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Principles of AAD surveillanceShould be tailored designed
wide variety of species cultured, the pathogens and management systems
Support to domestic production Aid to international trade (international
disease reporting, OIE standards)Moving toward Output based approaches
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Sabina Šerić-HaračićTCP consusltant
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Principles of AAD surveillanceMajor determinants of good AAD surveillance
strategyPlanning AAD surveillance – resources,
sampling, analysis
Presentation outline
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Principles of AAD surveillanceDifferent certification level
Farm accreditationNational/regional disease free statusMonitoring of diseases in environment
May target specific diseaseMay include all diseases (even previously
unknown/unseen)
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Principles of AAD surveillanceAAD surveillance objectives:
Early detection of diseaseDemonstrating freedom of diseaseControl/eradication of disease
AAD monitoring – detection of disease trendsDEALS with endemic diseases
DEALS with Exotic diseases
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Principles of AAD surveillance
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Principles of AAD surveillanceSurveillance to demonstrate freedom from
diseaseOn-going evidenceCertification for zones, regions and
compartmentsThreshold set by design prevalenceNo single survey is enough
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Principles of AAD surveillanceSurveillance for distribution and occurrence
of diseaseprevalence and incidenceMorbidity and mortalityRisk factorsDifferences between epi unitsDays from confirmation to control actions
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Principles of AAD surveillanceSurveillance for distribution and occurrence
of diseaseOn going evidenceDefinition of populationStructured survey/s
+other: sentinel, production records, biosecurity etc.
Representative sampling
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Principles of AAD surveillance
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Principles of AAD surveillance
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Principles of AAD surveillance
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Principles of AAD surveillanceDiagnosticsMore than having good testSurveillanceMore than having right sample size
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Principles of AAD surveillance
Major determinants of good AAD surveillance strategythe definition of population documentation of methodology/sampling diagnosticsdesign prevalence quality assurance systems
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Principles of AAD surveillanceData collection
Targeted and non targetedDisease focusSelection of units
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Principles of AAD surveillance
Methods of data collectionActive
primary purposes of surveillance activities Data tailored to surveillance needs Population based surveys (at slaughter or live
animals) Expensive
Passive Surveillance uses data from other sources (drug
use, farm records, etc.) May lack representativeness, completeness, timing
Collect all data you need, use all data you collected!!!
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Principles of AAD surveillanceIn addition to disease diagnosis data
Epidemiology of diseaseMovement of animals (cultured and wild)History of trade/importCompliance with health regulation
ALL DATA SOURCES SHOULD BE DESCRIBED!!!
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Principles of AAD surveillanceSources of data for AAD surveillance
Laboratory databasesField reporting systemNegative reporting systemProduction recordsSTRUCTURED SURVEYS!!!
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Principles of AAD surveillanceStructured surveys for both exotic and
endemic diseasesDesigned based on hypothesis testing (i.e.
Disease frequency = 0, or < designed prevalence)
Designed base on estimation of population parameters
Surveillance ≠ survey
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Principles of AAD surveillanceSingle stage survey (individual animals)
Certification of batches of animals for exportCertification of single establishment (one
pond/cage)Stratified surveys
By speciesBy regionBy production typeAllows multiple sampling methods to account
for differences
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Current status of diseasePurpose of surveillanceData type and sourcesPopulation, coverage, representativenessQualityApproach/methodology
Planning AAD surveillance
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Disease present Reliably measure disease frequency/trendsMake corrective actionsMonitor effectiveness
Disease absentDemonstrate disease freedomEarly diagnosis
Purpose of surveillance Dependent from disease presence/absenceDependent from certification level
(farm/region/country)
Planning AAD surveillance
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Presentation outlineChallenges of AAD surveillance comparing to
surveillance of terrestrial animal diseases (P9 17.7.2013.)
Principles of AAD surveillanceMajor determinants of good AAD surveillance
strategy (P13 17.7.2013.)Planning AAD surveillance – resources,
sampling, analysis (P13 17.7.2013.)Draft surveillance design (P16 18.7.2013.)
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Types of data collectedDiagnosis disease presence
Surveys, laboratory confirmationClassification – disease stageSyndrome, signs (morts, swimmers)Indirect indicators (growth, feed consumption,
reproductive performance)Risk factors (monitoring of environment)
Planning AAD surveillance
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Proving disease absence – never with 100%
Probabilistic approach i.e. Acceptable probability that surveillance
system will detect disease at designed prevalenceDifferent techniques – same output –
Confidence measure that diseases will be detected
Statistical basis – hypothesis testingNull hypothesis – i.e. Disease is present at equal
or greater than designed prevalenceEvidence to support null hypothesis
Planning AAD surveillance - Survey design -
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Design prevalenceat animal level (i.e. prevalence of infected
animals in a cage): between 1% and 5% for infections that are
present in a small part of the population over 5% for highly transmissible infections
first level of clustering (i.e. proportion of infected farms in a zone) is not greater than 2%
Planning AAD surveillance - Survey design -
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Probability of rejecting true null hypothesis = alfa (α)
1-α = strength of evidence confirming null hypothesis – measure of confidence ≥95% (account for test characteristics)
Reject Ho = disease freedom
Accept false Ho - no international standardsConsequence – more samples
Planning AAD surveillance - Hypothesis testing-
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Estimation of disease prevalence Confidence interval that includes true
prevalenceRepresentative sampleAllowed errorTest characteristics
New techniques still developingUse currently available tools, technically valid
and scientifically based
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Analysis of resultsAccount for survey designAccount for diagnostic test imperfectionAccount for prevalence
Planning AAD surveillance
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Sampling methodTest entire population – censusTest sample
Representative from population Non representative
Large populations - sampling frame not available -multi-stage sampling
Planning AAD surveillance
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Representative sampling – each individual in population has same and equal probability being selected into sampleSimple random saplingSystemic random samplingStratified sampling
Proportionally stratified samplingCluster sampling
Planning AAD surveillance
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Planning AAD surveillance
simple
systemic
stratified
cluster
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Planning AAD surveillance
Random sampling requires having sample framei.e. All individuals/units accessible and identifiedNOTHING IS RANDOM IN RANDOM SAMPLING!!!Alternative use systemic/spatial sampling
Used for wilages/farms/cagesFor individual aquatic animals – no sampling frameUse any method to achieve random selection –
documented and describedConvenience sampling never acceptable
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Non representative samplingSampling of available animals – convenience
samplingTargeted samplingMoribund /with lesions animals
Planning AAD surveillance
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Planning AAD surveillance
Why different sampling methods?Farm management
Size of groups/pools?With/without broodstock?
FeasibilityEnsure randomnessIdentify all animals/groups/farmsAccess to all animals/groups/farms
Disease biologyInfectious vs. Noninfectious diseases
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Sample size factors into account: Imperfection of diagnostic test/s (Se, Sp) the design prevalence/s the level of confidence
Other factors:Population size (acceptable to assume infinitely
large population)power of the survey
Before – standardized tablesNow – tailored calculation based on above
factors
Planning AAD surveillance
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Any procedure that help determining diseases better than by chance alone
Screening and diagnosticGold Standard
Planning AAD surveillance- diagnosistic tests -
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Planning AAD surveillance- diagnosistic tests -No perfect test!!!How much the test can be wrong
OverallDiagnosing diseaseDiagnosing health
Imperfection of test/s for interpretation of surveillance data
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Planning AAD surveillance
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D noD
T+ 45 3 48
T- 5 47 52
50 50 100
Planning AAD surveillance
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D noD
T+ 45 3 48
T- 5 47 52
50 50 100
Planning AAD surveillance
True Prevalence = 50/100Apparent Prevalence = 48/100
Ability of test to diagnose diseaseTest sensitivity=45/50
Ability of test to diagnose healthTest specificity=47/50
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Planning AAD surveillance
Se/Sp must be known for test used to demonstrate freedom
Ideally determined for same population where test is applied
Most use more than one test
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Planning AAD surveillance
Combination of tests increases specificity on the expense of sensitivity and visa verse
Independency of test results – use biologically independent tests
For pooled sample testing use relevant Se/Sp
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Quality of dataDisease present
Prevalence (precision and accuracy)Disease absent
Sensitivity - probability that surveillance sys. Will detect at least one case if disease is present at ≥ design prevalence
Planning AAD surveillance
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Planning AAD surveillance
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Planning AAD surveillance
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Planning AAD surveillance
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Approaches/methodologies of surveillancePassive disease reportingStructured surveys - population based
surveillanceSentinel surveillanceRisk factor surveillanceSyndromic surveillanceParticipatory disease surveillancePost harvest processing
Planning AAD surveillance
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Sentinel surveillanceAlternative for population based surveillanceSelected individuals/establishments
Fewer resources - restricted number of samplesRegular complete reportsOne or more diseasesNONREPRESENTATIVE for entire populationSuitable for high risk groups – exotic diseases, rare diseases
Planning AAD surveillance
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Risk base surveillanceTheory – EASY!
Looks where you expect disease to occurMore sensitivity with less samplesEfficient but cheaper
Practice – LITTLE COMPLICATED!?What is risk?Where does it apply?How to calculate sample size?
Planning AAD surveillance
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Relative risk 8High risk: 10% animals, 80% prevalenceLow risk: 90% animals, 20% prevalenceTrue prevalence 17%Apparent prevalence 80%
Planning AAD surveillance- risk based surveillance -
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Risk - likelihood of adverse eventLikelihood and consequences – result of risk
analysisRisk based surveillance
Risk factors (water temperature, age, moribund, ...)
Difference in risk (with and without risk factor) – relative risk
Sampling contribution of high risk subpopulation
Planning AAD surveillance- risk based surveillance -
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Relative Risk Risk of event (disease) relative to exposure
(risk factor)
Planning AAD surveillance- risk based surveillance -
D noD
Exposed
10 40 50
Non exposed
5 45 50
15 85 100
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Sampling type
High risk population %
Low risk population %
Sample size
%saving
Representative
20 80 331 0
Risk based (RR=3)
50 50 231 30
Risk based (RR=3)
90 10 165 50
Biased (non representative)
10 90 387 -17
Planning AAD surveillance- risk based surveillance -
Prevalence 1%, Test sensitivity 90%
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Planning AAD surveillance- risk based surveillance -
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Syndromic surveillanceEarly detection of outbreaks
a threshold number of early symptomatic casesWell-defined disease or clinical syndromes Indicates unusual clustering or sentinel casesTrends – size, spread and tempoUse existing health data
Planning AAD surveillance
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Participatory surveillanceGive stakeholders greater roleovercomes limitations of conventional
epidemiologydeveloped in small-scale applied to major
international disease control efforts – OIE – rinder pest
Provides: observations, semi-quantitative scores, quantitative data
Planning AAD surveillance