2 Definitions of biosecurity, zoning and compartments, infected zones and diseasefree zones

2.1 Biosecurity

Biosecurity can be broadly described as a strategic and integrated approach that encompasses both policy and regulatory frameworks aimed at analysing and managing risks relevant to human, animal and plant life and health, as well as associated environmental risks (FAO, 2007a). Biosecurity has direct relevance to the sustainability of aquaculture while ensuring protection of public health, the environment and biological diversity.

The term biosecurity in the context of aquatic animal health is used to specifically describe the measures used to prevent introduction of unwanted biological agents, particularly infectious pathogens, and to manage adverse effects associated with contagious agents.
It encompasses both farmed and wild aquatic animals; exotic, endemic and emerging diseases; and is applied from the farm to the ecosystem, and at national and international levels (Scarfe et al., 2009). Subasinghe and Bondad-Reantaso (2006) defined biosecurity in aquaculture as a collective term that refers to the concept of applying appropriate measures (e.g., proactive disease risk analysis) to reduce the probability of a biological organism or agent spreading to an individual, population or ecosystem and to mitigate the adverse impact that may result. Such analysis is done in a way that incorporates best available information on aspects of good husbandry, epidemiology and good science.
Where farming of aquatic animals takes place in open water systems connected to natural waterways, it may be impossible to exclude all infectious diseases.

Biosecurity practices provide the procedures that limit the impact of infectious diseases. These include the prevention, control and eradication of diseases (Scarfe et al., 2009). Farmers are increasingly encouraged, and in some cases mandated, to follow sound biosecurity practices. In order to have a predicted outcome, these should be formulated into a structured biosecurity plan that provides the framework for disease management on the farm and is implemented through documented standard operating procedures. Scarfe et al. (2009) from the source of a river to a barrier that effectively prevents introduction of specific infectious agents (OIE, 2016). Coastal areas and estuaries with precise geographic delineation may also comprise a zone (OIE, 2016). The boundaries of zones must be scientifically justifiable, and should not be based on administrative regions or industry/production-related convenience and needs (OIE, 2016). An integral part of a biosecurity strategy is the ability to identify, maintain and effectively manage subpopulations of aquatic animals relative to the presence or absence of disease within defined zones. For official disease control purposes, it is important that diseases restricted to zones are regulated and compulsorily notifiable under relevant legislation of the concerned country (Subasinghe, McGladdery and Hill, 2004).
propose the following essential elements to ensure that the biosecurity plan is effective, justifiable and useful:
• apply to a defined epidemiological unit or area (compartment) or geographical zone;
• identify specific disease hazards (infectious pathogens);
• evaluate the risk of these hazards to the unit;
• evaluate critical points where diseases can enter or leave the unit;
• evaluate and monitor disease status of the unit;
• have contingency plans in place if disease does break out;
• have written records for third-party auditing and certifying, particularly where markets require live animals or their products to be certified as free of disease or specific pathogens; and
• be transparent and credible.
For purposes of disease control, biosecurity principles should be applied to all levels of aquatic animal disease management, from farm to national and regional levels.
Export markets may source aquatic commodities from countries where establishing and maintaining freedom from a particular disease for the entire country may be difficult, or from regions of the world where countries may not have the infrastructure, expertise and resources needed to provide disease-status guarantees on a national level, as required by the importing countries. In such cases, distinct advantages are linked to applying biosecurity principles to subpopulations of aquatic animals restricted to compartments and zones within the country. With application of relevant biosecurity measures to compartments, trade from an individual farm unit becomes possible, even when a country as a whole is unable to provide guarantees of freedom from diseases of relevance.

 

2.2 Zones

The World Organisation for Animal Health (OIE) defines a zone as a portion of a contiguous water system with a distinct health status with respect to certain diseases (Corsin et al., 2009; OIE, 2016). The recognition of zones is thus based on geographical boundaries (OIE, 2016). A zone may comprise one or more water catchments, from the source of a river to an estuary or lake, or only part of a water catchment

 
2.3 Compartments

On a smaller scale, compartmentalization is based on the concept that animals sharing the waters within the same geographical location will share a common exposure risk to pathogens. The factors defining a compartment are based on management and biosecurity practices, and criteria are established by the relevant aquatic animal health services of a country with the objective of facilitating trade in aquatic animals and their products and as a tool for disease management (OIE, 2016). Compartments are epidemiologic units that define both the disease status of the population and the level of risk for entry of new pathogens (Corsin et al., 2009). For the purpose of international trade, a compartment is defined by the OIE as one or more aquaculture establishments under a common biosecurity management system, containing an aquatic animal population with a distinct health status with respect to a specific disease or diseases for which required surveillance and control measures are applied and basic biosecurity conditions are met (Corsin et al., 2009; OIE, 2016). One or more compartments within the same geographic delineation may make up a zone.

2.4 Infected Zones

An infected zone represents a clearly delineated area within a country or region with shared waterways in which a specific disease has been detected or is established as an endemic infection within the population of farmed or wild aquatic animals (Subasinghe, McGladdery and Hill, 2004). Where possible, eradication or control measures may be implemented. A zone will retain its status as infected until eradication of the disease has been proved through appropriate targeted surveillance.
If eradication is not possible, an infected zone may be surrounded by a clearly demarcated buffer zone that is subject to targeted surveillance for the disease in order to protect surrounding areas with a disease-free status (Subasinghe, McGladdery and Hill, 2004).

2.5 Disease-Free Zones

Zones free of specific diseases may constitute geographical or hydrological areas within which susceptible aquatic animal populations have been shown, through targeted surveillance and protection from exposure, to be free of a specific infectious disease.
Disease-free compartments constitute farms or aquaculture establishments with independent, protected water supplies that meet specific regulated biosecurity and surveillance measures that demonstrate absence of a specific infectious disease and guard against introduction of the disease. For trade purposes, such establishments must be officially registered with the relevant national authority. Facility-based, disease-free compartments may be located within an infected zone or within zones of unknown disease status.
The acceptance of disease-free compartments for international trade purposes opens opportunities for aquaculture producers to access international markets from countries where resources, skilled manpower, infrastructure or hydrologic limitations preclude collection of sufficient national surveillance data to prove absence of disease from larger zones.