Salmon lice (Lepeophtheirus salmonis) development times, body size and reproductive outputs follow universal models of temperature dependence

-Temperatures regulate metabolism of marine ectotherms and thereby influence development, reproduction, and, as a consequence, dispersal. Despite the importance of water temperatures in the epidemiology of marine diseases, for the parasitic copepod Lepeophtheirus salmonis, the effect of high and low temperatures has not been methodically investigated. Here, we examined the effects of a wide temperature range (3–20 °C) on L. salmonis larval development, adult body size, reproductive outputs, and infestation success. Further, we tested if dispersal of salmon lice differed with two temperature-dependent development times to the infective stage (30 and 60 degree-days) using an individual-based dispersal model. Development times followed universal models of temperature dependence described for other marine ectotherms. Water temperatures had a negative relationship with development times, adult body size, and reproductive outputs, except at 3 °C, where larvae failed to reach the infective stage and all parameters were decreased, indicating low temperatures are more detrimental than high temperatures. The predictable effect of temperatures on lice development and reproduction will have important applications, such as predicting dispersal and population connectivity, to assist in controlling lice epidemics

A metapopulation model reveals connectivity-driven hotspots in treatment resistance evolution in a marine parasite

In salmon aquaculture, the sustainable management of salmon lice (Lepeophtheirus salmonis) is limited by the adaptive capacity of the parasite. This is evident in the repeated evolution of pesticide resistance in the salmon louse population. To better prepare for resistance, we constructed a numerical metapopulation model that predicts the evolutionary dynamics of lice across an interconnected farm network. This model integrates within-farm population dynamics and between-farm louse dispersal, the latter using outputs from a state-of-the-art particle-tracking model. Distinct from previous metapopulation models, it also simulates spatial and temporal genetic variation arising from selection. The model was parameterized to simulate the evolution of resistance to the pesticide azamethiphos on farms in southern Norway. It successfully reproduced the rapid (within 10 years) evolution of azamethiphos resistance following extensive delousing treatments. It also identified strong spatial patterns in resistance, with regions of high farm connectivity being potential hotspots of louse adaptation. Rates of infestation and evolution were significantly reduced when highly connected farms were excluded from the simulation, compared to when low-connectivity or random sites were excluded. This model can be a valuable tool for coordinating pest management at a regional scale, in a way that slows or prevents the spread of resistance.A metapopulation model reveals connectivity-driven hotspots in treatment resistance evolution in a marine parasitepublishedVersio

Development of a new real-time PCR for the detection of pilchard orthomyxovirus (POMV) in apparently healthy fish

Pilchard orthomyxovirus (POMV) is a virus of concern to the Atlantic salmon aquaculture industry in Tasmania. First isolated from wild pilchards in southern Australia in 1998, the virus is now a recognised pathogen of farmed Atlantic salmon (Salmo salar) in Tasmania. While the current real-time PCR for POMV targets segment 5 of the viral genome, recent viral gene expression data suggests that other segments of the POMV genome presented higher transcription levels and thus may be better candidates for the early detection of the virus. This study aimed to design and begin validating a more sensitive reverse transcriptase real-time PCR (RT-qPCR) assay to detect POMV. Primers and probes were developed targeting two independent viral genes derived from segments 7 and 8, which presented higher transcription levels than segment 5 in both cell culture and infected fish. These were compared with the current POMV RT-qPCR. The POMV segment 8 assay had a higher analytical sensitivity than segment 7, detecting at least 1 plasmid copy μl−1, and was 10-fold more sensitive than both POMV segment 7 and 5 assays when analysing nucleic acid from a positive field sample. Both new assays also had high analytical specificity, detecting the 11 POMV isolates tested (inclusivity testing) and not amplifying nucleic acids from other viruses, including ISAV, a related orthomyxovirus. In the latent class model analysis, the diagnostic sensitivity of the segment 8 and 7 assays were higher than segment 5 in 93% and 92% of simulations, respectively. Seven samples (18.4%), all from subclinical fish infected with POMV, returned a positive result only with the segment 8 assay. Both new assays showed reproducible results when applied to aliquots of the same samples tested in three different laboratories. The new POMV segment 8 assay shows promising results as a surveillance tool for detecting POMV in fish without any symptoms.publishedVersio

Applying genetic technologies to combat infectious diseases in aquaculture

Disease and parasitism cause major welfare, environmental and economic concerns for global aquaculture. In this review, we examine the status and potential of technologies that exploit genetic variation in host resistance to tackle this problem. We argue that there is an urgent need to improve understanding of the genetic mechanisms involved, leading to the development of tools that can be applied to boost host resistance and reduce the disease burden. We draw on two pressing global disease problems as case studies—sea lice infestations in salmonids and white spot syndrome in shrimp. We review how the latest genetic technologies can be capitalised upon to determine the mechanisms underlying inter- and intra-species variation in pathogen/parasite resistance, and how the derived knowledge could be applied to boost disease resistance using selective breeding, gene editing and/or with targeted feed treatments and vaccines. Gene editing brings novel opportunities, but also implementation and dissemination challenges, and necessitates new protocols to integrate the technology into aquaculture breeding programmes. There is also an ongoing need to minimise risks of disease agents evolving to overcome genetic improvements to host resistance, and insights from epidemiological and evolutionary models of pathogen infestation in wild and cultured host populations are explored. Ethical issues around the different approaches for achieving genetic resistance are discussed. Application of genetic technologies and approaches has potential to improve fundamental knowledge of mechanisms affecting genetic resistance and provide effective pathways for implementation that could lead to more resistant aquaculture stocks, transforming global aquaculture

Applying genetic technologies to combat infectious diseases in aquaculture

Disease and parasitism cause major welfare, environmental and economic concerns for global aquaculture. In this review, we examine the status and potential of technologies that exploit genetic variation in host resistance to tackle this problem. We argue that there is an urgent need to improve understanding of the genetic mechanisms involved, leading to the development of tools that can be applied to boost host resistance and reduce the disease burden. We draw on two pressing global disease problems as case studies—sea lice infestations in salmonids and white spot syndrome in shrimp. We review how the latest genetic technologies can be capitalised upon to determine the mechanisms underlying inter- and intra-species variation in pathogen/ parasite resistance, and how the derived knowledge could be applied to boost disease resistance using selective breeding, gene editing and/or with targeted feed treatments and vaccines. Gene editing brings novel opportunities, but also implementation and dissemination challenges, and necessitates new protocols to integrate the technology into aquaculture breeding programmes. There is also an ongoing need to minimise risks of disease agents evolving to overcome genetic improvements to host resistance, and insights from epidemiological and evolutionary models of pathogen infestation in wild and cultured host populations are explored. Ethical issues around the different approaches for achieving genetic resistance are discussed. Application of genetic technologies and approaches has potential to improve fundamental knowledge of mechanisms affecting genetic resistance and provide effective pathways for implementation that could lead to more resistant aquaculture stocks, transforming global aquaculture.publishedVersio

Environmental transmission and connectivity of the ectoparasite Lepeophtheirus salmonis in marine ecosystems with intensive salmon aquaculture

© 2018 Dr Francisca SamsingLarge-scale epidemics are affecting ecologically and economically important marine species across the globe. For the most widely produced marine fish, salmon, parasitic sea lice are the most significant problem. Here I investigated the environmental transmission of farm-originated sea lice Lepeophtheirus salmonis in an area of intensive salmon aquaculture, and used empirical and modelling approaches to dissect the processes involved in parasite dispersal and connectivity. First, I empirically examined the effects of a wide range of biologically relevant water temperatures (3 – 20°C) on L. salmonis larval development and demonstrated that temperatures had a negative relationship with development times. My findings showed that larvae developed at a faster rate than was previously implemented in dispersal models, and changing this parameter had a significant impact on simulated dispersal patterns. In my next chapter, I used empirical data to validate the horizontal and vertical distribution of lice predicted by the dispersal model and tested different model parameterizations (changing larval development time and vertical behaviour) to see which best represented empirical data. The best model fit occurred with a development time to the infective stage that was faster than previously implemented in dispersal models, and the presence of temperature-driven vertical behaviour of lice early planktonic stages. This model also predicted that hindering fish from swimming close to the surface could reduce lice infection pressure by around 60%. In subsequent chapters, I used the validated dispersal model to construct networks of lice dispersal. In the first simulation, lice were released from all salmon farms in the southwest coast of Norway (~ 560 farms) for two seasons (spring and winter) from 2009 to 2014 to quantify spatial and temporal variations in connectivity and test the stability of dispersal networks. Lice dispersal patterns and network metrics varied greatly between seasons, but differences were consistent amongst years. Winter networks presented more connections between farms, and links were on average two times more distant than in spring (winter median connection ~ 37 km; spring median connection ~ 18 km). In the second simulation, lice particles were released from all farms in Norway (~ 1000 farms) for a whole simulation year (2016). Outputs from this simulation we used to test: 1) the best method to reduce overall connectivity; and 2) where firebreaks would be best located to disconnect the entire network. Sequential removal of key-positioned farms was more effective in fragmenting the network than removing farms with a high number of connections. I also demonstrated that the establishment of two firebreaks (at ~ 61° N and 67° N) could be achieved with the removal of 13 and 21 farms (~ 1.3 and 2.1% of all farms), fragmenting the entire network into three largely unconnected groups of farms. My work showed that dispersal models coupled to network analysis could aid future spatial planning of the salmon industry and identify strategies that keep lice infection pressure at a minimum. More broadly, the framework I used in my thesis is widely applicable to other diseases-causing agents within open-cage aquaculture in marine systems, and should be a key component in the study of marine epidemiology

High host densities dilute sea lice Lepeophtheirus salmonis loads on individual Atlantic salmon, but do not reduce lice infection success

Host density likely plays a key role in host−parasite interactions, but empirical evidence in marine ecosystems remains limited. Classical models predict a positive relationship between host density and parasite infection parameters, but this depends on the parasite transmission mode. Evidence from systems where mobile parasites actively seek hosts suggests that numbers of parasites per host decrease with increasing host density (‘dilution effect’). Copepodids, the infective stage of the salmon louse Lepeophtheirus salmonis, are mobile larvae that display a range of behaviours to detect their salmonid hosts. We hypothesized that high host density would decrease infection intensity, prevalence and degree of aggregation, but not infection success, which reflects parasite performance. We infected multiple groups of Atlantic salmon Salmo salar post-smolts at low (12 fish; 7.9 kg m−3) and high (96 fish; 68.5 kg m−3) densities, with the same number of L. salmonis copepodids in swimming chambers to enable more realistic swimming behaviours during infection. Infection intensity was 8.4 times higher in the low density treatment, but there were no differences in infection success and degree of aggregation. We observed 100% prevalence in the low density treatment, which was significantly higher than the high density treatment (68%). The dilution effect most likely explained the negative relationship between host density and infection intensity, as the individual risk of being ‘attacked’ by a parasite decreased as host density increased. Host density is crucial in salmon−sea lice infection dynamics, and opportunities may exist within production environments to use the dilution effect of density to improve fish welfare outcomes

High host densities dilute sea lice Lepeophtheirus salmonis loads on individual Atlantic salmon, but do not reduce lice infection success

publishedVersio