Biosecurity programs consist of bioexclusion, surveillance, and biocontainment.
The primary focus of bioexclusion is to limit the level of exposure to disease-causing agents below the threshold for infection or colonization. This requires a systematic approach to preventing pathogen movement across physical or imaginary barriers (protection zones), so as to eliminate or decrease the number of disease-causing organisms within the animal's environment. Sound epidemiologic principles should be used to establish zone boundaries while making use of existing physical/geographic barriers.
The global nature of the animal industry results in daily shipment of animals and animal products throughout the world. This trade is regulated by the World Organization for Animal Health (OIE), and such commerce must comply with OIE regulations, including those pertaining to farm-level control measures.
To control spread of disease, the OIE aids in the establishment of international agreement on animal and plant sanitary (SPS) measures. Such an agreement within the World Trade Organization establishes common definitions and describes OIE in-house procedures for dealing with international trade disputes. This treaty has also established guidelines and principles governing the transparent, objective, and defensible assessment of risk. More specifically, import risk analysis provides importing countries with an objective and defensible assessment of the disease risks associated with importation of such items as animals, animal products, biologic products, genetic material, feedstuffs, and pathologic material.
To account for the difficulties in controlling the disease status and management practices of animal populations across the vast expanse of large countries (eg, the USA), the Terrestrial Code makes allowance for zoning and compartmentalization. The code defines subpopulations based on animal health status, allowing member countries to limit damaging trade effects associated with disease outbreaks, without exposing the importing country to risk from disease spread. There are also “compartments” based on biosecurity procedures and “zones” based on geography. The requirements to establish these subpopulations vary based on disease specifics and the requirements of the trading partners, and they are best decided before disease outbreak. Factors such as disease epidemiology, environmental influences, natural/artificial boundaries, surveillance and monitoring, and applicable biosecurity measures (eg, movement controls and husbandry) are of particular interest. To establish local zones/compartments, the veterinary services of an exporting country must clearly define subpopulations as stipulated in the Code. Relevant claims must be reported to the veterinary services of an importing country and supported by official, detailed documentation.
Zone borders are based on natural, artificial, or legal boundaries, making them relatively easy to establish and officially communicate. Compartments based on biosecurity procedures are more difficult to define. The requirements for a compartment include defining adequate biosecurity plans, operating procedures, and management practices that are well documented (including documentation of compliance). This process involves developing a partnership and clearly stipulated responsibilities between animal owners and the appropriate veterinary authority.
Requirements of biosecurity plans include adequate and robust disease surveillance, animal identification, and traceability. This necessitates detailed records for factors such as animal movement and production, feed sources, sources of replacement stock, disease surveillance (including morbidity and mortality), vaccination and medication history, personnel training, and visitor logs. Risk mitigation also requires that the plan be regularly audited, reviewed, and adjusted as needed.
For purposes of biosecurity, companies operate as compartments within a country, state, or region. Minimal requirements of international trade require that companies conform to international standards, thereby meeting OIE requirements. As compartments within the biosecurity rubric, each trading company, region, state, or country should classify prescribed OIE diseases according to expected prevalence; design, document, and implement a biosecurity plan to prevent/control these diseases; and provide proof of plan compliance. This biosecurity plan forms the basis for all disease prevention, diagnosis, and control strategies. The plan should identify potential pathways for disease introduction/spread within the company and describe the measures being (or that will be) taken to minimize risk as prescribed within the OIE Terrestrial Code.
Bioexclusion begins at the animal confinement facility, which is the smallest epidemiologic unit within the company. The animals within this facility share a common environment, common management practices, and similar likelihood of pathogen exposure. The boundaries of these facilities represent well-defined barriers to entry and an ideal place for implementation of critical control procedures. Every crossing of the facility perimeter should be considered an “event” with potential for pathogen transfer or disease risk.
Reducing the risk from such events requires an “all-in, all-out” strategy. Thorough cleaning, decontamination, and chemical disinfection should be performed when the facility is empty. Once animals arrive, the focus shifts to limiting the number of events (in-house perimeter crossings), as well as the probability of pathogen transmission and infection during unavoidable events.
The next zone/compartment in the disease-control hierarchy is the farm or site. In this case, the epidemiologic unit is the farm rather than the containment facility. There is a defined boundary surrounding the farm, and the farm animals share a common environment with common management practices, thereby sharing a common likelihood of exposure to pathogens from nearby containment facilities.
The farm/site establishes real (eg, fence) or imaginary boundaries that serve as access points to the secondary control zone, which is a critical zone for disease control. From a biosecurity standpoint, the site is considered “closed” if the farmer/producer enforces full biosecurity with no uncontrolled access after disinfection. The site is considered “open” if general biosecurity is enforced at the start of transfer/depletion, with access granted only to necessary vehicular traffic. The site is “fully open” if routine control is enforced from the point of last animal removal. Sites involved in an outbreak should remain closed until the responsible veterinarian declares them clean.
Groups of animals that share a communal animal handling facility (ie, a complex) constitute a tertiary control zone. The complex represents its own epidemiologic unit, because sites/farms within it share facilities such as feed mills and processing plants. Production processes within the complex (eg, rearing farms and grow-out farms) similarly constitute separate epidemiologic units. All the defined areas may represent tertiary control zones for biosecurity purposes. Tertiary zones are seldom fenced, so access boundaries are typically imaginary.
As a cost-effectiveness measure, tertiary zones are often established around high-value sectors of the operation, such as the valuable stock used in breeding operations. To reduce transmission risk, critical control points such as transit facilities can be established beyond the site perimeter. At these points, procedures such as showering, changing into protective clothing, and transfer to site/zone-dedicated transport significantly reduce the probability of disease transmission onto the site.
The terms monitoring and surveillance are both used to describe ongoing data collection to estimate disease prevalence and severity in a population. However, a monitoring program is typically geared toward collecting statistically reliable prevalence data that can be used to track trends in disease incidence and severity over time. A surveillance program is based on prevalence data from a readily available sample of the population, with the goal being timely action to correct perceived increases in disease incidence. The importance of these programs increases when remote management-control is needed to keep up with increases in herd/flock size and production.
When catastrophic outbreaks require eradication, the surveillance program should be focused toward detecting source cases, allowing implementation of biocontainment through quarantine and slaughter. For situations in which eradication is not required, collection of prevalence data should be adjusted so as to differentiate background variation from effects of disease.
The data collection system should be designed so as to provide insightful and epidemiologically informative disease indicators. During design, it is good to keep in mind the need for important parameters (eg, sample size) that can be used to calculate other animal-health estimates, such as prevalence, incidence, morbidity, mortality, and herd/flock immunity (as indicated by antibody titers, farm production records, etc). In the absence of random sampling, such statistical estimates cannot be taken as absolutely accurate but may serve as adequate markers of the need for intervention.
For purposes of disease eradication and trade, it may be necessary to demonstrate freedom from infection in the country, zone, or compartment/company. Although there may be no obvious evidence of infection in the population, it is impossible to definitively prove freedom from infection unless a perfect (100% sensitive/specific) test is used to examine every member of the population. In this situation, the surveillance system should be able to statistically estimate to an acceptable level of confidence that infection is below a specified prevalence level.
As part of health tracking, individuals should be monitored for disease at regular intervals. Any change in prevalence suggests a change in incidence that may require corrective action to prevent disease spread. The frequency at which animals are monitored depends on disease epidemiology and the level of biocontainment (eg, need for quarantine or slaughter). Factors such as latent period, mode of transmission (eg, vertical or horizontal), potential for animal dispersion and tracking, and test sensitivity are important considerations.
Biocontainment strategies reduce the consequence of disease challenge by limiting the opportunity for challenge (bioexclusion), enhancing resistance (immunization), and preventing spread (quarantine). When eradication is required, quarantine is usually followed by emergency slaughter. Control measures are routinely applied for endemic diseases but more sporadically for epidemic outbreaks.
The term quarantine refers to the practice of enforced isolation of animals exposed to infectious agents, as well as the place in which these animals are isolated and the time period of isolation. Quarantine is routinely required when live animals or their products are imported. To avoid disease entry into a country, region, zone, compartment, or population, potentially infectious animals/material must be isolated until they have been shown to be disease free.
Enforced isolation is the first biocontainment step when potentially infected animals are brought to a production setting. Movement within or through the control area is restricted and monitored. The size/nature of the control zone depends on disease risk but usually involves a containment facility, farm, site, or complex within a particular company. The control zone may be expanded to a 2-mile radius for diseases of national/regional importance.
Disease monitoring is used to establish the extent of an outbreak, first within the quarantine zone and then in a well-defined surrounding contact zone. The relevant veterinary authority assumes control in cases of foreign/notifiable disease. All vested parties (eg, countries, states, regions, companies, owners) should prearrange an emergency response plan sufficient to address relevant details of containment and eradication.
Medication may be added to feed or water to reduce disease risk. Vaccination is commonly used to reduce the risk and/or consequence of infection among exposed individuals or populations. The main purpose of immunization is to prevent clinical illness by raising the ID50 of the population. Vaccinations can be used to protect individuals from disease or to protect the next generation by limiting direct vertical transmission and enhancing maternal antibody transfer.