A Gregory

Small- and large-scale network structure of live fish movements in Scotland.

Networks are increasingly being used as an epidemiological tool for studying the potential for disease transmission through animal movements in farming industries. We analysed the network of live fish movements for commercial salmonids in Scotland in 2003. This network was found to have a mixture of features both aiding and hindering disease transmission, hindered by being fragmented, with comparatively low mean number of connections (2.83), and low correlation between inward and outward connections (0.12), with moderate variance in these numbers (coefficients of dispersion of 0.99 and 3.12 for in and out, respectively); but aided by low levels of clustering (0.060) and some non-random mixing (coefficient of assortativity of 0.16). Estimated inter-site basic reproduction number R(0) did not exceed 2.4 at high transmission rate. The network was strongly organised into communities, resulting in a high modularity index (0.82). Arc (directed connection) removal indicated that effective surveillance of a small number of connections may facilitate a large reduction in the potential for disease spread within the industry. Useful criteria for identification of these important arcs included degree- and betweenness-based measures that could in future prove useful for prioritising surveillance.

Distribution of infectious pancreatic necrosis virus (IPNV) in wild marine fish from Scottish waters with respect to clinically infected aquaculture sites producing Atlantic salmon, Salmo salar L.

This study represents the first large-scale investigation of IPNV in Scottish wild marine fish. Kidney samples were taken from 30 627 fish comprising 37 species and 45 isolations were made from nine different species, illustrating these as reservoirs of IPNV in Scottish waters. The estimated prevalence of IPNV in the Scottish marine environment was low at 0.15% (90% confidence intervals, (CI) of 0.11-0.19%). This was significantly greater in fish caught less than 5.0 km from IPN-positive fish farms in Shetland, at 0.58% (90% CI of 0.45-0.77%). This prevalence persisted and did not significantly decrease over the 16-month period of study. The estimated prevalence of IPNV for each positive species was less than 1% with the statistically non-significant exceptions of flounder, Platichthys flesus (L.), at 12.5% (90% CI of 0.64-47.06%) and saithe, Pollachius virens (L.), at 1.11% (90% CI of 0.49-2.19%). The 45 isolates were titrated and all but two were below the detection limit of the test (<55 PFU g(-1)). Titres of 3.8 x 10(2) PFU g(-1) and 2.8 x 10(1) PFU g(-1) were calculated from common dab, Limanda limanda (L.), and saithe, respectively. This study provides evidence that clinical outbreaks of IPN in farmed Atlantic salmon may cause a localized small increase in the prevalence of IPNV in wild marine fish.

Application of network analysis to farmed salmonid movement data from Scotland

The movement of live animals plays an important role in the spread of diseases. In the farming industry, this role is evident in both terrestrial systems, for example, the 2001 UK foot and mouth (FMD) outbreak (Kao 2002; Mansley, Dunlop, Whiteside & Smith 2003), and aquatic systems, for example, the 1998 UK infectious salmon anaemia (ISA) outbreak (Murray, Smith & Stagg 2002). Diseases of aquatic animals can also be spread by a variety of routes other than direct movement of live fish, for example, with the movement of water (Gustafson, Ellis, Beattie, Chang, Dickey, Robinson, Marenghi, Moffett & Page 2007) or by the movement of wellboats (Murray et al. 2002). However, the focus of this study is spread of disease by live fish movements, which is likely to be the most effective means of spreading disease within aquaculture, especially over longer distances (Murray & Peeler 2005). In the UK aquaculture industry, there are substantial movements of live fish from hatcheries to on-growing sites and ultimately onwards to processing plants, broodstock sites or fisheries. This study presents research on live salmonid movements within Scotland, where Marine Scotland, Marine Laboratory, Aberdeen is the official authority, regulation of the industry in the UK being devolved. To date, fish movement data have been used to model pathogen spread with the aim of informing contingency plans (Sharkey, Fernandez, Morgan, Peeler, Thrush, Turnbull & Bowers 2006; Thrush & Peeler 2006). The study described here represents the first attempt to apply social network analysis to the structure of live fish movements within Scotland. Social network analysis is well established in the social and physical sciences (Liljeros, Edling, Amaral, Stanley & Aberg 2001; Albert & Barabási 2002) and has been applied to animal health issues following the 2001 foot and mouth outbreak (Christley, Robinson, Lysons & French 2005; Kiss, Green & Kao 2006). The aim of this type of analysis is to identify sites with high vulnerability to infection and therefore enhance the management and risk-based surveillance (Stärk, Regula, Hernandez, Knopf, Fuchs, Morris & Davies 2006) of aquatic diseases. The contact models developed also offer an opportunity to calculate network-based measures for incorporation in epidemiological models. In this study, a database of movement records from Scottish registered fish farm sites from 2003 was created and analysis conducted on a component of both Atlantic salmon, Salmo salar L. and rainbow trout, Oncorhynchus mykiss (Walbaum), industries in Scotland. At Marine Scotland, the Fish Health Inspectorate (FHI) holds paper records of all live fish movements that take place on and off all registered sites in Scotland. Each record contains date of movement, species, number or weight, development stage (e.g. fry), source and destination, supplier, method of transportation and name of carrier. A database was created in Microsoft Access from the paper records of movements occurring between 1 January 2003 and 31 December 2003. A Journal of Fish Diseases 2009, 32, 641–644 doi:10.1111/j.1365-2761.2009.01076.x

Plasma Membrane-Located Purine Nucleotide Transport Proteins Are Key Components for Host Exploitation by Microsporidian Intracellular Parasites

Microsporidia are obligate intracellular parasites of most animal groups including humans, but despite their significant economic and medical importance there are major gaps in our understanding of how they exploit infected host cells. We have investigated the evolution, cellular locations and substrate specificities of a family of nucleotide transport (NTT) proteins from Trachipleistophora hominis, a microsporidian isolated from an HIV/AIDS patient. Transport proteins are critical to microsporidian success because they compensate for the dramatic loss of metabolic pathways that is a hallmark of the group. Our data demonstrate that the use of plasma membrane-located nucleotide transport proteins (NTT) is a key strategy adopted by microsporidians to exploit host cells. Acquisition of an ancestral transporter gene at the base of the microsporidian radiation was followed by lineage-specific events of gene duplication, which in the case of T. hominis has generated four paralogous NTT transporters. All four T. hominis NTT proteins are located predominantly to the plasma membrane of replicating intracellular cells where they can mediate transport at the host-parasite interface. In contrast to published data for Encephalitozoon cuniculi, we found no evidence for the location for any of the T. hominis NTT transporters to its minimal mitochondria (mitosomes), consistent with lineage-specific differences in transporter and mitosome evolution. All of the T. hominis NTTs transported radiolabelled purine nucleotides (ATP, ADP, GTP and GDP) when expressed in Escherichia coli, but did not transport radiolabelled pyrimidine nucleotides. Genome analysis suggests that imported purine nucleotides could be used by T. hominis to make all of the critical purine-based building-blocks for DNA and RNA biosynthesis during parasite intracellular replication, as well as providing essential energy for parasite cellular metabolism and protein synthesis.

An experimental investigation on aspects of infectious salmon anaemia virus (ISAV) infection dynamics in seawater Atlantic salmon, Salmo salar L.

This study investigated infection dynamics of infectious salmon anaemia virus (ISAV) by conducting two experiments to examine minimum infective dose and viral shedding of ISAV. In terms of minimum infective dose, the high variability between replicate tanks and the relatively slow spread of infection through the population at 1 x 10(1) TCID(50) mL(-1) indicated this dose is approaching the minimum infective dose for ISAV in seawater salmon populations. A novel qPCR assay incorporating an influenza virus control standard with each seawater sample was developed that enabled the quantity of ISAV shed from infected populations to be estimated in values equivalent to viral titres. Viral shedding was first detected at 7 days post-challenge (5.8 x 10(-2) TCID(50) mL(-1)kg(-1)) and rose to levels above the minimum infective dose (4.2 x 10(1) TCID(50) mL(-1)kg(-1)) on day 11 post-challenge, 2 days before mortalities in ISAV inoculated fish started. These results clearly demonstrate that a large viral shedding event occurs before death. Viral titres peaked at 7.0 x 10(1) TCID(50) mL(-1)kg(-1) 15 days post-infection. These data provide important information relevant to the management of ISA.

Estimation of infectious dose and viral shedding rates for infectious pancreatic necrosis virus in Atlantic salmon, Salmo salar L., post-smolts.

Infectious dose and shedding rates are important parameters to estimate in order to understand the transmission of infectious pancreatic necrosis virus (IPNV). Bath challenge of Atlantic salmon post-smolts was selected as the route of experimental infection as this mimics a major natural route of exposure to IPNV infection. Doses ranging from 10(2) to 10(-4) 50% end-point tissue culture infectious dose (TCID(50)) mL(-1) sea water were used to estimate the minimum infectious dose for a Scottish isolate of IPNV. The minimum dose required to induce infection in Atlantic salmon post-smolts was <10(-1) TCID(50) mL(-1) by bath immersion (4 h at 10 degrees C). The peak shedding rate for IPNV following intraperitoneal challenge using post-smolts was estimated to be 6.8 x 10(3) TCID(50) h(-1) kg(-1) and occurred 11 days post-challenge. This information may be incorporated into mathematical models to increase the understanding of the dispersal of IPNV from marine salmon sites.