CHAPTER 1
Expert consensus
G. Rees, J. Bartram and D. Kay
1.1 CONTEXT OF THE WORKSHOP
Bivalve shellfish are filter-feeding organisms. They can concentrate microbial pollutants in marine waters including pathogenic species capable of producing disease outbreaks in consuming populations. Control of this disease risk requires integrated management of the water environment used for shellfish growing and harvesting together with post-harvest product processing which might involve depuration and/or heat treatment where appropriate. Perhaps uniquely, therefore, sustainable utilization of this food resource requires continued excellence in the quality of "natural" harvesting waters as well as appropriate management interventions designed to correct any short-term deteriorations in environmental quality. All centres of human population produce the microbial pollutants impacting on shellfish compliance with food quality standards and also contribute the pathogens which can generate disease outbreaks. Sustainable shellfish management, therefore, presents a complex challenge of integrated environmental management encompassing both effluent streams and receiving water quality, together with related food processing and regulation, to achieve end-product quality for consumer protection.
Harvesting shellfish on a global scale is increasing. From a series of regional concentrations, the industry has increased to a total production of 12 million tonnes in 2002, equivalent to 9.4% of the total seafood market, with exports totalling $1.4 billion in 2002. Although, in terms of global trade dollars, this is a relatively small industry, it presents disproportionate health risks because shellfish are often eaten raw or only lightly cooked.
Levels of wild source exploitation for commercial use have remained fairly constant over recent years with the increase in harvested product coming from a growth in aquaculture which comprises 84% of the total bivalve market (2002 figures).
The major global market for shellfish is Asia. The People's Republic of China is responsible for 68% of global production. Import/export of shellfish usually takes place within regional limits. For example, the bulk of live bivalve commerce in Europe is between members of the European Union (EU). China, China (Province of Taiwan), Japan, Malaysia and Thailand are key shellfish trade partners which present the potential for transboundary transport of pathogens.
Food safety is the primary issue in bivalve shellfish trade. The nature of the end product and the associated risks are significant as outlined in chapter 3 of this volume. These are compelling justifications to ensure that bivalve shellfish products are properly tracked through the food chain especially where they cross national borders. Whilst the commercial trade can be regulated relatively easily, there are issues with the "casual" trade which characterizes this product, including its quantification and the undoubted existence of illegal harvesting. The reliability of trade statistics may be sound, but the quantum of casual exploitation is probably impossible to define.
Global experts met at a workshop held in Kuala Lumpur, Malaysia to:
• identify infectious disease risks associated with the consumption of contaminated bivalve shellfish;
• assess water quality management approaches that may reduce the risk of infectious disease; and
• examine and suggest strategies to reduce the risk from pathogens derived from human and/or animal excreta.
The workshop set out to provide guidance to health agencies, water quality and shellfish regulatory agencies and other stakeholders worldwide in recognition of existing and potential future infectious disease problems associated with the consumption of contaminated bivalve shellfish. The efficacy of current practices in protecting human health was assessed and the need for the deployment of new approaches evaluated.
In delimiting the scope of the workshop, initial discussions centred on which shellfish and which contaminants to consider. The workshop elected to maintain an exclusive focus on bivalve shellfish – effectively filter-feeding shellfish predisposed to transmit bacterial and viral pathogens. Throughout this volume, where reference may be made, on occasion, to naturally-occurring pathogens and biotoxins, the reader will be referred to sources of authoritative information.
The workshop also addressed contaminant sources and means of transmission to bivalve shellfish, where possible identifying options to interrupt the cycle. Transmission routes were identified from land- or water-derived contamination (fresh or sea) of harvested products (including harvest for subsistence, recreational, non-market or local sale, or commercial harvest). For the purposes of this publication, post-harvest issues are considered the domain of food safety and post-infection issues the domain of health care and treatment, thus, the focus is specifically on water management aspects and strategies.
1.2 PUBLIC HEALTH FACTORS
Shellfish have been a source of food for thousands of years, as indicated by shellfish middens near ancient human habitation. Human illness caused by infectious agents translated from human or animal sources through shellfish consumption has long been identified.
Minor, self-limiting complaints predominate, although more serious illness may occur in some cases. These include cholera and hepatitis A (HAV) in less developed countries and a range of infections associated with exposure of the immunocompromised. Such illness can occur in populations dependent on shellfish as a subsistence protein source, or in populations far from the point of origin through intra-regional or international trade, i.e. potentially transmitting pathogens from endemic areas to other locations. Primary prevention of the transmission of infectious disease through shellfish requires:
1. ensuring that shellfish are only collected at places and times that minimize or eliminate the likelihood of contamination with relevant pathogens, AND;
2. methods that prevent contamination of shellfish during harvesting and transport.
OR
3. collection at places at times of potential risk, AND;
4. depuration or post-harvest processing using procedures proven to reduce risk to tolerable levels.
Using either route, the protection of public health requires active monitoring of the source waters and the end-product in order to ensure that controls are adequate.
1.3 HARVESTING AREA MANAGEMENT OPTIONS AND RESPONSES
The workshop concluded that commercial exploitation and casual, artisanal collection of bivalve shellfish need similar levels of health protection. It was also considered important that policies are in place to ensure implementation of monitoring and regulatory regimes in commercial contexts as well as provision of unambiguous information to casual collectors on likely health risks.
Management approaches must cover the spectrum of need from highly technologically (prevalent in developed countries) to intermediate technology applications (especially in less-developed countries). Management steps can be incremental and aspirational as resources sequentially become available; i.e. not a one size fits all approach. Responses to the differing threats should therefore be upgradeable.
In terms of the transmission cycle, land and/or water-based contamination sources, that may affect product quality up to harvest, were considered. Identifying probable sources of contamination, both point (such as sewage discharges) and non-point (such as septic tanks and livestock), as well as potential management responses was considered critical to success. It should be noted that post-harvest processes are outside the scope of this monograph and are not, therefore, considered.
Exploration of available management interventions included:
Site management
• positioning harvest sites remote from known contaminant sources and provision of advice to facilitate this intervention;
• planning controls, applied at the outset of harvesting, site development, to prevent adverse effects of subsequent developments on harvest areas;
• assessment and prioritization of urban sewage and agricultural waste management actions, using studies to quantify the various source of microbial pollution and devise mitigation strategies;
• monitoring for the correct sentinels in the correct locations and with the appropriate frequency, identification and dissemination of good practice, including pollution prevention and mitigation strategies;
• sewage treatment processes to control point sources; quantification of and reduction in diffuse pollution from agricultural and other sources;
• forecasting to be applied on monitoring and other data based on the likelihood of events that may compromise shellfish integrity such as rainfall;
• creation/restoration of natural buffers between contaminant source and shellfisheries, such as wetlands.
Harvesting management
• applying the most appropriate means of purifying contaminated shellfish (such as relaying and depuration).
Education and information
• tracking shellfish from outbreak back to harvest site;
• effective communication leading up to and emanating from notices to modify practices in shellfish areas (including closure and opening notices);
• educating producers/harvesters and consumers in health-related issues;
• harmonizing systems to ease or support international trade through agreed environmental and product standards; and
• development of guidelines for commercial and recreational vessels to govern disposal of on-board contamination.
1.4 SOURCE IDENTIFICATION, SANITARY SURVEYS AND PROFILING
Contamination is often first identified through end-product (i.e. shellfish flesh) and/or environmental water sampling in harvesting areas. Remediation requires the sources of this pollution to be identified and quantified followed by practical management measures designed to reduce the pollutant flux from point and diffuse sources.
Point sources of microbial pollution, traditionally associated with end-of-pipe delivery of human and/or animal effluents, are readily identifiable and attributable. Point sources of particular relevance include livestock slaughterhouse and processing effluent; overflow of manure lagoons; crude and treated sewage effluent; stormwater runoff and combined sewer overflows (CSOs).
Non-point sources, i.e. the diffuse delivery of pathogens and indicator bacteria, are far less easy to identify and include contaminated freshwater inflow or coastal movement of contaminated waters; runoff from pasture or cropland; untreated or partially treated sewage spread to land and seepage from septic tanks; leachate from landfills and fly-tipping sites. In addition, there may be a potential for faecal contamination to be mobilized from contaminated sediment (by processes such as dredging, large ship propeller wake, anchor pulling and seasonal thermocline turnover).
Intermittent sources, both point and non-point, include recreational, fishing boat and other vessel waste; large ship bilge dumping; seasonal tourist concentrations, as well as livestock and wildlife migration.
Sanitary surveys and profiling (i.e. formal assessment of pollutant sources and estimates of magnitude) are invaluable, particularly for initial selection of shellfish harvest areas. Periodic updates in survey profiles should be undertaken to assess impacts of changes and impacts of potential developments. In such surveys, there must be appropriate collation of information covering sewage outfalls; CSOs; riverine inputs; livestock; wild animals; tidal factors and currents; prevailing winds; susceptibility to and frequency of severe storm events. The data collected should provide information that can lead to decisions on when to open and close sites and such decisions must be communicated in a clear and timely fashion. These data will also inform pre-emptive closure after severe storm events and the appropriate time interval after severe events for safe reopening.
1.5 MONITORING: CHALLENGES AND OPPORTUNITIES
The basic science underpinning the monitoring processes must be reappraised and evaluated, particularly exploring the relationships between water quality measures and shellfish flesh quality. Once such relationships have been identified, it may be possible to establish relationships between indicators and pathogens in flesh and water. In addition, concerted efforts to understand species differences and the environmental drivers contributing to pathogen uptake and release will also lead to better evaluation of approaches to mitigation of shellfish product contamination, such as relay and depuration.
There is widespread agreement on the need to classify growing sites, but there is less convergence on the most appropriate methods; i.e. either by water column classification (which is potentially easier and cheaper, but relies on clear understanding of the relationship between contaminants in water and shellfish flesh) or by shellfish flesh contamination (which is more expensive, destroys product, but provides a good surrogate risk measure and is presently required by most regulators).
It is impossible to test water or flesh for all possible contaminants. It is also generally agreed that thermotolerant (faecal) coliforms are not comprehensive indicators of health risk as their presence in water does not correlate well with the presence of bacterial or viral pathogens in flesh. The workshop participants strongly advocated that thermotolerant coliforms should be replaced by E. coli. Investigation of alternative and potentially more representative indicators, such as F+RNA phages, was recommended alongside E. coli to provide insight into risks from viral pathogens for an appropriate appraisal period. In addition, differential indicators for water quality, shellfish flesh determinations or efficacy of management procedures such as depuration should be explored. The goal should be to provide the tools for the design of acceptable standards and interventions that can be taken to better protect public health in a range of situations.
The variations between tropical and temperate situations may also require different indicators. To identify such indicators, more data on indicators appropriate to tropical countries must be collected and approaches that can be applied to developing countries explored. In developing this approach, there should be a real effort to expand and enhance the existing sparse empirical data resource on pathogen concentrations and survival in tropical and developing countries.
Real time prediction should be applied to the modelling/forecasting of cause and effects. Parameters which could drive this approach include: salinity; rainfall; changes in turbidity; stream flows; wind direction/speed; sewage overflows; relation to sewer outfalls; and CSOs riverine inputs. Worst case scenarios and impacts of likely adverse events should be established for both baseline water quality and shellfish flesh quality.
Progressing these issues should lead to the development of risk based numerical standards, as applied in recreational waters, and could exemplify further integration of good practice in water quality assessment through modern drainage-basin management and regulation as exemplified in the US Clean Water Act and the EU Water Framework Directive.
The expert workshop participants felt strongly that a pragmatic approach to standards should be adopted – stricter standards should be in force for shellfish which are generally eaten raw as opposed to those that are eaten after cooking.
1.6 POST-CONTAMINATION PURIFICATION PROCESSES
Relaying shellfish in clean environmental waters, or in tanks of specifically treated water, produces a cleaning of pathogens from the shellfish flesh, a process commonly known as depuration. Depuration processes should be optimized with and based on technologies demonstrated to be effective in virus removal. In addition, an appropriate surrogate for viral presence (potentially F+RNA phage) should be used to monitor the process.
Regulators should ensure that depuration is applied during the highest risk periods based on historic data used to characterize seasonal and other factors (such as those collected via sanitary survey). Some sites may require frequent depuration.
The diverse responses of different species to depuration processes must also be factored into the management regime.
