principles of aad surveillance should be tailored designed wide variety of species cultured, the...

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

Sabina Šerić-HaračićTCP consusltant

Principles of AAD surveillanceMajor determinants of good AAD surveillance

strategyPlanning AAD surveillance – resources,

sampling, analysis

Presentation outline

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)

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

Principles of AAD surveillance

Principles of AAD surveillanceSurveillance to demonstrate freedom from

diseaseOn-going evidenceCertification for zones, regions and

compartmentsThreshold set by design prevalenceNo single survey is enough

Principles of AAD surveillanceSurveillance for distribution and occurrence

of diseaseprevalence and incidenceMorbidity and mortalityRisk factorsDifferences between epi unitsDays from confirmation to control actions

Principles of AAD surveillanceSurveillance for distribution and occurrence

of diseaseOn going evidenceDefinition of populationStructured survey/s

+other: sentinel, production records, biosecurity etc.

Representative sampling

Principles of AAD surveillanceDiagnosticsMore than having good testSurveillanceMore than having right sample size


Major determinants of good AAD surveillance strategythe definition of population documentation of methodology/sampling diagnosticsdesign prevalence quality assurance systems

Principles of AAD surveillanceData collection

Targeted and non targetedDisease focusSelection of units


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!!!

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!!!

Principles of AAD surveillanceSources of data for AAD surveillance

Laboratory databasesField reporting systemNegative reporting systemProduction recordsSTRUCTURED SURVEYS!!!

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

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

Current status of diseasePurpose of surveillanceData type and sourcesPopulation, coverage, representativenessQualityApproach/methodology

Planning AAD surveillance

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)


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.)

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)


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 -

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%


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-

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


Analysis of resultsAccount for survey designAccount for diagnostic test imperfectionAccount for prevalence


Sampling methodTest entire population – censusTest sample

Representative from population Non representative

Large populations - sampling frame not available -multi-stage sampling


Representative sampling – each individual in population has same and equal probability being selected into sampleSimple random saplingSystemic random samplingStratified sampling

Proportionally stratified samplingCluster sampling



simple

systemic

stratified

cluster


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

Non representative samplingSampling of available animals – convenience

samplingTargeted samplingMoribund /with lesions animals



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

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


Any procedure that help determining diseases better than by chance alone

Screening and diagnosticGold Standard

Planning AAD surveillance- diagnosistic tests -

Planning AAD surveillance- diagnosistic tests -1 2 3 4 5 6 7 8 9 10

1

2

3

4

5

6

7

8

9

10

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

Planning AAD surveillance- diagnosistic tests -1 2 3 4 5 6 7 8 9 10

1

2

3

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5

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7

8

9

10

D noD

T+ 45 3 48

T- 5 47 52

50 50 100


D noD

T+ 45 3 48

T- 5 47 52

50 50 100


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


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


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

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


Approaches/methodologies of surveillancePassive disease reportingStructured surveys - population based

surveillanceSentinel surveillanceRisk factor surveillanceSyndromic surveillanceParticipatory disease surveillancePost harvest processing


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


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?


Relative risk 8High risk: 10% animals, 80% prevalenceLow risk: 90% animals, 20% prevalenceTrue prevalence 17%Apparent prevalence 80%

Planning AAD surveillancerisk based surveillance -

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


Relative Risk Risk of event (disease) relative to exposure

(risk factor)


D noD

Exposed

10 40 50

Non exposed

5 45 50

15 85 100

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


Prevalence 1%, Test sensitivity 90%

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


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


principles of aad surveillance should be tailored designed wide variety of species cultured, the...

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