Terrestrial Animal Health Code
Animal health surveillance
Introduction and objectives
In general, surveillance is
aimed at demonstrating the absence of disease or infection,
determining the presence or distribution of disease or infection or
detecting as early as possible exotic or emerging diseases.
The type of surveillance applied
depends on the outputs needed to support decision-making. The following
recommendations may be applied to all diseases or infections and
all susceptible species (including wildlife).
The general recommendations in this chapter may be refined by the
specific approaches described in the disease chapters. Where detailed
disease or infection-specific
information is not available, suitable approaches should be based
on the recommendations in this chapter.
Animal health surveillance is
also a tool to monitor disease trends, to facilitate the control
of disease or infection, to
provide data for use in risk analysis,
for animal or public health purposes, and to substantiate the rationale
for sanitary measures.
Both domestic animals and wildlife are
susceptible to certain diseases or infections. However, the
presence of a disease or infection in wildlife does
not mean it is necessarily present in domestic animals in
the same country or zone or
vice versa. Wildlife may
be included in a surveillance system
because they can serve as reservoirs of infection and
as indicators of disease risk to humans
and domestic animals. Surveillance in wildlife presents
challenges that may differ significantly from those in surveillance in
Prerequisites to enable a Member Country
to provide information for the evaluation of its animal health status
that the Member Country complies
with Chapter 3.1.;
that, where possible, surveillance data
be complemented by other sources of information, such as scientific publications,
research data, documented field observations and other non-survey
The objectives of this chapter are to:
The following definitions apply for the purposes
of this chapter:
’Bias’: means a tendency of an estimate to deviate in one direction from a true value.
’Confidence’: means the probability that the type of surveillance applied would detect the presence of infection if the population were infected and is equivalent to the sensitivity of the surveillance. Confidence depends on, among other parameters, the assumed prevalence of infection.
means a sampling strategy in which every unit has a known non-zero
probability of inclusion in the sample.
’Sample’: means the
group of elements (sampling units) drawn from a population, on which
tests are performed or parameters measured to provide surveillance information.
means the unit that is sampled, either in a random survey or in
This may be an individual animal or a group
of animals, such as
an epidemiological unit.
Together, they comprise the sampling frame.
the proportion of truly positive units that are correctly identified
as positive by a test.
’Specificity’: means the proportion of truly negative units that are correctly identified as negative by a test.
means the population from which surveillance data
are derived. This may be the same as the target population or a
subset of it.
means a method of surveillance that
may involve one or more component activities that generates information
on the health or disease status of animal populations.
’Survey’: means an
investigation in which information is collected systematically usually
carried out on a sample of a defined population group, within a
defined time period.
means the population about which conclusions are to be inferred.
’Test’: means a procedure
used to classify a unit as either positive, negative or suspect
with respect to a disease or an infection.
’Test system’: means 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:
In this chapter, surveillance activities are classified as being based on:
structured population-based surveys, such as:
structured non-random surveillance activities, such as:
disease reporting or notifications;
control programmes or health schemes;
targeted testing or screening;
ante-mortem and post-mortem inspections;
laboratory investigation records;
biological specimen banks;
farm production records;
wildlife disease data.
In addition, surveillance data
should be supported by related information, such as:
data on the epidemiology of
the disease or infection, including
environmental, host population distribution, and climatic information;
data on animal movements, including transhumance and natural wildlife migrations;
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
biosecurity measures in place; and
the likelihood and consequence of disease or infection introduction.
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 sources of the data, when the data were collected, and a consideration of any biases that may be inherent in the system.
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.
Time frame (or temporal values of surveillance
The relevant epidemiological units for the surveillance system should be defined to ensure that it is appropriate to meet the objectives of surveillance. 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.
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 or flock, 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.
Where one exists, the case definition in the specific chapter of the Terrestrial Code should be used. If the Terrestrial Code does not give a case definition, a case should be defined using clear criteria for each disease or infection under surveillance. For wildlife disease or infectionsurveillance, it is essential to correctly identify and report host animal taxonomy (including genus and species).
Surveillance data should be analysed using appropriate methodologies, and at the appropriate organisational level 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 different host species, pathogenic
agents, production systems and surveillance systems,
and types and amounts of data and information available.
The methodology used should be based on the best
information available. It should also be in accordance with this
chapter, 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.
Surveillance involves the detection of disease or infection in accordance with appropriate case definitions and 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, its specificity and predictive values. Imperfect sensitivity 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 sensitivity and specificity values of the tests used should be specified for each species in which they may be used, and the method used to estimate these values should be documented. Alternatively, where sensitivity or specificity values of a particular test are specified in the Terrestrial Manual, these 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
Surveillance systems should incorporate the principles of quality assurance. They should 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.
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; this requires effective collaboration
among all stakeholders, such as governmental or non-governmental
organisations, and others, particularly for data involving wildlife;
the ability of the data processing system
to detect missing, inconsistent or inaccurate data, and to address
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 discussed in Article 1.4.3., the following should be considered 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 two ways:
non-probability based sampling methods, such as:
probability based sampling methods, such as:
simple random selection;
Periodic or repeated surveys conducted in order to document disease freedom should be conducted 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 given to any biases that may be inherent in the survey design.
The population of epidemiological units should first be clearly defined; hereafter appropriate sampling units should be defined for each stage, depending on the design of the survey.
The design of the survey will depend on the size, structure and degree of understanding of the population being studied, the epidemiology of the infection and the resources available.
Data on wildlife population size often do not exist. However, they should be determined to the extent possible before the survey is designed. The expertise of wildlife biologists may be sought in the gathering and interpretation of such population data. Historical population data should be updated since these may not reflect current populations.
The objective of sampling from a population is to select a subset of units that is representative of the population of interest with respect to the objective of the study. Sampling should provide the best likelihood that the sample will be representative of the population, within the practical constraints imposed by different environments and production systems.
Specimens of wildlife may be available from sources such as hunters and trappers, road-kills, wild animalmeat markets, sanitary inspection of hunted animals, morbidity-mortality observations by the general public, wildlife rehabilitation centres, wildlife biologists and wildlife agency field personnel, farmers, and other landholders, naturalists and conservationists. Wildlife data such as census data, trends over time, and reproductive success can be used in a manner similar to farm production records for epidemiological purposes.
When selecting epidemiological units from within a population, probability sampling, such as 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.
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, such as disease reporting systems, sentinel sites and testing schemes.
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).
Whenever the responsibility for disease notification
falls outside the scope of the Veterinary Authority,
for example in some countries for diseases in wildlife,
effective communication and data sharing should be established with
the relevant authorities to ensure comprehensive and timely disease
Control programmes and 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 and 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 slaughterhouses/abattoirs may provide valuable surveillance data. The sensitivity and specificity of slaughterhouse/abattoir inspection for detecting the presence of specified diseases should be pre-determined for the inspection system in place. The accuracy of the inspection system will be influenced by:
the training, experience and number of the inspection staff;
the involvement of the Competent Authority in the supervision of ante-mortem and post-mortem inspections;
the quality of construction of the slaughterhouse/abattoir, speed of the slaughter chain, lighting quality, etc.; and
staff morale and motivation for efficient performance.
Slaughterhouse/abattoir inspections are likely to provide good coverage for particular age groups and geographical areas only. Slaughterhouse/abattoirsurveillance data are subject to biases in relation to target populations (e.g. only animals of a particular class and age are likely to be slaughtered for human consumption in significant numbers). Such biases need to be recognised when analysing surveillance data.
For traceback and analysis of spatial and herd or flock-level coverage, there should be, if possible, an effective identification system that relates animals in the slaughterhouse/abattoir to their 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 slaughterhouse/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 or sites involve the identification and regular testing of one or more of animals of known health or immune status in a specified geographical location to detect the occurrence of disease or infection (usually serologically). They are particularly useful for surveillance for diseases or infections which have a strong spatial component, such as vector-borne diseases or infections. 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 or infection.
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 standardised case definition is applied. Education of potential field observers in application of the case definition and reporting is important. 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 or infection 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.
Specimens from wildlife for disease or infectionsurveillance may be available from sources such as hunters and trappers, road-kills, wild animalmeat markets, sanitary inspection of hunted animals, morbidity and mortality observations by the general public, wildlife rehabilitation centres, wildlife biologists and wildlife agency field personnel, farmers and other landholders, naturalists and conservationists. Wildlife data such as census data, trends over time, and reproductive success can be used in a manner similar to farm production records for epidemiological purposes.
Critical elements for structured non-random surveillance
There are a number of critical factors which should be taken into account when using structured non-random surveillance data. These include 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 sources can, however, be a cost-efficient method of early detection, and may increase the level of confidence or detect a lower level of prevalence compared to random sampling surveys.
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 a shorter period of time.
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 or infection
Requirements to declare a country or a zone free from disease or infection without pathogen specific surveillance
This article provides general principles for declaring a country or a zone free from disease or 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 Article 1.4.3. and the following premises:
in the absence of disease and vaccination, the animal population would become susceptible over a period of time;
the pathogenic 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 or infection over a long period of time in a susceptible population can be substantiated by effective disease investigation and reporting by a Member Country.
there has never been occurrence of disease, or
eradication has been achieved or the disease or infection has ceased to occur for at least 25 years,
provided that for at least the past 10 years:
the disease has been a notifiable disease;
an early detection system has been in place for all relevant species;
Last occurrence within the previous 25 years
Countries or zones that have achieved eradication (or in which the disease or 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, countries should follow the general recommendations on surveillance outlined in this chapter provided that for at least the past 10 years:
the disease has been a notifiable disease;
an early detection system has been in place;
measures to prevent the introduction of the disease or infection introduction have been in place;
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:
the disease is a notifiable disease;
an early detection system is in place;
measures to prevent the introduction of the disease or infection are in place;
vaccination against the disease is not applied;
infection is known not to be established in wildlife. It can be difficult to collect sufficient epidemiological data to prove absence of disease or infection in wild animal populations. In such circumstances, a range of supporting evidence should be used to make this assessment.
Self declaration of freedom from disease or infection
A Member Country may make a self declaration in accordance with Chapter 1.6. that its entire territory, a zone or a compartment is free from a listed disease, based on the implementation of the provisions of the Terrestrial Code and the Terrestrial Manual. The Veterinary Authority may wish to transmit this information to OIE Headquarters, which may publish the information.
International recognition of disease or infection free status
For diseases for which procedures exist whereby the OIE can officially recognise the existence of a disease or infection free country or zone, a Member Country wishing to apply for recognition of this status should, via its Permanent Delegate, send to the OIE all the relevant documentation relating to the country or zone concerned. Such documentation should be presented in accordance with the recommendations prescribed by the OIE for the appropriate animal diseases.
Demonstration of freedom from infection
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. Therefore, demonstrating freedom from infection involves providing sufficient evidence to demonstrate (to a level of confidence acceptable to Member Countries) that infection with a specified pathogenic agent, if present, is present in less than a specified proportion of the population.
However, finding evidence of infection at any prevalence in the target population automatically invalidates any freedom from infection claim unless otherwise stated in the relevant disease chapter. The implications for the status of domestic animals of disease or infection present in wildlife in the same country or zone should be assessed in each situation, as indicated in the relevant chapter on each disease in the Terrestrial Code.
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 the distribution and occurrence of infection, disease or of other relevant health-related events is used to assess progress and aid in decision making in the control or eradication of selected diseases or infections. It also has relevance for the international movement of animals and products.
In contrast to surveillance to demonstrate freedom from infection, surveillance used to assess progress in control or eradication of selected diseases or infections is usually designed to collect data about a number of variables such as:
prevalence or incidence of infection;
morbidity and mortality rates;
frequency distribution of antibody titres;
farm production records;
nb: first adopted in 2005; most recent update adopted in 2012.
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