Based on a closer examination of the top eight farms of each list, for every year , one list always included at least the top three elements of the other. In other words, each list contained the top three farms in terms of out degree and the top three farms in terms of outcloseness. Further iterations of this model could exploit the similarity between ranks of farms based on these two centrality measures, for example evaluating the effect of targeting a lower number of farms based on a list created from the top elements of both rankings. For the case presented here , either centrality measure could be used. Being this the case, we would advocate for the use of out degree over outcloseness, given its simplicity of estimation and understanding. The proportion of farms connected via live fish movements varied in a cyclical manner, with spikes during the periods of January-April, July, and October-December, which is consistent with results from our previous descriptive study of the network of live fish movements in Ireland . Interventions could be considered that specifically apply at these times of higher connectivity between farms, to take account of this observed cyclicity. The remaining between-farm prevalence levels observed after the implementation of this targeted strategies is due to residual infectious pressure and local spread, clone rack where PMCV is not fully cleared from the environment between generations of fish, allowing its transmission to newly stocked fish and locally between neighboring farms.
Similarly, the lower performance of the reactive approach, even if all transmission via fish movements is halted suggests that eradication of PMCV is virtually impossible in Ireland, as it seems that after elimination of transmission via fish movements, the agent is consistently sustained by local spread . The lack of complete production records for all Irish Atlantic salmon farms was the main reason for using movement records to recreate fish population dynamics. Nevertheless, we consider that the rules as applied in this study were realistic. For example, if a farm ships fish in excess of the total fish population at the time of the shipment, it is reasonable to assume that these fish must have originated at a previous time. The options for this origin are either non-recorded, incoming fish shipments or hatching of new fish. In the case of the latter, this is perfectly reasonable if the fish deficit at the farm is due to a shipment of eggs. However, if the deficit is due to a shipment of older fish , assigning an enter event for this age groups is not realistic. Nevertheless, in the absence of records accounting for the origin of fish sent in these age groups, this seemed like a better approach than arbitrarily imputing their origin to another farm, which in turn would have created fish deficits in other farms cascading to the rest of the network. Arguably, the availability of complete production records from all Irish salmon farms would minimize this issue, although making such records available for a 9-year time period would pose a hefty burden on fish farmers.
Additionally, we assert that the impact of our imputation is marginal, considering that only 90 enter events were imputed during the study period , mostly at the beginning of the simulation , and involving mainly fertilized eggs in freshwater farms. This is further evident when evaluating the generated population dynamics, like the number of fish in each age group and the timing of fish enter events , where the abundance of each age group and the enter events follow a seasonal pattern that would be expected given the life cycle of farmed Atlantic salmon. Assigning exit events the day before the last fish shipment of a fish cohort was a simplification necessary for allowing farms not to overpopulate as the simulation proceeded. The impact of assuming all fish within a cohort were present until the day before shipping is hard to gauge, but we think it would be a small effect, especially considering the large fish populations involved in salmon farming. Future iterations of this model could include a mortality function fitted from the data, or even better, real mortality data from fish farm production records, if available. One of the assumptions of the intervention strategies used in this study is that they are 100% effective in eliminating transmission between farms via fish movements. In order to achieve a similar level of effectiveness in the field, it would require screening of all fish shipments with a highly sensitive test before they exit the origin farm, and elimination of all positive batches . The sensitivity of currently used diagnostic methods is not reported in the literature, but one could arguably assume that the RT-PCR method for detection of the virus has a high sensitivity given its capacityto measure viral RNA, which may or may not be present within a virion that is able to replicate.
Currently there are no confirmatory tests for PMCV, and diagnosis of the clinical disease is based on clinical observations, necropsy, and histopathological findings . As for diagnosing latently or subclinically infected fish, this would pose a great challenge today, as there are no cell cultures or other methods that could assist in such a task, which is particularly important for the correct diagnosis of the agent on the early stages of fish life, namely eggs, juvenile fish, and smolts. Further, even if accurate diagnostic tests were available, the feasibility of discarding all infected fish consignments is doubtful, as it would impose a heavy burden on fish farmers, especially considering the modeled current levels of PMCV prevalence. This indicates that for PMCV it is already too late for this type of action to be taken. Nonetheless, it does suggest a clear path to prevent the spread of exotic infectious agents in Ireland, such as ISA virus, piscine reovirus , and others. For these agents, targeted surveillance strategies could be implemented based on the top ranked farms in terms of out degree as described above, which would allow for a timely detection and prevention of further spread across the country. In conclusion, in this study we highlight the importance of human-assisted live fish movement for the dissemination of PMCV across the country, and demonstrate a means, using centrality based targeted surveillance strategies, to prevent this type of spread in the future for other infectious disease agents. These strategies should be applied early on in the epidemic process, before country-wide dissemination of the agent has taken place. The Irish salmon farming industry would benefit from this approach, as it would help in the early detection and prevent the spread of exotic viral agents which have the potential to severely impact local farms and the livelihoods of people that depend on them. This in turn would make Irish salmon farming a more robust and sustainable industry, capable of dealing with infectious agents in a timely and effective way, minimizing socioeconomic and environmental losses, and maximizing fish health and welfare.California leads the nation in production of vegetables, fruits and nuts and its fresh produce industry is composed of an exceptional diversity of crops and farm types. Beginning in the late 1990s and accelerating after prominent incidents such as the 2006 outbreak of E. coli O157:H7 linked to bagged spinach, new rules and best practice guidelines to mitigate foodborne pathogen contamination propagated rapidly through the fresh produce industry . The multiple layers of rules and guidance developed over the past decade present growers with a complex landscape of pressures to adopt and intensify on-farm practices intended to improve food safety, 4×8 tray grow some of which may conflict with efforts to conserve natural resources. Alongside continually developing expectations for food safety, growers are also expected to conserve water and soil and are legally obligated to protect water quality as well as wildlife and its habitat. Many of the state’s major agricultural regions, such as the San Joaquin, Sacramento, Santa Clara and Salinas River valleys are located in or near ecologically sensitive river corridors and floodplains.
These ecosystems host fertile soils, are sources of fresh water, and also provide habitat for many species of birds, amphibians and other wildlife. On-farm practices, including those related to food safety, can have direct and indirect consequences to the benefits provided from these ecosystems .Early evidence suggested that pressures to improve food safety after 2006 led growers to adopt on-farm practices with substantial economic and environmental costs . Surveys and interviews of Central Coast growers conducted in 2007 and 2009 revealed that “as a condition to sell their produce, growers report yielding to tremendous pressure exerted by auditors, inspectors, and other food safety professionals to take measures that are potentially damaging to the environment” . Such measures included clearing vegetation , removing ponds or water bodies, setting poison bait traps for wild animals and installing extensive wildlife exclusion fences. In a separate study of land use change using aerial imagery, Gennet et al. confirmed that approximately 13% of the remaining riparian habitat in the Salinas Valley was removed between 2005 and 2009. Further, emerging evidence suggests that removal of non-crop vegetation fails to reduce, and may even increase, pathogen prevalence on leafy-green vegetable farms in the California Central Coast , while degrading important ecosystem benefits such as natural pest control services . Removing vegetation is expensive and at times conflicts with landowners’ acknowledged environmental stewardship responsibilities . Furthermore, such approaches may conflict with California’s regulatory targets for surface water quality. They may also conflict with standards for USDA’s Natural Resources Conservation Service conservation practices, limiting growers’ access to Farm Bill or other federal sources of funding. In addition, activities specifically adopted to remove or deter wildlife from entering fields — such as poison bait, trapping and extensive wildlife exclusion fences — may expose growers to criticism from wildlife conservation interests and public expectations of farmers to protect native plants, animals and environmental quality. The pressures on growers to improve food safety originate from government regulators such as the Food and Drug Administration , from private third-party auditors and certifiers, and from retail and food service companies that purchase produce. The cumulative effect of these multiple layers of pressure on growers and the food safety measures they feel obligated to implement has not been assessed industry-wide. Food safety regulation at the farm level is a recent development. Before FDA finalized its Produce Safety Rule in 2015 pursuant to the Food Safety Modernization Act of 2011, there was no direct federal oversight. The rule was written in general terms to provide growers with the flexibility to adopt the food safety measures that they deem most appropriate for their farm. For example, with respect to how growers should manage animal intrusion, the rule only requires that growers visually monitor the growing area prior to harvest and take “measures reasonably necessary” in case an animal does find its way into the field. It leaves precise interpretation of what those measures should entail open to the discretion of growers, inspectors, auditors and produce buyers. The rule sets more specific standards for detectable amounts of bacteria in biological soil amendments and agricultural water. But while it does detail approved composting and pathogen testing methodologies, the rule gives growers latitude to choose their own irrigation and soil amendment practices and technologies. FDA, USDA and various partner organizations are actively developing additional guidance and training resources to assist growers in interpreting and implementing the rule, but it will take years to reach everyone. In the meantime, it is unclear precisely how growers will respond. That said, while much attention has been given to the minimum legal requirements set by government regulators, it is critical to understand that government oversight will not account for all, or even necessarily the primary, pressure on growers to improve food safety on their farms. Because food safety rules as written are frequently open-ended and regulators have very limited resources to monitor or enforce compliance, many produce buyers seek additional assurance by requiring their growers to receive certification to one or more private standards. Maintained by third-party certifiers, these private standards add an additional burden of compliance to government regulation . Regulations and private standards are generally publicly available, but there are two further layers of more opaque pressure. First, government inspectors and third-party auditors decide whether or not the on-farm practices used by a given grower comply with regulations or private standards, respectively.