Pathogen Risk Analysis for Biosecurity and the Management of Aquatic Animal Movements

Training Course

Introduction to the Use of Risk Analysis in Aquaculture

1Part 8: Pathogen Risk Analysis Risk AssessmentAn ounce of prevention is worth a pound of cure.[ancient proverb]

Henry de Bracton, De Legibus, 1240 2Risk AssessmentHow likely is it to go wrong?The process of evaluating the likelihood that a potential hazard will gain entry into the importing country andEstimating the biological and economic consequences of its entry, establishment and/or spread.

Import Risk Analysis Framework

4Modified from OIE (2008).

The framework consists of four major components.Simplified Hypothetical Risk Analysis Process5. Identify Stakeholders.6. Inform Stakeholders of Project & Seek Comments on Preliminary Hazard Identification7. Conduct the Detailed Hazard Identification8. Conduct the Risk Assessment Release assessmentExposure assessmentConsequence assessmentRisk estimation5Release assessment has apparently been renamed as Entry assessment in the 2012 version of the Code.Risk AssessmentConsists of four interrelated steps:release assessmentexposure assessmentconsequence assessmentrisk estimation

Release is the movement of the pathogen from the source in the exporting country to the border of the importing country.

Exposure is the movement of the pathogen from the border of the importing country to its establishment and/or spread in aquacultured and/or wild populations.

Consequence assessment estimates the biological, social and economic consequences of release and exposure.

Risk estimation combines the estimates of likelihood of release, likelihood of exposure and magnitude of consequences to give a final risk estimate of unmitigated risk (i.e.the level of risk before application of risk management measures).6Risk AssessmentRisk Assessments can be:Qualitative (e.g., high, medium, low)orQuantitative (e.g., 1 outbreak of disease/per 100,000 tonnes of imported fish/year)Both processes are equally valid - therefore one is not innately better than the other.Note that one short-coming of qualitative risk assessment is that it is difficult to incorporate the effect of volume of trade into the risk estimate.

Qualitative risk analysis uses non-numerical estimators, while Quantitative RA uses numbers.

The volume of trade is an important component of risk.

There is also an intermediate method semi-quantitative risk analysis.7Qualitative Risk AnalysisRelease and Exposure Assessment use various terms to describe the likelihood of an event occurring. Example with 5 terms: High: Event would be expected to occurModerate: There is less than an even chance of the event occurringLow: Event would be unlikely to occurVery low: Event would rarely occurNegligible: Chance of event occurring is so small that it can be ignored in practical termsSome risk analyses use fewer or more categories. Its important to define what you mean by each term at the beginning of the risk analysis.

These are the categories that we will use in this training course and the associated Working Group activities.8Risk AssessmentPreparation of scenario trees aid in outlining pathways and estimating likelihoods for release and exposureNote that risks are assessed individually for each hazard identified

A scenario tree shows all the possible pathways a pathogen could move by.

A pathway is a single means of movement.

Each pathway will have a number of steps that must occur, and each step will have an associated estimate of likelihood of being completed.

9Release AssessmentIf we bring in the commodity, how likely is it that the pathogen will enter with it?

Involves a detailed discussion of the biological pathways necessary for the commodity to become infected and estimating the likelihood of these events occurring.As previously mentioned, the end point of release is usually considered to be arrival at the border of the importing country. 10Release AssessmentThis means consideration of:biological factorsinfectivityvirulence and stabilityroute(s) of infectionmeans of transmission,susceptibility of animalsoutcomes of infection, etc.11Release AssessmentThis means consideration of:country factorsevaluation of exporting countrys Competent Authoritypresence of surveillance, control & eradication programszoningprevalence and incidenceanimal demographics, etc. etc.In practice, much information is typically lacking.

Prevalence and Incidence have quite different meanings and should not be used interchangeably. Veterinarians should know this, but fisheries officers may not.

Prevalence is the percentage of infected animals occurring in a given population at one point in time. Example, 15 of 25 fish collected on March 2, 1998 were infected with Clonorchis sinensis, giving a prevalence of 60%.

Incidence is the number of new outbreaks of a disease occurring in a population over a specified time period. Example, 5 outbreaks of whitespot disease were reported in a population of shrimp over the three-month period 1 March 31 May, 2008.

12Release AssessmentThis means consideration of:commodity factorslife cycle stage to be movedease of infection (e.g., host resistance)proposed sourcestorage and transport methods, etc.

The analysis is concluded if the likelihood of release is negligible

Life cycle stage is highly important some pathogens are not carried by e.g. fertilized eggs or larval stages.13Simplified Release ScenarioTree for for a Virus

The probability that release will occur is: P = (L1 x L2 x L3 x L4) + (L1 x L2 x L5).

14A Matrix of Rules for Combining Descriptive LikelihoodsNegligibleVery LowLowModerateHigh

High

Negligible Very Low

Low

Moderate

High

ModerateNegligible

Very Low

Low

Low

LowNegligible

Very Low

Very Low

Very Low

Negligible

Negligible

Negligible

Negligible

From the various pathways steps in a scenario tree, this table can be used to calculate the overall likelihood for the release estimate and the exposure estimate. This table provides guidance on how to combine descriptive likelihoods. Modified from AFFA 2001 draft Guidelines for Import Risk Analysis Table 5, p. 41.15Example Calculation Based on the Hypothetical Pathway for Viral ReleaseStep

QualitativeDescriptorProduct of Likelihoods

L1 Selection of Infected Spawners

LowL2 Selection of Infected PLs

High>

Low

L3 Selection of Infected Subsample

High....>

Low

L4 Pathogens not detected by genomic testing (and shipped to Cook Islands)Very Low..>

Very Low

Assumptions:

prevalence in broodstock is low transmission from female to PL is high diagnostic test has very high sensitivity

16Exposure Assessment Now that the pathogen has successfully gained entry to the country, what are the chances than susceptible native or cultured stocks will be exposed to it?

Involves a detailed discussion of the biological pathways necessary for aquatic animals in the importing country to become infected17Exposure AssessmentFor each potential hazard:Describe the biological pathway(s) necessary for exposure of aquatic animals in the importing countryEstimate the likelihood of these exposure(s)If the likelihood of exposure is negligible, the risk assessment for the hazard is completed.Note that exposure does not imply infection of the exposed stockThe procedure is essentially the same as that used for Release Assessment.18Exposure AssessmentRelevant factors include:Biological factorsCountry factorsCommodity factorsSimilar to those discussed under Release Assessment.19Simplified Exposure Scenario Tree for a Virus

The diagram shows 2 possible pathways of movement, via the cultured animals (A) and via the culture water (B). 20A Matrix of Rules for Combining Descriptive LikelihoodsNegligibleVery LowLowModerateHigh

High

Negligible Very Low

Low

Moderate

High

ModerateNegligible

Very Low

Low

Low

LowNegligible

Very Low

Very Low

Very Low

Negligible

Negligible

Negligible

Negligible

Note that this is the same table used for estimating pathogen releaseThis table (the same as previously given) provides guidance on how to combine descriptive likelihoods. From the various pathways steps in a scenario tree, it can be used to calculate the overall likelihood for the release estimate and the exposure estimate.

Modified from AFFA 2001 draft Guidelines for Import Risk Analysis Table 5, p. 41.21Consequence AssessmentWhat would be the consequences of its going wrong?Identify the consequences of the pathogens entry, establishment & spread (i.e. potential biological, environmental & economic impacts)Estimate the likelihood of these potential consequences occurring.Consequence AssessmentConsequences can be both direct & indirect:Direct consequences include:outcome of infections in local fish stocks (morbidity, mortality, production losses, public health impacts, environmental impacts, etc.)Indirect consequences, include:economic considerations such as control & eradication costs, compensation, surveillance costs, potential trade losses, etc.Consequence Assessment : Qualitative RankingsExample 5 category system:

CatastrophicHighModerateLowNegligible

Terms are defined in terms of the biological and economic consequences and the difficulty of controlAgain, there may be fewer or more categories depending on the risk analyst.

In all cases, the terms need to be defined.24Consequence Assessment: Qualitative RankingsCatastrophic: Disease would significantly harm economic performance at an industry level &/or cause serious & irreversible environmental harm.

High: Disease would have serious biological consequences (e.g. high mortality or morbidity) & would not be amenable to control or eradication. It could significantly harm economic performance at an industry level &/or cause serious environmental harm.

Moderate: Diseases would have less pronounced biological consequences & may be amenable to control or eradication. It could harm economic performance at an industry level &/or cause some environmental effects, which would not be serious or irreversible.

Again, there may be fewer or more categories depending on the risk analyst.

In all cases, the terms need to be defined.25Consequence Assessment : Qualitative RankingsLow: Diseases would have mild biological consequences & would normally be amenable to control or eradication. Effects on economic performance & the environment would not be serious or irreversible.Negligible: Diseases would have no significant biological , ecological or economic consequences & would not require control or eradication.Again, there may be fewer or more categories depending on the risk analyst.

In all cases, the terms need to be defined.

26Steps in Consequence AssessmentEstimate the likelihood that at least one animal becomes infected.Identify the biological, environmental and economic consequences associated with the entry, establishment or spread of the hazard and their likely magnitude.Estimate the likelihood of the occurrence of these consequences.Note that The first consequence of interest is the successful infection of at least one animal..

See OIE handbook, vol. I.

27Consequence AssessmentNote that there are various methods for estimating the magnitude and likelihood of a consequence, and for combining consequences. Outbreak scenarios can be useful in estimating likelihood and magnitude.Similarly, consequences can be examined at different levels: farm/village, district, regional, national.In some cases it may be necessary to have the assistance of experts in economics and social sciences to estimate the potential magnitude of consequences.Again, there may be fewer or more categories depending on the risk analyst.

In all cases, the terms need to be defined.

28Risk EstimationIntegrates the results of release assessment, exposure assessment & consequence assessment to produce an overall estimate of the risk posed by the hazard.

First we combine the likelihood or release with the likelihood of exposure, then we combine the result with the estimated level of consequence.Combining Likelihood of Release and Probability of ExposureNegligibleVery LowLowModerateHigh

High

Negligible Very Low

Low

Moderate

High

ModerateNegligible

Very Low

Low

Low

LowNegligible

Very Low

Very Low

Very Low

Negligible

Negligible

Negligible

Negligible

Likelihood of Exposure

Likelihood of ReleaseThis table combines the release estimate and the exposure estimate. (its content is the same as the matrix previously presented). Since we are combining likelihoods (probabilities), the result will be equal or lower that the lower of the two estimates. There is an element of subjectivity in this table.

Note that where Release or Exposure was determined to be Negligible the risk analysis for the hazard would have been terminated at that point.

Modified from AFFA 2001 draft Guidelines for Import Risk Analysis, Table 5, p. 41.30Estimating Total Risk posed by a HazardNegligibleLow Moderate High Catastrophic

High

Negligible RiskLow Risk

Moderate Risk

HighRiskExtremeRisk

ModerateNegligibleRiskLowRiskModerateRisk

High Risk

Extreme Risk

LowNegligible RiskVery Low Risk

LowRisk

ModerateRisk

High Risk

Very Low

Negligible RiskNegligible Risk

Very LowRisk

LowRisk

ModerateRisk

Negligible

Negligible RiskNegligible RiskNegligible RiskNegligible RiskNegligibleRiskEstimated Likelihood of Release & Exposure

Estimated Consequence of Release & ExposureLikelihood combines the release estimate and the exposure estimates.Note that where Release or Exposure was determined to be Negligible the risk analysis for the hazard would have been terminated earlier.

Modified from AFFA 2001 draft Guidelines for Import Risk Analysis, Table 13, p. 64.31Practical Example: Risk Assessment for WSSV in Giant River PrawnRelease Assessmentcircumstantial evidence indicates Fijian stock unlikely to be exposedmaintained in relative isolation since importation from Tahiti in 1977No reports of WSSV from TahitiFijian stock believed never to have been in contact with penaeid shrimpno evidence that wild or cultured penaeids in Fiji have been exposed to WSSVTherefore, likelihood of WSSV being present in the stock of origin is LOWNote that determination of likelihood is quite subjective and the risk analysts have erred on the side of caution.32Practical Example: Risk Assessment for WSSV in Giant River PrawnExposure AssessmentIf PL introduced to Rarotonga are infected with WSSV, the likelihood of the pathogen escaping the culture facility & gaining access to susceptible populations is estimated as LOW, because:animals will be maintained in covered concrete tanks, making escape unlikelyeffluent waters will not enter natural waters

Later events (a typhoon) were to show the fallacy of these assumptions. catastrophic events occur with higher frequency than might be assumed.

Remember, Exposure and Release were not based on scenario trees but on logical discussion.33Practical Example: Risk Assessment for WSSV in Giant River PrawnConsequence AssessmentWSSV does not cause overt disease or mortalities in river prawnno penaeid culture in Cook Islands; thus any impact will be on natural stocks (which are very limited)potential exists for WSSV to become endemic, with possible negative impacts on future aquaculture developmentThe consequences of this occurring were considered Moderate

Practical Example: Risk Assessment for WSSV in Giant River PrawnEstimating riskLikelihood of Release LowLikelihood of Exposure LowLikelihood of Release and Exposure LowEstimate of Consequence ModerateEstimate of overall risk - Moderate

In this risk analysis, we were very conservative in combining likelihoods. Using the previous tables for combining likelihoods would give a total estimated risk as Very Low.

In this risk analysis, we were more conservative in combining likelihoods. Using the previous tables for combining likelihoods would give a total estimated risk as Very Low.

Risk Assessment is completed when the overall risk estimate has been calculated. Note that we havent decided if the risk is acceptable or unacceptable. This will happen during Risk Management.35Key PointsRisk Assessment is done individually for each hazard.If likelihood of release is negligible, the assessment stops.If it is non-negligible, we proceed to exposure assessment.If the likelihood of exposure is negligible, the assessment stops.If it is non-negligible, we proceed to consequence assessment.If the consequence is negligible, the assessment stopsKey PointsIf it is non-negligible, we proceed to risk estimation.Risk estimation gives us an estimate of the total risk posed by the hazard. Preparing scenario trees and pathways help the risk analyst to clarify thinking and to estimate release and exposure likelihoods. They are also useful for consequence assessment and in communicating the process and results of risk analysis to stakeholders.

KLIA Quarantine Station, Kuala Lumpur, Malaysia (R. Arthur)38

Subsample uninfected (L5)

Uninfected PL selected

Uninfected broodstock selected

Selection of Spawners at Naduraloulou, Fiji

Infected broodstock selected (L1)

Infected PL Shipped to Cook Islands

Yes

No (L4)

Selection of Subsample for Pathogen Testing

Pathogen(s) Detected by Genome-based Testing

Infected subsample selected (L3)

Batch Rejected -No Pathogen Transfer

Selection of Postlarvae for Shipment

Infected PL Shipped to Cook Islands

Infected PL selected (L2)

No Pathogen Transfer

No Pathogen Transfer

Pathogen Present in Culture Facility

Water not disposed into natural waterway

Contact with susceptible species (B-L2)

Infective Pathogen Present in Culture Water (B)

Pathogen Present in Cultured Animals (A)

Contact with susceptible species (A-L2)

Escape or release of infective animal(s) occurs (A-L1)

No Exposure

No contact

Exposure

Infective Virions Free in Environment

No Exposure

Water containing viable pathogen disposed into natural waterway (B-L1)

Exposure

No Exposure

No contact

Infective Animal Free in Environment

No escape or release occurs

No Exposure