Terrestrial Animal Health Code
Terrestrial Animal Health Code |
Introduction and objectives
In general, surveillance is aimed at demonstrating the absence of disease or infection, determining the occurrence or distribution of disease or infection, while also detecting as early as possible exotic or emerging diseases. The type of surveillance applied depends on the desired outputs needed to support decision-making. The following recommendations may be applied to all diseases, their agents and susceptible species as listed in the Terrestrial Code, and are designed to assist with the development of surveillance methodologies. Except where a specific surveillance method for a certain disease or infection is already described in the Terrestrial Code, the recommendations in this Chapter may be used to further refine the general approaches described for a specific disease or infection. Where detailed disease/infection-specific information is not available, suitable approaches should be based on the recommendations in this Chapter.
Animal health surveillance is an essential component necessary to detect diseases, to monitor disease trends, to control endemic and exotic diseases, to support claims for freedom from disease or infection, to provide data to support the risk analysis process, for both animal health and/or public health purposes, and to substantiate the rationale for sanitary measures. Surveillance data underpin the quality of disease status reports and should satisfy information requirements for accurate risk analysis both for international trade as well as for national decision-making.
Essential prerequisites to enable an OIE Member to provide information for the evaluation of its animal health status are:
that the particular Member complies with the provisions of Chapter 3.1. of the Terrestrial Code on the quality and evaluation of the Veterinary Services;
that, where possible, surveillance data be complemented by other sources of information (e.g. scientific publications, research data, documented field observations and other non-survey data);
that transparency in the planning and execution of surveillance activities and the analysis and availability of data and information, be maintained at all times, in accordance with Chapter 1.1. of the Terrestrial Code.
The objectives of this Chapter are to:
provide guidance to the type of outputs that a surveillance system should generate;
provide recommendations to assess the quality of disease surveillance systems.
Definitions
The following definitions apply for the purposes of this Chapter:
Bias: A tendency of an estimate to deviate in one direction from a true value.
Case definition: A case definition is a set of criteria used to classify an animal or epidemiological unit as a case.
Confidence: In the context of demonstrating freedom from infection, confidence is the probability that the type of surveillance applied would detect the presence of infection if the population were infected. The confidence depends on, among other parameters, the assumed level of infection in an infected population. The term refers to confidence in the ability of the surveillance applied to detect disease, and is equivalent to the sensitivity of the surveillance system.
Early detection system: A system for the timely detection and identification of an incursion or emergence of disease/infection in a country, zone or compartment. An early detection system should be under the control of the Veterinary Services and should include the following characteristics:
representative coverage of target animal populations by field services;
ability to undertake effective disease investigation and reporting;
access to laboratories capable of diagnosing and differentiating relevant diseases;
a training programme for veterinarians, veterinary para-professionals and others involved in handling animals for detecting and reporting unusual animal health incidents;
the legal obligation of private veterinarians in relation to the Veterinary Authority;
timely reporting system of the event to the Veterinary Services;
a national chain of command.
Outbreak definition: An outbreak definition is a set of criteria used to classify the occurrence of one or more cases in a group of animals or units as an outbreak.
Probability sampling: A sampling strategy in which every unit has a known non-zero probability of inclusion in the sample.
Sample: The group of elements (sampling units) drawn from a population, on which tests are performed or parameters measured to provide surveillance information.
Sampling units: The unit that is sampled, either in a random survey or in non-random surveillance. This may be an individual animal or a group of animals (e.g. an epidemiological unit). Together, they comprise the sampling frame.
Sensitivity: The proportion of truly positive units that are correctly identified as positive by a test.
Specificity: The proportion of truly negative units that are correctly identified as negative by a test.
Study population: The population from which surveillance data are derived. This may be the same as the target population or a subset of it.
Surveillance: The systematic ongoing collection, collation, and analysis of data, and the timely dissemination of information to those who need to know so that action can be taken.
Surveillance system: A method of surveillance that may involve one or more component activities that generates information on the health, disease or zoonosis status of animal populations.
Survey: An investigation in which information is systematically collected, usually carried out on a sample of a defined population group, within a defined time period.
Target population: The population about which conclusions are to be inferred.
Test: A procedure used to classify a unit as either positive, negative or suspect with respect to an infection or disease.
Test system: A combination of multiple tests and rules of interpretation which are used for the same purpose as a test.
Principles of surveillance
Types of surveillance
Surveillance may be based on many different data sources and can be classified in a number of ways, including:
the means by which data are collected (active versus passive surveillance);
the disease focus (pathogen-specific versus general surveillance); and
the way in which units for observation are selected (structured surveys versus non-random data sources).
In this Chapter, surveillance activities are classified as being based on:
EITHER
structured population-based surveys, such as:
systematic sampling at slaughter;
random surveys;
OR
structured non-random surveillance activities, such as:
disease reporting or notifications;
control programmes/health schemes;
targeted testing/screening;
ante-mortem and post-mortem inspections;
laboratory investigation records;
biological specimen banks;
sentinel units;
field observations;
farm production records.
In addition, surveillance data should be supported by related information, such as:
data on the epidemiology of the infection, including environmental, host population distribution, and climatic information;
data on animal movements and trading patterns for animals and animal products;
national animal health regulations, including information on compliance with them and their effectiveness;
history of imports of potentially infected material; and
biosecurity measures in place.
The sources of evidence should be fully described. In the case of a structured survey, this should include a description of the sampling strategy used for the selection of units for testing. For structured non-random data sources, a full description of the system is required including the source(s) of the data, when the data were collected, and a consideration of any biases that may be inherent in the system.
Critical elements
In assessing the quality of a surveillance system, the following critical elements need to be addressed over and above quality of Veterinary Services (Chapter 3.1.).
Populations
Ideally, surveillance should be carried out in such a way as to take into account all animal species susceptible to the infection in a country, zone or compartment. The surveillance activity may cover all individuals in the population or part of them. When surveillance is conducted only on a subpopulation, care should be taken regarding the inferences made from the results.
Definitions of appropriate populations should be based on the specific recommendations of the disease Chapters of the Terrestrial Code.
Epidemiological unit
The relevant epidemiological unit for the surveillance system should be defined and documented to ensure that it is representative of the population. Therefore, it should be chosen taking into account factors such as carriers, reservoirs, vectors, immune status, genetic resistance and age, sex, and other host criteria.
Clustering
Infection in a country, zone or compartment usually clusters rather than being uniformly or randomly distributed through a population. Clustering may occur at a number of different levels (e.g. a cluster of infected animals within a herd, a cluster of pens in a building, or a cluster of farms in a compartment). Clustering should be taken into account in the design of surveillance activities and the statistical analysis of surveillance data, at least at what is judged to be the most significant level of clustering for the particular animal population and infection.
Case and outbreak definitions
Clear and unambiguous case and outbreak definitions should be developed and documented for each pathogen under surveillance, using, where they exist, the standards in the Terrestrial Code.
Analytical methodologies
Surveillance data should be analysed using appropriate methodologies, and at the appropriate organisational levels to facilitate effective decision making, whether it be planning interventions or demonstrating status.
Methodologies for the analysis of surveillance data should be flexible to deal with the complexity of real life situations. No single method is applicable in all cases. Different methodologies may be needed to accommodate the relevant pathogens, varying production and surveillance systems, and types and amounts of data and information available.
The methodology used should be based on the best available information that is in accord with current scientific thinking. The methodology should be in accordance with this Chapter and fully documented, and supported by reference to the scientific literature and other sources, including expert opinion. Sophisticated mathematical or statistical analyses should only be carried out when justified by the proper amount and quality of field data.
Consistency in the application of different methodologies should be encouraged and transparency is essential in order to ensure fairness and rationality, consistency in decision making and ease of understanding. The uncertainties, assumptions made, and the effect of these on the final conclusions should be documented.
Testing
Surveillance involves the detection of disease or infection by the use of appropriate case definitions based on the results of one or more tests for evidence of infection or immune status. In this context, a test may range from detailed laboratory examinations to field observations and the analysis of production records. The performance of a test at the population level (including field observations) may be described in terms of its sensitivity and specificity and predictive values. Imperfect sensitivity and/or specificity will have an impact on the conclusions from surveillance. Therefore, these parameters should be taken into account in the design of surveillance systems and analysis of surveillance data.
The values of sensitivity and specificity for the tests used should be specified, and the method used to determine or estimate these values should be documented. Alternatively, where values for sensitivity and/or specificity for a particular test are specified in the Terrestrial Manual, these values may be used as a guide.
Samples from a number of animals or units may be pooled and subjected to a testing protocol. The results should be interpreted using sensitivity and specificity values that have been determined or estimated for that particular pool size and testing procedure.
Quality assurance
Surveillance systems should incorporate the principles of quality assurance and be subjected to periodic auditing to ensure that all components of the system function and provide verifiable documentation of procedures and basic checks to detect significant deviations of procedures from those documented in the design.
Validation
Results from animal health surveillance systems are subject to one or more potential biases. When assessing the results, care should be taken to identify potential biases that can inadvertently lead to an over-estimate or an under-estimate of the parameters of interest.
Data collection and management
The success of a surveillance system is dependent on a reliable process for data collection and management. The process may be based on paper records or computerised. Even where data are collected for non-survey purposes (e.g. during disease control interventions, inspections for movement control or during disease eradication schemes), the consistency and quality of data collection and event reporting in a format that facilitates analysis, is critical. Factors influencing the quality of collected data include:
the distribution of, and communication between, those involved in generating and transferring data from the field to a centralised location;
the ability of the data processing system to detect missing, inconsistent or inaccurate data, and to address these problems;
maintenance of disaggregated data rather than the compilation of summary data;
minimisation of transcription errors during data processing and communication.
Structured population-based surveys
In addition to the principles for surveillance discussed above, the following recommendations should be used when planning, implementing and analysing surveys.
Types of surveys
Surveys may be conducted on the entire target population (i.e. a census) or on a sample. A sample may be selected in either of the two following ways:
non-probability based sampling methods, such as:
convenience;
expert choice;
quota;
probability based sampling methods, such as:
simple random selection;
cluster sampling;
stratified sampling;
systematic sampling.
Non-probability based sampling methods will not be discussed further.
Periodic or repeated surveys conducted in order to document disease freedom should be done using probability based sampling methods so that data from the study population can be extrapolated to the target population in a statistically valid manner.
The sources of information should be fully described and should include a detailed description of the sampling strategy used for the selection of units for testing. Also, consideration should be made of any biases that may be inherent in the survey design.
Survey design
The population of epidemiological units should first be clearly defined; hereafter sampling units appropriate for each stage, depending on the design of the survey, should be defined.
The design of the survey will depend on the size and structure of the population being studied, the epidemiology of the infection and the resources available.
Sampling
The objective of sampling from a population is to select a subset of units from the population that is representative of the population with respect to the object of the study such as the presence or absence of infection. Sampling should be carried out in such a way as to provide the best likelihood that the sample will be representative of the population, within the practical constraints imposed by different environments and production systems. In order to detect the presence of an infection in a population of unknown disease status, targeted sampling methods that optimise the detection of infection can be used. In such cases, care should be taken regarding the inferences made from the results.
Sampling methods
When selecting epidemiological units from within a population, probability sampling (e.g. simple random selection) should be used. When this is not possible, sampling should provide the best practical chance of generating a sample that is representative of the target population.
In any case, the sampling method used at all stages should be fully documented and justified.
Sample size
In general, surveys are conducted either to demonstrate the presence or absence of a factor (e.g. infection) or to estimate a parameter (e.g. the prevalence of infection). The method used to calculate sample size for surveys depends on the purpose of the survey, the expected prevalence, the level of confidence desired of the survey results and the performance of the tests used.
Structured non-random surveillance
Surveillance systems routinely use structured non-random data, either alone or in combination with surveys.
Common non-random surveillance sources
A wide variety of non-random surveillance sources may be available. These vary in their primary purpose and the type of surveillance information they are able to provide. Some surveillance systems are primarily established as early detection systems, but may also provide valuable information to demonstrate freedom from infection. Other systems provide cross-sectional information suitable for prevalence estimation, either once or repeatedly, while yet others provide continuous information, suitable for the estimate of incidence data (e.g. disease reporting systems, sentinel sites, testing schemes). Surveillance systems routinely use structured non-random data, either alone or in combination with surveys.
Disease reporting or notification systems
Data derived from disease reporting systems can be used in combination with other data sources to substantiate claims of animal health status, to generate data for risk analysis, or for early detection. Effective laboratory support is an important component of any reporting system. Reporting systems relying on laboratory confirmation of suspect clinical cases should use tests that have a high specificity. Reports should be released by the laboratory in a timely manner, with the amount of time from disease detection to report generation minimized (to hours in the case of introduction of a foreign animal disease).
Control programmes / health schemes
Animal disease control programmes or health schemes, while focusing on the control or eradication of specific diseases, should be planned and structured in such a manner as to generate data that are scientifically verifiable and contribute to structured surveillance.
Targeted testing / screening
This may involve testing targeted to selected sections of the population (subpopulations), in which disease is more likely to be introduced or found. Examples include testing culled and dead animals, swill fed animals, those exhibiting clinical signs, animals located in a defined geographic area and specific age or commodity group.
Ante-mortem and post-mortem inspections
Inspections of animals at abattoirs may provide valuable surveillance data. The sensitivity and specificity of the particular slaughterhouse inspection system for detecting the presence of infectious agents of surveillance interest under the particular inspection arrangements applying in a country should be pre-determined by the Competent Authority if the data is to be fully utilised. The accuracy of the inspection system will be influenced by:
the level of training and experience of the staff doing the inspections, and the ratio of staff of different levels of training;
the involvement of the Competent Authorities in the supervision of ante-mortem and post-mortem inspections;
the quality of construction of the abattoir, speed of the slaughter chain, lighting quality, etc.; and
staff morale/motivation for accurate and efficient performance.
Abattoir inspections are likely to provide good coverage only for particular age groups and geographical areas. Abattoir surveillance data are subject to obvious biases in relation to target and study populations (e.g. only animals of a particular class and age may be slaughtered for human consumption in significant numbers). Such biases need to be recognized when analysing surveillance data.
Both for traceback in the event of detection of disease and for analysis of spatial and herd-level coverage, there should be, if possible, an effective identification system that relates each animal in the abattoir to its locality of origin.
Laboratory investigation records
Analysis of laboratory investigation records may provide useful surveillance information. The coverage of the system will be increased if analysis is able to incorporate records from national, accredited, university and private sector laboratories. Valid analysis of data from different laboratories depends on the existence of standardised diagnostic procedures and standardised methods for interpretation and data recording. As with abattoir inspections, there needs to be a mechanism to relate specimens to the farm of origin.
Biological specimen banks
Specimen banks consist of stored specimens, gathered either through representative sampling or opportunistic collection or both. Specimen banks may contribute to retrospective studies, including providing support for claims of historical freedom from infection, and may allow certain studies to be conducted more quickly and at lower cost than alternative approaches.
Sentinel units
Sentinel units/sites involve the identification and regular testing of one or more of animals of known health/immune status in a specified geographical location to detect the occurrence of disease (usually serologically). They are particularly useful for surveillance of diseases with a strong spatial component, such as vector-borne diseases. Sentinel units provide the opportunity to target surveillance depending on the likelihood of infection (related to vector habitats and host population distribution), cost and other practical constraints. Sentinel units may provide evidence of freedom from infection, or provide data on prevalence and incidence as well as the distribution of disease.
Field observations
Clinical observations of animals in the field are an important source of surveillance data. The sensitivity and specificity of field observations may be relatively low, but these can be more easily determined and controlled if a clear, unambiguous and easy to apply standardised case definition is applied. Education of potential field observers in application of the case definition and reporting is an important component. Ideally, both the number of positive observations and the total number of observations should be recorded.
Farm production records
Systematic analysis of farm production records may be used as an indicator of the presence or absence of disease at the herd or flock level. In general, the sensitivity of this approach may be quite high (depending on the disease), but the specificity is often quite low.
Critical elements for structured non-random surveillance
There is a number of critical factors which should be taken into account when using structured non-random surveillance data such as coverage of the population, duplication of data, and sensitivity and specificity of tests that may give rise to difficulties in the interpretation of data. Surveillance data from non-random data sources may increase the level of confidence or be able to detect a lower level of prevalence with the same level of confidence compared to structured surveys.
Analytical methodologies
Different methodologies may be used for the analysis of non-random surveillance data.
Different scientifically valid methodologies may be used for the analysis of non-random surveillance data. Where no data are available, estimates based on expert opinions, gathered and combined using a formal, documented and scientifically valid methodology may be used.
Combination of multiple sources of data
The methodology used to combine the evidence from multiple data sources should be scientifically valid, and fully documented including references to published material.
Surveillance information gathered from the same country, zone or compartment at different times may provide cumulative evidence of animal health status. Such evidence gathered over time may be combined to provide an overall level of confidence. For instance, repeated annual surveys may be analysed to provide a cumulative level of confidence. However, a single larger survey, or the combination of data collected during the same time period from multiple random or non-random sources, may be able to achieve the same level of confidence in just one year.
Analysis of surveillance information gathered intermittently or continuously over time should, where possible, incorporate the time of collection of the information to take the decreased value of older information into account. The sensitivity, specificity and completeness of data from each source should also be taken into account for the final overall confidence level estimation.
Surveillance to demonstrate freedom from disease/infection
Requirements to declare a country, zone or compartment free from disease/infection without pathogen specific surveillance
This Article provides general principles for declaring a country, zone or compartment free from disease/infection in relation to the time of last occurrence and in particular for the recognition of historical freedom.
The provisions of this Article are based on the principles described in Article 1.4.3. of this Chapter and the following premises:
in the absence of disease and vaccination, the animal population would become susceptible over a period of time;
the disease agents to which these provisions apply are likely to produce identifiable clinical signs in susceptible animals;
competent and effective Veterinary Services will be able to investigate, diagnose and report disease, if present;
the absence of disease/infection over a long period of time in a susceptible population can be substantiated by effective disease investigation and reporting by a Member.
Historically free
Unless otherwise specified in the relevant disease Chapter, a country, zone or compartment may be recognised free from infection without formally applying a pathogen-specific surveillance programme when:
there has never been occurrence of disease, or
eradication has been achieved or the disease/infection has ceased to occur for at least 25 years,
provided that for at least the past 10 years:
it has been a notifiable disease;
an early detection system has been in place;
measures to prevent disease/infection introduction have been in place; no vaccination against the disease has been carried out unless otherwise provided in the Terrestrial Code;
infection is not known to be established in wildlife within the country or zone intended to be declared free. (A country or zone cannot apply for historical freedom if there is any evidence of infection in wildlife. However, specific surveillance in wildlife is not necessary.)
Last occurrence within the previous 25 years
Countries, zones or compartments that have achieved eradication (or in which the disease/infection has ceased to occur) within the previous 25 years, should follow the pathogen-specific surveillance requirements in the Terrestrial Code if they exist. In the absence of specific requirements for surveillance in the Terrestrial Code, countries should follow the general recommendations on surveillance to demonstrate animal health status outlined in this Chapter provided that for at least the past 10 years:
it has been a notifiable disease;
an early detection system has been in place;
measures to prevent disease/infection introduction have been in place;
no vaccination against the disease has been carried out unless otherwise provided in the Terrestrial Code;
infection is not known to be established in wildlife within the country or zone intended to be declared free. (A country or zone cannot apply for freedom if there is any evidence of infection in wildlife. However, specific surveillance in wildlife is not necessary.)
Recommendations for the discontinuation of pathogen-specific screening after recognition of freedom from infection
A country, zone or compartment that has been recognised as free from infection following the provisions of the Terrestrial Code may discontinue pathogen-specific screening while maintaining the infection-free status provided that:
it is a notifiable disease;
an early detection system is in place;
measures to prevent disease/infection introduction are in place;
vaccination against the disease is not applied;
infection is known not to be established in wildlife. (Specific surveillance in wildlife has demonstrated the absence of infection.)
International recognition of disease/infection free status
For diseases for which procedures exist whereby the OIE can officially recognise the existence of a disease/infection free country, zone or compartment, a Member wishing to apply for recognition of this status shall, via its Permanent Delegate, send to the OIE all the relevant documentation relating to the country, zone or compartment concerned. Such documentation should be presented according to the recommendations prescribed by the OIE for the appropriate animal diseases.
Demonstration of freedom from infection
A surveillance system to demonstrate freedom from infection should meet the following requirements in addition to the general requirements for surveillance outlined in Article 1.4.3. of this Chapter.
Freedom from infection implies the absence of the pathogenic agent in the country, zone or compartment. Scientific methods cannot provide absolute certainty of the absence of infection. Demonstrating freedom from infection involves providing sufficient evidence to demonstrate (to a level of confidence acceptable to Members) that infection with a specified pathogen is not present in a population. In practice, it is not possible to prove (i.e., be 100% confident) that a population is free from infection (unless every member of the population is examined simultaneously with a perfect test with both sensitivity and specificity equal to 100%). Instead, the aim is to provide adequate evidence (to an acceptable level of confidence), that infection, if present, is present in less than a specified proportion of the population.
However, finding evidence of infection at any level in the target population automatically invalidates any freedom from infection claim unless otherwise stated in the relevant disease Chapter.
Evidence from targeted, random or non-random data sources, as stated before, may increase the level of confidence or be able to detect a lower level of prevalence with the same level of confidence compared to structured surveys.
Surveillance for distribution and occurrence of infection
Surveillance to determine distribution and occurrence of infection or of other relevant health related events is widely used to assess progress in the control or eradication of selected diseases and pathogens and as an aid to decision making. It has, however, relevance for the international movement of animals and products when movement occurs among infected countries.
In contrast to surveillance to demonstrate freedom from infection, surveillance used to assess progress in control or eradication of selected diseases and pathogens is usually designed to collect data about a number of variables of animal health relevance, for example:
prevalence or incidence of infection;
morbidity and mortality rates;
frequency of disease/infection risk factors and their quantification;
frequency distribution of herd sizes or the sizes of other epidemiological units;
frequency distribution of antibody titres;
proportion of immunised animals after a vaccination campaign;
frequency distribution of the number of days elapsing between suspicion of infection and laboratory confirmation of the diagnosis and/or to the adoption of control measures;
farm production records, etc.
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