waters, recent warm summer temperature has increased the frequency of spawning events. Although correlation between the increasing abundance of Pacific oyster reefs in Northern European waters and climate change is documented, persistence of wild populations may also be influenced by external recruitment from farmed populations and other wild oyster populations, as well as on competition for resources with aquaculture sites. Our understanding of the combined impact of the spawning frequency, external recruitment, and competition on wild population persistence is limited. This study applied an age-structured model, based on ordinary differential equations, to describe an oyster population under discrete temperature-related dynamics. The impact of more frequent spawning events, external recruitment, and changes in carrying capacity on Pacific oyster density were simulated and compared under theoretical scenarios and two case studies in Southern England. Results indicate that long term persistence of wild oyster populations towards carrying capacity requires a high frequency of spawning events but that in the absence of spawning, external recruitment from farmed populations and other wild oyster populations may act to prevent extinction and increase population density. However, external recruitment sources may be in competition with the wild population so that external recruitment is associated with a reduction in wild population density. The implications of model results are discussed in the context of wild oyster population management.

Experimental evidence that local interactions select against selfish behaviour.

How social behaviours evolve remains one of the most debated questions in evolutionary biology. An important theoretical prediction is that when organisms interact locally due to limited dispersal or strong social ties, the population structure that emerges may favour cooperation over antagonism. We carry out an experimental test of this theory by directly manipulating population spatial structure in an insect laboratory model system and measuring the impact on the evolution of the extreme selfish behaviour of cannibalism. We show that, as predicted by the theory, Indian meal moth larvae that evolved in environments with more limited dispersal are selected for lower rates of cannibalism. This is important because it demonstrates that local interactions select against selfish behaviour. Therefore, the ubiquitous variation in population structure that we see in nature is a simple mechanism that can help to explain the variation in selfish and cooperative behaviours that we see in nature.

The effectiveness of fishery net dips: Advice for the improvement of biosecurity measures.

Disease poses a major threat to aquaculture and commercial and recreational fisheries globally. Biosecurity measures have been implemented; however, empirical evidence of their efficacy in situ is lacking. Here, we present the results from a study conducted to examine the effectiveness of disinfectant net dips. Samples were collected from disinfectant net dips at 25 recreational fisheries in south-west England and assessed to determine (a) the level of bacterial contamination and (b) the reduction in titre of a target virus (infectious pancreatic necrosis virus, IPNV) following a contact time of 2 and 5 min. In addition, the study examined the reduction in target virus titre following exposure to laboratory prepared Virkon® , representing "clean," "dirty" and "diluted and dirty" conditions, for 2 and 5 min. Bacterial contamination was high in 64% of disinfectant samples, and, 76% of disinfectant samples did not effectively reduce the target virus titre in 2 or 5 min. Virus titre was successfully reduced following exposure to laboratory prepared Virkon® for 2 or 5 min, although dilution and contamination reduced the effectiveness. These results suggest that disinfectant net dips may not be working effectively on a high proportion of fishery sites. We provide recommendations for improving biosecurity.

The skin immune response of rainbow trout, Oncorhynchus mykiss (Walbaum), associated with puffy skin disease (PSD).

Puffy skin disease (PSD) is an emerging skin condition which affects rainbow trout, Oncorhynchus mykiss (Walbaum). The transmission pattern of PSD suggests an infectious aetiology, however, the actual causative infectious agent(s) remain(s) unknown. In the present study, the rainbow trout epidermal immune response to PSD was characterised. Skin samples from infected fish were analysed and classified as mild, moderate or severe PSD by gross pathology and histological assessment. The level of expression of 26 immune-associated genes including cytokines, immunoglobulins and cell markers were examined by TaqMan qPCR assays. A significant up-regulation of the gene expression of C3, lysozyme, IL-1ß and T-bet and down-regulation of TGFß and TLR3 was observed in PSD fish compared to control fish. MHCI gene expression was up-regulated only in severe PSD lesions. Histological examinations of the epidermis showed a significant increase in the number of eosinophil cells and dendritic melanocytes in PSD fish. In severe lesions, mild diffuse lymphocyte infiltration was observed. IgT and CD8 positive cells were detected locally in the skin of PSD fish by in situ hybridisation (ISH), however, the gene expression of those genes was not different from control fish. Total IgM in serum of diseased animals was not different from control fish, measured by a sandwich ELISA, nor was significant up regulation of IgM gene expression in PSD lesions observed. Taken together, these results show activation of the complement pathway, up-regulation of a Th17 type response and eosinophilia during PSD. This is typical of a response to extracellular pathogens (i.e. bacteria and parasites) and allergens, commonly associated with acute dermatitis.

The evolutionary dynamics of within-generation immune priming in invertebrate hosts.

While invertebrates lack the machinery necessary for 'acquired immunity', there is increasing empirical evidence that exposure to low levels of disease may 'prime' an invertebrate's immune response, increasing its defence to subsequent exposure. Despite this increasing empirical data, there has been little theoretical attention paid to immune priming. Here, we investigate the evolution of immune priming, focusing on the role of the unique feedbacks generated by a newly developed susceptible-primed-infected epidemiological model. Contrasting our results with previous models on the evolution of acquired immunity, we highlight that there are important implications to the evolution of immunity through priming owing to these different epidemiological feedbacks. In particular, we find that in contrast to acquired immunity, priming is strongly selected for at high as well as intermediate pathogen virulence. We also find that priming may be greatest at either intermediate or high host lifespans depending on the severity of disease. Furthermore, hosts faced with more severe pathogens are more likely to evolve diversity in priming. Finally, we show when the evolution of priming leads to the exclusion of the pathogens or hosts experiencing population cycles. Overall the model acts as a baseline for understanding the evolution of priming in host-pathogen systems.

The epidemiological consequences of immune priming.

Exposure to low doses of pathogens that do not result in the host becoming infectious may 'prime' the immune response and increase protection to subsequent challenge. There is increasing evidence that such immune priming is a widespread and important feature of invertebrate host-pathogen interactions. Immune priming clearly has implications for individual hosts but will also have population-level implications. We present a susceptible-primed-infectious model-in contrast to the classic susceptible-infectious-recovered framework-to investigate the impacts of immune priming on pathogen persistence and population stability. We describe impacts of immune priming on the epidemiology of the disease in both constant and seasonal environments. A key result is that immune priming may act to destabilize population dynamics. In particular, when the proportion of individuals becoming primed rather than infected is high, but this priming does not confer full immunity, the population may be strongly destabilized through the generation of limit cycles. We discuss the implications of our model both in the context of invertebrate immunity and more widely.

Within and transgenerational immune priming in an insect to a DNA virus.

Invertebrates mount a sophisticated immune response with the potential to exhibit a form of immune memory through 'priming'. Increased immune protection following early exposure to bacteria has been found both later in life (within generation priming) and in the next generation (transgeneration priming) in a number of invertebrates. However, it is unclear how general immune priming is and whether immune priming occurs in response to different parasites, including viruses. Here, using Plodia interpuctella (Lepidoptera) and its natural DNA virus, Plodia interpunctella granulosis virus, we find evidence for both within generation and transgeneration immune priming. Individuals previously exposed to low doses of virus, as well as the offspring of exposed individuals, are subsequently less susceptible to viral challenge. Relatively little is known about the mechanisms that underpin viral immunity but it is probable that the viral immune response is somewhat different to that of bacteria. We show that immune priming may, however, be a characteristic of both responses, mediated through different mechanisms, suggesting that immune memory may be a general phenomenon of insect immunity. This is important because immune priming may influence both host-parasite population and evolutionary dynamics.