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What is disease surveillance?

Surveillance is a mechanism applied to collect and interpret data on the health of animal populations, to accurately describe their health status with respect to specific diseases of concern. This can be based on historic scientific evidence for absence, under certain circumstances, of clinical cases of a virulent disease of the susceptible species. Targeted surveillance to prove absence of infection by specific pathogens may be used to reinforce inconclusive general (passive) and/or historic evidence. As indicated in the definition section, the term surveillance is used for the detection of new or exotic diseases, while monitoring is aimed at detecting increases in established or endemic infection levels that may signal the recurrence of a disease outbreak. The term surveillance programme is often used to incorporate both surveillance and monitoring activities.

The concept of surveillance is shown diagrammatically in Figure 1. Stakeholders include people whose livelihoods depend on consistent aquatic animal productivity, government regulators with the responsibility for protection of trade and wild and cultured resources, and environmental protection interests. One of the most critical factors behind this overview is the feedback loop. Rapid and transparent communication between the stakeholders and surveillance managers is essential for data accuracy and effectiveness for disease control use. Many countries and international organizations make mandatory reporting of disease outbreaks a legal requirement to ensure diseases cannot occur and go unreported. Regardless of legislative support, however, it is in the interest of all stakeholders to report any health concern immediately, since the cost of delayed intervention always outweighs the cost of early intervention. In countries where mandatory reporting is not yet legislated, Good Management Practices (GMPs) at the farm level can be used to assist effective health management. Good Management Practices are particularly effective where farms operating under such programmes can command stronger market positions or site licensing costs. This can address “self-policing” concerns and promotes rapid voluntary reporting.

Figure 2 shows the relationships among the components of a surveillance program, including effective surveillance, host population and environmental factors.

Surveillance and monitoring require trained expertise, suitably equipped laboratories, legal support structures, transport and communication networks, etc. Effective application of this support infrastructure requires a good knowledge of susceptible/carrier host populations and their local environments. Building on this foundation are the various surveillance and monitoring activities which lead to accurate data and knowledge of the location and pathologic significance of pathogens of concern. Last, but not least, the information collected and analysed must be communicated to relevant stakeholders, including surveillance personnel. This completes the feedback required for reducing the risk of disease transfer with movements of live aquatic animals for all purposes.

Figure 1: Overview of disease surveillance.

Figure 2: Relationships among different components of a surveillance programme.

Importance of disease surveillance

An ongoing problem for setting guidelines for low risk trade practices is that the disease situation is never static. A number of factors contribute to changes in disease status, proliferation and spread. These include:

Globalization: There is a continuing integration and inter-dependence of world markets and economies. This includes the current increase in trade of live animals, their products, animal feeds and related food items. Concomitant with faster and higher volume shipping, transportation, and human travel, is a significant increase in opportunity for introduction of “new”[5] diseases and infections.

Increasing aquaculture production: Aquaculture is presently the fastest growing food sector in the world, with production almost trebling since 1990 to reach 45.71 million tonnes in 2000. Developing countries are providing the bulk of this increase in production. Pressures from continued growth at this rate raise the risks due to rapid changes in production systems and the emergence and spread of “new” diseases.

Microbial adaptation: Micro-organisms have a remarkable ability to adapt to changes in their environment. For example, the widespread use of antibiotics in the treatment of human and animal diseases has led to the appearance of problematic drug resistant organisms. Similarly, global warming and other climatic changes are beginning to show evidence of facilitating alterations in the geographical distribution of some pathogens and their vectors.

These issues suggest that the disease situation in aquaculture will continue to change in an unpredictable way. Some recent examples of diseases that have emerged as aquaculture has developed are shown in Table 1.

Table 1. Examples of important diseases that have emerged for finfish, crustaceans and molluscs.




1. Epizootic Ulcerative Syndrome (EUS)
2. Viral Nervous Necrosis (VNN)
3. Koi Herpes Virus (KHV)

1. White Spot Disease (WSD)
2. Yellowhead Disease (YHD)
3. Taura Syndrome (TS)

1. Multinucleate Sphere X (MSX) disease
2. Bonamiosis

Enhanced trade in aquatic animal commodities (live, fresh and frozen product) has lead to increasing scrutiny of the risks of spread of diseases along with this trade. This, in turn, has highlighted the need for more effective systems for investigating, reporting and responding to significant aquatic animal disease threats. Reliable evidence for freedom from particular diseases is a major challenge behind development of such programmes. Current international animal trade agreements (notably the WTO-SPS Agreement) require scientific justification for restrictions on trade on animal health grounds, such as a clear risk to the freedom of a country or territory from a particular disease or a potential threat to the effectiveness of an official control programme for the disease. Supporting international standards for aquatic animal diseases (OIE Aquatic Animal Health Code and OIE Manual of Diagnostic Tests for Aquatic Animal Diseases) recommend certain requirements for surveillance to support a country’s declaration of freedom from a disease[6]. This is necessary for any country wishing to impose protective measures to prevent exposure via imports from countries where the disease of concern exists. In order for a country, regions or other zone, to make informed decisions on preventive or remedial actions, it is essential to have effective means of identifying and tracking diseases, as well as assessing their effects.

Disease surveillance is a fundamental component of any official aquatic animal health protection programme. Such surveillance forms the basis for early warning of imminent or emerging disease outbreaks; planning and maintaining disease control programmes; provision of sound (data-based) aquatic animal health advice to farmers and environmental interests; certification of exports; international reporting and verification of freedom from diseases of concern. Proactive surveillance, prior to any emergency disease outbreak, provides the data essential to respond immediately and effectively to isolate the source and identify the extent of the problem. Knowing the source and extent of a disease outbreak saves valuable time and effort in focusing control efforts on the areas most required.

Purpose and objectives of surveillance

A clear definition of objective(s) is of prime importance for effective surveillance. The following are summaries from Consultation discussions, along with pertinent reports on the issue.

The primary purpose of aquatic animal disease surveillance is to provide scientifically accurate, cost-effective, information for assessing and managing risks of disease transfer associated with trade (intra- and international) in aquatic animals and animal production efficiency and public health. This statement of purpose is consistent with the WTO-SPS Agreement, the OIE Aquatic Code, and international understanding of what disease surveillance is meant to achieve in both terrestrial and aquatic production systems.

Diseases that warrant surveillance programmes should be those that pose a significant threat to trade, productivity (wild or cultured) and/or public health. These may be diseases listed by the OIE, or other diseases of special concern within a country. The objectives which define surveillance for aquatic animal diseases are:

These objectives define what surveillance is meant to achieve, whether undertaken to describe the distribution and prevalence of an important disease, to ensure that disease zones are maintained, or to assess success of eradication, fallowing or other disease control measures.

Types of surveillance

Many terms have been applied to describe different types of surveillance, reflecting the various objectives of surveillance. Terms such as passive surveillance, active surveillance, general surveillance, targeted surveillance and, more recently, scanning surveillance (Scudamore 2002) are used throughout the literature. Since they are frequently interchanged, or used without clear definition, a brief explanation for each is given below. A more complete discussion on passive and active surveillance is provided by Cameron (2002). A comprehensive surveillance programme can comprise of a combination of many approaches to the gathering of surveillance data.

Passive surveillance is the secondary use of data routinely collected for some other purpose. This specific disease information is a “by-product” of more general disease investigations, e.g. routine gathering of information on disease incidents reported by farmers and field officers, or results from specimens submitted to diagnostic laboratories, or examined for research purposes. General surveillance is very useful for early detection of emerging diseases and often provides a general picture of the disease situation in a population. However, it provides negligible quantifiable data on infection levels, or reliable data on the full geographic distribution, of the disease. This type of surveillance cannot be used to reliably demonstrate absence of a particular disease from a given area. Because of this its broad non-specific nature, general surveillance is sometimes called “passive” or “scanning” surveillance.

Targeted surveillance involves planned collection of precise field data on the presence of a specific disease or pathogen within a defined population. Active disease surveillance programmes may be (i) “catch all” - aimed at detecting any significant disease occurrences; (ii) may target specific diseases; or (iii) may monitor the progress of specific disease control or eradication efforts. This kind of surveillance can provide the data required to prove that the specified populations are free of a specific disease. In order to maximize the value of targeted surveillance, it should be based on survey techniques which provide representative samples of the susceptible population of interest. Sampling techniques are aimed at maximizing the likelihood of pathogen detection, based on available epidemiological information.

Since general surveillance is not always completely “passive” and targeted surveillance can include activities other than planned active surveillance (e.g. investigation of disease outbreak reports), clear understanding of surveillance activities can be complicated and hybrid terms such as targeted active surveillance may appear. To avoid confusion in the context of this report, the definitions and terminology below are used throughout the rest of this document.

General surveillance can be used to develop targeted surveillance programs. For example, routine health checks of dying oysters in Atlantic Canada revealed the presence of MSX disease for the first time in Canadian waters. This then triggered a targeted surveillance programme to define the extent of the spread of the disease in Canadian oyster populations. The design of the programme was strongly based on historic data as well as current oyster transfers throughout the Atlantic region (Stephenson et al. 2003).

Box 1. General and targeted surveillance

General (passive) surveillance is an ongoing observation of the endemic disease profile of a susceptible population, so that unexpected and/or abnormal changes can be detected and acted upon as rapidly as possible. In addition, laboratory diagnostic data may be used to define a threshold level of undiagnosed syndromes which would trigger in-depth investigations to try and characterize them. For example, if gill disease in fish exceeded a given prevalence, this could trigger a diagnostic investigation to determine whether or not this is indicative of a "new" disease. Such surveillance of disease syndromes (common clinical signs) could also be collected by fisheries officers or harvesters/farmers.

Targeted surveillance collects information on a specific disease or condition so that its presence within a defined population can be measured, or its absence can be substantiated.

What is zoning and what are zones?

Disease zoning is a tool that can be used to facilitate domestic, as well as international trade, whilst preventing spread of diseases of concern. Zones defined by appropriate surveillance mechanisms as being free of such diseases (uninfected) may be used to facilitate trade and to protect against the introduction of their causative pathogens. Zones defined as having the presence of a specific pathogen may also have unrestricted transfers to zones positive for the same pathogen (“like-to-like” disease profiles). Thus, a zone which is positive for a disease is not, necessarily subject to cessation of trade, although it could necessitate mitigative conditions, such as movement of surface disinfected eggs only, to prevent spread of vertically-transmitted viral agents of diseases of concern to uninfected zones or countries.

Disease zoning is the process of delineating infected and uninfected populations within a country or group of countries. “Infected zone” and “uninfected zone” usually applies to specific diseases, except on the rare occasion where a range of different diseases share common epidemiological characteristics or can be detected using common diagnostic (non-disease-specific) techniques. An uninfected zone can be established within a country using the health status of a susceptible host species for a specific disease within a particular geographic or hydrographic area. The OIE provides an outline of the zoning concept in Chapter 1.4.4 of the Code, in the section on import risk analysis. Zoning is particularly relevant to control of aquatic animal diseases, since these do not readily respond to disease control measures used for isolation and containment in land-based facilities or for terrestrial animals.

Disease zones are usually clearly delineated geographical areas within a country; but they can also cross country borders. Catchment areas and rivers may be used to define continental zones, whereas coastal zones can be based on tidal and oceanographic water movements (that may span large areas). Coastal zonation for specific diseases is often further complicated by migratory hosts or poorly understood reservoir species. The tools used for delineation of zones must be relevant to the purpose of zoning, i.e., ability to detect infections early (sensitive), thereby

As different diseases have different means of spread, effective delineation of a zone depends on applying tools that are relevant for the particular disease of concern.

Importance of zoning

Historically, the occurrence of a disease within a nation’s borders has lead to suspension or restrictions on trade of that species (or products from it) from the whole country. Recently, however, geographical, hydrological and climatic barriers have been recognized as being just as effective in delineating and controlling the spread of disease from an affected area - effectively isolating it within a zone within a given country. Furthermore, recognition of the biological basis for variations in disease occurrence is a first step in the concept of zoning for aquatic animal health management. An effective zoning scheme can allow surrounding uninfected zones within the country to continue trading while the “infected” zone is placed under appropriate disease control measures, including trade or movement restrictions. Zoning is equally applicable and effective for preventing spread and reducing economic losses due to diseases of concern within a country.

How are zone boundaries defined?

For terrestrial animals, an infected zone may simply be defined as an area of a specified radius around an infected property. For aquatic animals, however, delineation of zone boundaries is more difficult. The simplest freshwater zonation system is farms that obtain their incoming water from an unshared river system, an independent reservoir of surface water, spring or borehole supply. In such situations, zoning can effectively be facility-based, thus, animal transfers can continue even when neighbouring (unconnected hydrographically) facilities are infected. In inland situations, however, most aquaculture facilities are connected to common river systems or other shared waterways, through which the disease agent can be transmitted to wild aquatic animal populations, or to other farms located downstream. The minimum “zone” that can be applied to a freshwater aquatic animal disease, in such circumstances, would be the entire river system or water catchment area. Some massive water catchment areas, such as the Mekong Delta and Great Lakes often require consideration of multi-national and regional political jurisdictions. Disease management on one side of a shared water body, where none exists on the other side, may not be an effective way to manage disease. Therefore, political cooperation (intra- or international) is required.

Zoning in marine and estuarine areas is also complicated, depending on oceanographic characteristics, and vector/host distribution and characteristics, as well as, in many instances, shared political boundaries and unrelated human activities (recreational, shipping, etc.).

Types of zones

In a country wishing to establish zones for controlling a particular aquatic animal disease, the disease must be compulsorily notifiable. This is necessary to prevent “hidden” or “unreported” outbreaks of the disease detracting from the efficacy of surveillance and the disease response mechanisms associated with it. Both the WTO and OIE base their standards on the assumption that disease control is always more effective with rapid and open reporting from affected stakeholders. Where such notification is not compulsory, or clearly legislated, however, surveillance can still be initiated with stakeholders who agree with the objectives. In such an instance, “self-policing” provides a temporary balance until mandatory reporting, can develop legislative support. The reporting can also be built into good management practices (GMPs) or formal registration and licensing of farms/sites.

The size, location, delineation and management requirements for different types of zones vary with the disease they are meant to control. The extent of zones and their limits should be established by the CA of the country and enforced by national legislation, but also clearly delineated by natural, artificial or legal boundaries, which are scientifically justifiable. Strict conditions for disease surveillance and data management must be met to support the disease-free status, including mandatory reporting or equivalent mechanism that ensures all significant disease outbreaks are rapidly investigated by a laboratory capable of diagnostics that meet national or international standards (directly or though regional/national reference laboratories). This includes appropriate immediate reporting to the CA for aquatic animal disease management and control, when necessary.

Zones free of specific diseases

Aquaculture facilities - farms or establishments located within an infected zone, but having a protected independent water supply, and meeting other stocking conditions. Each facility can demonstrate freedom from the disease of concern, thus, can supply other farms free of the specific disease(s) within that country or in other countries officially free of the disease. Strict conditions for disease surveillance and data management must be met to support the disease-free status for aquaculture establishments within an area endemic for the disease. Facility-based zonation can be applied equally to diseases of national or regional concern within a country, but cannot be applied to facilities in open-estuaries or coastal waters, where isolation from wild populations or other cultured stocks is impossible.

Hydrographic areas - areas for within which susceptible aquatic animal populations (cultured or wild) can be demonstrated to be free of a specific disease of concern (national, regional or OIE listed) through targeted surveillance and protection from exposure to populations or stocks from “infected zones” (as described below).

Buffer zones for specific diseases

A zone - often referred to as a “surveillance zone” (somewhat confusingly, since all zones de facto require some degree/level of surveillance) - that is established in an uninfected zone surrounding an infected zone. Surveillance within this zone helps maintain accurate delineation of the uninfected zone. In a disease outbreak situation, a buffer zone can be established around an identified infected zone, to control spread of the disease while surveying for the actual extent of spread from the known infected area.

Infected zones for specific diseases

An “infected zone” is a zone where a specific disease:

If eradication is possible, e.g. in an aquaculture facility; control measures to attempt to eradicate the infectious agent may be undertaken. The zone will maintain its “infected” status until eradication of the disease agent is proven through targeted surveillance appropriate for demonstration of disease absence. In open-water/flow-through situations where eradication is impossible, delineation of the infected zone is maintained by general surveillance of the zone and targeted surveillance of the surrounding buffer zone.

Movement of animals between zones

Ongoing management is essential to prevent live aquatic animals from being transported from infected to uninfected zones, including into buffer zones. Likewise, it is necessary to control shipments of other known vectors of the disease, e.g., genetic material, vaccines, pathological material and aquatic animal feedstuffs, between infected and uninfected zones.

Obviously, “susceptibility” can span the range of non-clinical carriers of the disease agent (disease tolerance), to true resistance (uninfected). This acknowledges that the same species may have disease resistant/tolerant stocks, as well as naïve, or vulnerable stocks. Thus, targeted surveillance of individual populations is an essential pre-requisite for assessing true susceptibility. The general principle for movement between different zones is shown in Figure 3.

Relationship of surveillance and zoning to import risk analysis

Countries that are members of WTO are obliged to follow various multilateral agreements, including the WTO-SPS Agreement. The WTO-SPS Agreement recognises the OIE as the international organisation responsible for the development and promotion of standards applicable to international animal health recommendations affecting trade in live animals and animal products. The OIE Code provides guidelines for national CA’s for addressing the principles laid out in the SPS Agreement for aquatic animal diseases of trade concern. Section 1.4 of the OIE Code provides a framework for analysing the risks of international transfer of disease deemed to be of trade significance by the OIE Aquatic Animal Health Standards Commission (AAHSC, formerly known as the Fish Disease Commission). Diseases that pose the highest assessed risk are those that: (a) are highly infectious to species (wild or cultured) of economic or ecological importance; (b) have few, if any, effective control options; and (c) have a high risk of establishing endemic infections either in susceptible and/or reservoir host species. The threat can be direct, causing significant disease loss, or indirect, affecting domestic or international markets for live or processed products. Arrows indicate the direction of low risk transfers consistent with general principles of disease control, while broken lines indicate movements that may have mitigative measure options to reduce disease risks, or risks posed by unknown disease status. Surveillance zone/farm = Buffer zone/farm[7].

Another, often underestimated, economic impact is that of devaluation of product value by consumer perception. That is that product from “diseased populations” is of poor/lower quality than that of “disease-free” sources, even where the disease has no human health or seafood quality significance.

Figure 3. Transfers of live aquatic animals between countries, zones and farms of different health knowledge status (Diagram reproduced from Anon. 2000).

All these risk factors are of pivotal importance in selecting the diseases that warrant surveillance and zonation. The OIE Code lists several criteria by which a disease is assessed as being of sufficient risk to warrant listing in the Code (OIE 2003b). These criteria are:

Diagnosis - A repeatable, robust means of detection/diagnosis exists.

In addition, the OIE Code provides criteria for urgent notification of aquatic animal diseases (OIE 2003b). They are:

A. For listed diseases:

B. For non-listed diseases:

Emerging disease/pathogenic agent if there are findings that are of epidemiological significance to other countries

Under the OIE Code, risk analysis has four major components. These are:

Hazard identification and risk assessment are most meaningful when based on an accurate understanding of the health status of the relevant aquatic animal populations in both importing and exporting waters (zones, regions or countries). This requires data from comprehensive and effective targeted and/or general surveillance programs. Likewise, the disease status for defining individual zones can only be established by accurate analysis and interpretation of the surveillance data. Once zones are established, some level of ongoing surveillance of uninfected zones is required to verify the uninfected status of the zone and ensure early detection of any change in that status.

The relationships among these components are shown in Figure 4.

Figure 4. Import risk assessment components and risk assessment steps.

[5] Previously unobserved, unreported or undocumented infections.
[6] 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 -
[7] Anon. 2000. Aquaplan Zoning Policy Guidelines. Agriculture, Fisheries, Forestry - Australia, Canberra, Australia. 41 pp.

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