Surveillance should be an integral component of all official aquatic animal health management programmes. This ensures that activities are in place that support early warning of diseases of concern, contingency planning and monitoring of disease control measures. Likewise, surveillance provides the basis for sound aquatic animal health advice for farmers, processors and other stakeholders involved in the handling of live aquatic animals; and accurate certification of exports, international health status reporting and verification of freedom from diseases. Surveillance is particularly important for effective aquatic animal emergency disease preparedness.
Both general and targeted surveillance are necessary. Solely targeted programmes are not cost effective, and can only be applied to a few selected diseases. General surveillance is useful for detection of new and exotic diseases, as well as for monitoring outbreaks of endemic diseases. General surveillance increases farmers (and other field personnels) awareness of disease, and establishes working links to expertise providing clinical and preventive health care. General surveillance can also reassure farmers and other stakeholders that disease monitoring is not an automatic trigger for emergency disease responses that can mean stock destruction or transfer/trade controls. Conversely, such surveillance can provide the evidence required for those nervous of such activity that proactive management can reduce negative economic or productivity impacts. Surveillance can also serve as an indirect consumer/business value-enhancement label where competitors may not have embraced proactive health management or related good management practices. Initial efforts, especially where resources or infrastructure are under development (or limited), should focus on facilities, sites or aquatic populations at greatest risk of exposure to known pathogens/diseases of concern.
Investigations of suspected disease occurrences, or to prove freedom from those diseases, require:
appropriately trained and dedicated personnel;
standardised field and laboratory methods supported by quality control systems; and
access to manuals and ongoing training opportunities.
The basis of effective surveillance programmes is observant and skilled people, who understand normal health patterns, are alert to changes, and can describe the abnormalities they see. The precise design and structure of a surveillance programmes vary with their exact purpose, but all share some basic common features:
clearly stated objective(s);
a list of diseases of concern;
the capability to recognize a disease outbreak with general surveillance activities to the required level of diagnostic certainty;
specified protocols for collection of the information required; and
a system to record and collate the data collected, as well as report findings.
The following outline the approach to meeting the four general objectives of surveillance:
1. Rapidly detect new and exotic infectious diseases in aquatic animals.
As a minimum requirement, all countries should have a system in place that gives early warning of new and exotic aquatic animal diseases. Such surveillance is based on comprehensive general surveillance activities aimed at spotting endemic pathogen problems. If this system is working well, new or exotic disease can be detected using Levels I and II diagnostic methods, although many diseases will require first time confirmation using Level III methods and/or confirmation by a reference laboratory.
Once a new or exotic pathogen is detected, targeted surveillance will be required to define its distribution and the magnitude of the problem, track its spread, assess feasible control options and, where appropriate, demonstrate successful eradication.
2. Provide evidence of freedom from diseases relevant to domestic and international movement of aquatic animals and products.
The existence of comprehensive general surveillance activities, which have the ability to diagnose the pathogen(s) of interest, provide the initial evidence of freedom from diseases of national/international concern. Historical records may be used to reinforce the hypothesis of freedom being tested by the current surveillance program, or to develop preliminary surveillance programs. It should be recognised, however, that environmental conditions or human activities may have changed host susceptibility since historic records were collected. At present, there are no specific guidelines on how to quantify historic evidence, but methods of analysis are currently being developed. A combination of Levels I to III diagnostic methods may be necessary, depending on how characteristic the signs of the particular disease are, and whether or not sub-clinical carriers are suspected. This is because Level I can rarely provide conclusive evidence of freedom of an exotic disease, and Level II and III diagnostic tests frequently require validation from Level I observations.
For significant diseases (high risk), targeted surveillance may be required in addition to general surveillance in order to prove freedom from the causative agent. Cameron (2002) describes various methods that can be used to enhance comprehensive and scientifically-justifiable coverage of aquatic animal populations.
3. Describe the distribution and occurrence of diseases relevant to disease control and domestic and international movement of aquatic animals and products.
Defining the geographic distribution of specific diseases can often be done using general surveillance, provided it is sufficiently comprehensive to include adequate sample sizes from all geographical areas where susceptible host populations occur. Such surveillance, however, does not delineate the precise geographic distribution of the disease agent, nor infection levels present. This information requires targeted surveillance and specific diagnostic techniques (usually Levels II and III).
4. Assess progress in control or eradication of selected diseases and pathogens.
Having undertaken a surveillance program, and delineated a positive zone where control or eradication of the disease is possible, it is important to know how successful the control or eradication measures are. Without this capability, redefining the zone as negative, or accurately protecting surrounding negative populations from spread of the disease is impossible. A necessary prerequisite for any control or eradication measure to be successful is complete participation by all affected stakeholders (farmers, commercial harvesters, processors, and regulatory authorities). If one or more sites within a zone do not participate, controls within the zone as a whole may be jeopardised. In addition, the cost of disease control measures (on top of losses to the disease itself) means that non-compliance by some stakeholders may be a catalyst for conflict.
Assessment of control of spread requires targeted surveillance of the buffer zone surrounding the affected area. The buffer zone must include the susceptible host species and lie outside the immediate hydrographic influences of the zone containing the infected stocks. As long as the populations in the buffer zones remain uninfected, the disease has been successfully contained within the positive zone. If the disease spreads to the buffer zone, and cannot be eradicated, the positive zone will need to be expanded to include the affected populations. A new buffer zone is then established to protect the next neighbouring susceptible populations. Once again, hydrographic influences on the expanded positive zone need to be taken into account for selecting the new buffer zone.
Assessment of success in eradicating a disease from a positive zone can be challenging and there are few examples for aquatic animal diseases. Scotland successfully suppressed clinical infections of Infectious Salmon Anaemia (ISA) with early intervention, salmon stock depopulation, fallowing and stringent biosecurity measures. However, eradication of the causative agent from the affected area is not assured, so biosecurity measures have been maintained, along with close monitoring for any disease reoccurrence. Norway attempted to eradicate the salmon ectoparasite Gyrodactylus salaris, using chemical sterilisation of entire river ecosystems. Although successful, such an extreme measure requires serious cost-benefit analysis.
Where eradication has potential for success, and measures to remove the disease agent are undertaken, surveillance can be impeded by the fact that all susceptible animals have to be removed to break the infection cycle (fallowing). In such cases, a small number of susceptible animals could be used to test the area, following the fallowing period. If the disease does not appear in these animals, more intense stocking with disease-free animals can be considered. If the disease does occur in the test animals, the fallowing period may need to be extended, or the area may have to be zoned as positive. In the latter instance, efforts would then concentrate on controlling disease spread, rather than on its eradication.
Each country will have its own specific diseases of concern. A minimum list would be those notifiable to the OIE which are relevant to the particular countrys aquatic resources, or trade interests, or other infectious diseases of regional concern. OIE listed diseases have been identified as posing a risk with trade in susceptible aquatic species, therefore, demonstration of freedom or equivalent infection status may be necessary for trade with countries that also have susceptible populations.
A model plan for the development of disease surveillance capabilities and capacity at national and regional levels is provided in Appendix I.
In the event of an emergency disease outbreak, the following information is required:
the disease(s) suspected;
the exact geographical location(s)/extent of the outbreak(s);
contact information for affected growers or sites, responsible authorities (aquaculture leasing/management; wild resource/fisheries protection);
species and life-history stages affected;
approximate numbers (estimated percentages) of sick and dead animals, where this can be calculated. Other measures of the extent of disease impact can be used, e.g. number of ponds or cages affected;
brief description of history, clinical signs and lesions observed;
date(s) when the disease was first noticed at the initial outbreak site as well as at any subsequent sites;
details of movements of susceptible animals to or from the affected site(s)/facility(ies);
any other key epidemiological information, such as temperature, salinity, turbidity, disease status of surrounding wild populations, abnormal environmental events (floods, drought, pollutant contamination), and possible vectors (birds, human activities); and
initial disease control actions taken.
All exotic and other significant aquatic animal disease emergencies should be reported immediately to the national authority responsible for aquatic resources within that country. This requires a communication network to be in place that ensures that a fishery resource user or aquaculturist has access (direct or indirect) to the national authority. In some countries this is achieved through mandatory reporting legislation or policy. This requires diagnostic laboratories or field veterinarians and extension officers to report specific diseases or mortalities fitting specified criteria to the national authority immediately upon detection. In other countries, this may be achieved through education of the resource users, rather than legislation, but this has a higher risk of delayed or non-reporting, than legislated reporting. Even with immediate reporting, most aquatic animal diseases are difficult to eradicate from open water or flow-through sites, so delayed reporting renders the option of eradication even more challenging (if not impossible).
For endemic diseases, reporting may be limited to recording presence or absence of disease events within a particular area or, in more sophisticated systems, recording estimates of prevalence of the particular disease for specified time periods (e.g., annually or quarterly).
To provide access to surveillance findings, some form of information repository is required, from which various reports can be produced. This can be at a national or regional level, as required.
A national disease data management system is necessary to collect, store, and use the data needed to establish and maintain zones for diseases of national or trade concern. As a minimum, this should include presence/absence data for reportable diseases within the country. Data required for risk analysis or epidemiological research, may be stored at a regional or local aquatic animal support facility (veterinary, government, or research). In general, corporate or individual client information is retained by the direct aquatic animal health service provider (extension officer, local veterinarian or government aquatic animal health services). This can be independent of, or linked to, the data repositories used for zonation. Where such information is kept separate from the national database, some form of communication network must be in place to ensure that client or local information can be accessed quickly when a disease emergency arises.
Aquatic animal health information systems may range from information gathered by stock-owners, passed by word of mouth, recorded on local, regional or national computerized data bases, or managed via networks linking a broad number of government agencies and diagnostic laboratory resources.
Effective disease management requires transparent declaration of significant disease problems. Historic establishment of many aquatic animal diseases has undoubtedly been due to inadequate surveillance, but more recent history indicates another reason that is more difficult to address. Some stakeholder believe that rapid reporting of disease losses (regardless of diagnosis) poses a significant risk of immediate market loss, devaluation of product and related disease management credibility issues. In addition, disease reporting is often linked to stock destruction - especially for detection of exotic diseases. Such beliefs are easy to understand, in light of terrestrial disease examples, such as Foot and Mouth Disease (FMD) and Bovine Spongioform Encephalopathy (BSE). However, reticence to report poses significant challenges to effective disease control in open-water and flow-through aquatic systems. This is why mandatory reporting is an OIE requirement for both terrestrial as well as aquatic animals. It assists accurate pin-pointing of disease incursions, and provides credible trade certification status. If a disease breaks out post-import and the country has a surveillance system with transparent reporting and associated export certification, that importation can be examined and (where appropriate) ruled out as the source of the infection. This is covered by the chapter on veterinary ethics in the OIE Code.
National disease reporting
Special emergency disease reporting mechanisms must be in place for serious disease outbreaks or suspect cases. These reporting mechanisms (usually part of a more comprehensive contingency plan) must allow critical information (as outlined under the first 4 bullets under Basic Information above) to be transmitted quickly and accurately to the national authorities responsible for aquatic animal disease control (preferably, the same day of detection/presumptive diagnosis). This means that field and laboratory staff, who are involved in surveillance, need to have the necessary contact information (with a list of alternatives) so emergency disease reports can be acted upon with minimal delay. Mandatory reporting is useless if information required to pinpoint the outbreak is inaccessible.
A national disease reporting system should be based, where possible, on the day-to-day disease investigation activities of field officers and diagnostic laboratories. Such a reporting system, by necessity, requires feedback loops, as shown in Figure 7. Routine data collection is aimed at helping the industry stakeholder with any disease problems, so only a small proportion of the field information is required by the next administrative level, and likewise up the line. Some kind of tracking system is required, however, to permit rapid feedback, when required.
Figure 7. Example of information flows in a national disease reporting system
International disease reporting
There are various international levels of formal reporting, the most important for aquatic animal diseases of trade significance being through the OIE. The NACA/FAO and OIE Quarterly Aquatic Animal Disease Reporting System provide similar reporting programs for geographic regions such as Asia-Pacific countries. The North Atlantic countries report to the International Council for the Exploration of the Sea (ICES) and European countries provide reports to the European Inland Fisheries Advisory Commission (EIFAC).
World Organisation for Animal Health (OIE). The OIE has disease reporting requirements for member countries that need to be addressed within any national aquatic animal disease control program. A staff member in the national office of the Competent Authority for each country should be delegated the official responsibility of preparing international disease reports, which are submitted to the OIE by the national delegate (usually the Chief Veterinary Officer [CVO]) of the OIE Member Country. The same person can prepare reports for other organizations with aquatic animal disease interests, e.g. NACA/FAO, ICES, EIFAC, but these do not require submission via the CVO for the country.
Obligations of the OIE Aquatic Code require notification to be sent to the OIE Central Bureau within 24 hours of confirmation of any of the following events:
for diseases listed by the OIE, the first occurrence or re-occurrence of a disease in a country or zone of the country, if the country or zone of the country was previously considered to be free of that particular disease; or
for diseases listed by the OIE, if the disease has occurred in a new host species; or
for diseases listed by the OIE, if the disease has occurred with a new pathogen strain or in a new disease manifestation; or
for diseases listed by the OIE, if there is potential for international spread of the disease; or
for diseases listed by the OIE, if the disease has newly recognised zoonotic potential; or
for diseases not listed by the OIE, if there is a case of an emerging disease or pathogenic agent should there be findings that are of epidemiological significance to other countries.
Thereafter, monthly reports outlining the disease situation are sent to the OIE, until the disease has been eradicated, or the situation has been brought under control, e.g. by surveillance supported zonation.
Annual disease status reports are sent to the OIE from member countries with presence/absence records, information on changes in status of diseases listed by the OIE, or findings of epidemiological importance to other countries for diseases that are not listed by the OIE. These reports are aimed at providing information on significant changes in the status of infected zones that are of relevance to trade partners or neighbouring countries with confluent hydrographic boundaries.
Regional organizations. Regional Inter-Governmental Organizations (IGOs) with a mandate for aquatic animal health management could be used to assist international cooperation on aquatic animal health issues and development of infrastructure to reduce risks associated with trade in live aquatic animals and their products (e.g. the European Commission, Council Directive 91/67/EC, its amendments, or related Directives and Commission Decisions, and the ICES Code of Practice on the Introductions and Transfers of Marine Organisms (ICES 1995a)). These mechanisms may include diseases of significance to the region, which are not included in the OIE list. The NACA/FAO and OIE Quarterly Aquatic Animal Disease Reporting System is an example of such cooperation in the Asian region. The NACA/FAO and OIE list includes the OIE listed diseases and other diseases of significance to Asia-Pacific Region trade. Since aquatic animal diseases do not respect borders, close collaboration between neighbouring countries is required. This includes early warning, rapid and transparent sharing of information on new disease occurrences, the spread of existing epidemic diseases to areas with shared water bodies, and information on related control strategies. This information should be shared by: (i) respective Competent Authorities; (ii) responsible government agencies; (iii) local district, provincial or regional management offices and laboratory personnel/scientists; as well as (iv) industry associations along shared borders.
Mechanisms to ensure rapid and transparent flow of disease information between the responsible government agencies of major trading partner countries, is also essential for effective disease control in aquatic animals and their products.
Most countries can conduct basic surveillance using existing communication networks but this should be reinforced, where necessary, by a system of formal reporting and record keeping. Use of existing communications systems ensures that historical records, even if qualitative, are available for back-up information. Such historical data is also useful for setting interim zones until targeted data can be generated to either confirm the zone or identify where the zone may need to be revised. Potential sources of qualitative surveillance information are listed in Table 7.
Table 7. Potential sources of surveillance information on aquatic animal diseases.
· Farm/Lease Workers
· Village Elders/Chiefs
· Industry Associations
· Post-Larvae/Fry/Seed Salesmen
· Salesmen/Delivery Men
· Material Suppliers
· Fisheries Officers
· Extension Officers
· Research Workers
· Research Institutes
· Government/Private Laboratories
· Research Literature
· Grey Literature
A diversity of information sources may be included in the surveillance system, and may span anecdotal information, farm records, hatchery records, private and government laboratory reports, certification records, research investigation and fishery stock assessment data. Such diversity inevitably includes varying levels of reliability and quality. All the information is valuable but, less reliable data will require some degree of verification before it can be used to make management decisions. Basic principles behind use of variable sources of surveillance information are given below.
In Country A, the occurrence of sudden mass mortalities is noted in mussel farms located in a single lagoon within a single province. This information, along with other related observations, is communicated to the National Coordinator (NC) for aquatic animal health who immediately notifies authorities in neighbouring provinces. If the NC is not the Competent Authority (CA) for aquatic animal health for the country, the NC also will notify the CA. At the same time the NC requests information from any mussel/mollusc disease specialists in other institutes on any similar occurrences in mussel populations elsewhere. An "affected zone" is established, based on the initial reports, which is the area delineated by human activity and hydrographic links to the mussel stocks known to be affected (e.g. the lagoon). If human activities create links to more distant locations, these must also be investigated.
Once the disease or pathogen is identified, and confirmed as being a new/exotic infectious threat to mussel stocks in the country, surveillance is critical for delineating the actual geographic distribution of infection, this is reported to the NC who then notifies the fisheries officials responsible for mussel culture and processing and requests that they implement controls that will prevent transfer or spread of the infection out of the affected area. At this point the CA should be informed of the disease situation and control measures implemented. If a reportable disease, the appropriate authorities should also be informed within the required time period (for OIE listed diseases, this is 24 hours). More intensive surveillance is undertaken to ensure no sub-clinical infections have been missed in the initial surveillance for clinical infections. Any samples that are uninfected but within the positive zone (delineated by positive cases and hydrographic continuity) can be managed as "uninfected" sub-zones if controls can be put in place that isolate the site from infected sites.
A fisherman in a small village notices an unusually high number of dead crabs in the creek where he normally fishes. He tells a friend who informs a rural crop extension worker who visits the village every three months. As a result of a government information initiative, the extension worker knows he should communicate information on unusual losses or diseases of aquatic animals to the district fisheries office.
When the district fisheries officer gets the information, he sends a note to his supervisor who, in turn, submits it to the National Coordinator (NC) for aquatic animal health. If the NC is not the Competent Authority (CA) for aquatic animal health for the country, the NC will include this information in their regular report to the Competent Authority (CA), and bulletin reports to other district fisheries offices. The NC will also check records and request information on any similar occurrences from other district fisheries officers.
At the same time, movement of live crabs from affected areas to markets in nearby provinces for use as bait, comes under regulatory prohibition. District fisheries officers in the affected and neighbouring provinces are also asked to distribute notices informing local communities of the situation and request that alternative types of bait be used until further notice.
Surveillance to establish zones: National surveillance programmes entail significant investment in the required support infrastructure, so it is recommended that diseases for such programs be selected by a thorough risk analysis. Criteria for OIE listed diseases are provided in the Code (OIE 2003b).
Evidence of freedom: Evidence of freedom from infection may be based on a number of different sources, including:
structured surveys using one or more tests for the presence of the agent;
knowledge of physiological tolerance limits of the infectious agent;
history of no imports of potentially infected material;
existence of appropriate biosecurity measures;
evaluation of historic or general surveillance data by the Competent Authority;
structured, non-random surveillance (e.g. sentinel sites, accreditation programmes, etc.); or
any other sources that provide evidence that lowers the probability that infection is present.
For diseases listed in the OIE Code, the general requirements for surveillance for recognition of freedom from infection are presented in the OIE Aquatic Manual, Chapter 1.1.4. In brief, these requirements differ depending on the previous infection status of the country, zone or aquaculture establishment, namely absence of susceptible species; historically free; last known occurrence within the previous 25 years; or previously unknown infection status.
To maintain evidence of freedom from a disease, a surveillance system that ensures early detection and pathogen identification is required. This system should be managed or coordinated through the Competent Authority and include:
representative coverage of susceptible populations by fishery or aquatic veterinary services capable to detecting, investigating and reporting disease incidents;
access to laboratories capable of diagnosing the disease of concern;
training for fish health specialists to ensure they can detect and identify the disease agent; and
import requirements to prevent the introduction of disease/infection into the country or zone, from known infected areas.
The decision to invest in surveillance depends on whether or not there is a risk of introduction or spread of the disease of concern. Some basic questions behind assessing this risk are:
1. Are susceptible species present?
Where a species is present that is known to be susceptible to an infectious disease of concern, the risk from exposure to the disease agent needs to be evaluated. If there is no history of unexplained mortalities and no detection of the infectious agent through general surveillance (where present), it may be assumed that the populations present are naïve and at risk. The degree of risk depends on whether or not human activities expose the populations present to stocks from areas where the disease is known to occur. Likewise, current or potential trade with other countries, or zones within the country, that are considered or proven to be free of the disease, also needs to be taken into consideration.
If the species is present, and likely to be exposed to risk, targeted surveillance to confirm the presumed negative status is required. Before embarking on such a program, it is recommended that susceptibility be confirmed. This is necessary to ensure that the populations present are truly susceptible, as opposed to being tolerant or resistant to infection. In the latter case, no surveillance for protection may be required, although surveillance for sub-clinical infections would be needed. In some cases, countries have undertaken challenge experiments using animals from the presumed negative zone, in collaboration with countries where the disease of concern is endemic.
2. Can the pathogen of concern survive the environmental conditions present in the area being proposed for zonation?
Many of the most significant aquatic animal pathogens are serious because of their ability to cause disease outbreaks in susceptible hosts over a broad geographic range. Timing and duration of infections may be affected by temperature and salinity differences, but most diseases are suppressed, rather than eradicated under extremes in environmental conditions. Some tropical pathogens may show a limited distribution, but where the host species is/are cultured outside their natural geographic range, culture conditions usually replicate the optimum temperatures and salinities for growth, that will be conducive to pathogen proliferation if present.
3. Do susceptible stocks straddle political borders (national or provincial/state)?
Before embarking on targeted surveillance to define zones containing susceptible populations, it is necessary to ensure that these zones cover the entire ecological range of the species. Where this range straddles political borders (internal or international), zonation will only be effective if all jurisdictions participate or support the surveillance efforts.
4. Is data available to define zones?
Data from general surveillance activities that include the susceptible populations can provide a strong foundation for establishing preliminary zones. These can then be refined as data from targeted surveillance is generated. General surveillance can be especially useful for providing evidence of freedom, where the diagnostic tests required to detect infection are not pathogen-specific. Such data normally requires reinforcement from targeted surveillance, where the objective is to prove (certify) freedom from infections within the zone.
Where there is insufficient data to establish a preliminary zone, targeted surveillance of susceptible populations can be started. Consideration should be given to prioritising those stocks or populations at greatest risk, selecting samples of the most susceptible size/age group from discrete populations, and timing collections to match the season of peak outbreaks in endemic zones.
5. What sample sizes are required?
Statistical tables require samples sizes close to 100 percent if 0 percent prevalence (disease absence) is assumed. Since this would defeat the purpose of zonation, most surveillance programmes work towards samples giving 95 percent confidence of detecting a single infection at 2 percent prevalence (n = 150 for populations >1 000 000). Any sub-sampling of widespread but homogenous stocks needs epidemiological review (Cameron 2002). Also, some stocks may be too valuable or rare for intense sampling, so non-lethal sampling or lower confidence levels may need to be considered, possibly over a longer period.
Smaller samples (n = 30-60) can be collected from any stocks experiencing abnormal growth or mortalities during the period of establishing the zone, in addition to the targeted surveillance samples.
At least two representative samples per year, over a two year period, from each discrete population, is considered by the EU and other individual countries to be the minimum amount of data required to define the zones applicable to those populations. The OIE Aquatic Manual stipulates that the number of units to be sampled from a population should be calculated using a statistically valid technique that takes at least the following factors into account: the sensitivity and specificity of the diagnostic test, or test system; the design prevalence; and the level of confidence that is desired of the survey results. The specific sampling requirements will need to be tailor-made for each individual disease, taking into account its characteristics and the specificity and sensitivity of the accepted testing methods for detecting the disease agent in host populations. The sample size calculations are performed using the FreeCalc software (FreeCalc-Cameron, A.R. Software for the calculation of sample size and analysis of surveys to demonstrate freedom from disease). Available for free download from http://www.ausvet.com.au).
6. Surveillance within established zones
Positive Zones - If eradication is determined to be unfeasible, minimal surveillance is required, except to monitor for recurrent outbreaks that require management intervention to minimise losses.
Buffer Zones - Similar to surveillance in free zones, but with samples concentrated on susceptible populations closest to, or with human activity links to, positive zones.
7. Assessment of risk associated with movements of other species out of a positive zone
The question of host specificity of the pathogen is becoming increasingly important as more species and greater number of animals get transported live for aquaculture, processing and marketing. This is especially pertinent to molluscs and crustaceans that have serious pathogens with broad host-specificities. The ability to accurately assess the potential of other species to act as carriers or reservoirs of infection is, therefore, essential for effective zonation. This may be achieved by using historic evidence of transfers of other species, with no associated spread of disease, or it may require targeted challenge experiments.
Accurate risk assessments are essential for identifying true risks of pathogen transfer, versus suspected risk of carrier transfer. Suspicion of potential carrier species must focus on links to naïve susceptible populations; otherwise the scope of surveillance or challenge experiments required could encompass all species within an infected zone. Although this may be logistically challenging when resources are focussed on controlling the disease impact on the recognised host species, protection of surrounding vulnerable populations may necessitate impacting other resource users until a risk analysis can be done.
A basic assumption for any diagnostic analysis of potential carrier species is that the characteristics of infection in the primary susceptible host are unlikely to be present in the carrier/transport host. Visual inspections, such as histology are, therefore, of limited use for checking carrier status of other species in positive zones. Vertebrates can be screened for serious finfish viruses using sensitive fish cell-lines, but no such tools are currently available for invertebrate pathogens. Recent development of molecular probes, however, has significantly increased our ability to screen potential sub-clinical carriers.
In 1995, the question of assessment of carrier species was raised by the ICES Working Group on Pathology and Diseases of Marine Organisms (WGPDMO) (ICES 1995b). More recently, FAO hosted a workshop to discuss the use and interpretation of molecular-based diagnostics, which included discussion of sensitivity-specificity field validation for screening potential carrier/reservoir hosts, as well as sub-clinical infections in the normal host species (Walker and Subasinghe 2000).
General considerations recommended by the WGPDMO for assessing whether or not other species pose a risk of transfer of a significant pathogen from a positive zone to a negative zone are as follows:
Determine the geographic range of pathogen A in known susceptible host species
Determine which species within that range is/are likely to be transferred live to other areas that have uninfected susceptible species populations present.
Examine these species for the presence of pathogen A, using diagnostic tools known to be sensitive and specific for the pathogen, and sample sizes appropriate for detection of low level, sub-clinical, infections.
If evidence of pathogen A is detected, assess the viability of the pathogen in the alternate host. If tests are molecular assays, can the pathogen be isolated in tissue samples using in situ hybridisation? Is there any evidence of pathogen proliferation?
Assess the transmissibility of the infection from the suspect carrier species to the known susceptible host, using e.g. laboratory-based challenges where susceptible hosts are held in contact with the suspect carriers. Although useful for validating laboratory challenges, especially for pathogens with unknown intermediate hosts, field exposure of naïve host species and suspect carriers is not recommended. Such experiments are difficult to control and risk raising infection loads in open-water.
If the results from the challenge experiment are positive, then the alternate species presents a high risk for transfer of the pathogen out of the infected zone. The species should be reported to the national authority as being a carrier of viable disease agent, so appropriate control measures can be put in place.
The WGPDMO further recommended that simple inoculation-based challenges not be used alone to assess alternate host susceptibility. Proximity challenges and pathogen viability also need to be examined.
 Client confidentiality
is protected except where action is required to prevent exotic disease spread to
surrounding wild resources or neighbouring farm animals.|
 Dedicated human resources (field and laboratory), including legal or policy frameworks for mandatory reporting, diagnostic quality management systems, intergovernmental or jurisdictional policy agreements, etc.
 See Chapter 1.1.4 Requirements for surveillance for international recognition of freedom from infection; OIE Manual of Diagnostic Tests for Aquatic Animals, 4th ed, 2003 - - http://www.oie.int/eng/normes/en_amanual.htm
 See Chapter 1.1.4 Requirements for surveillance for international recognition of freedom from infection; OIE Manual of Diagnostic Tests for Aquatic Animals, 4th ed, 2003 - - http://www.oie.int/eng/normes/en_amanual.htm