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1.Introduction to quantitative risk aement

2.Risk analysis is a valuable tool in the management of

microbial food safety iues and can provide a systematic

approach for the regulatory authorities and the food industry

to control the risk posed by a pathogen in a particular

food commodity.Risk analysis consists of three elements:

risk aement, risk management and risk communication.Risk aement is the scientific part of the proce in which

the hazards are identified and the risk posed by that particular

hazard(i.e.pathogen)is calculated.The principles of

risk aement including the four stages involved(hazard

identification, exposure aement, hazard characterisation

and risk characterisation)are outlined by the Codex

Alimentarius Commiion(Codex, 1999).Each of the stages is summarised below.1.1.Hazard identification

A hazard is defined as an agent having an adverse effect

on the public health of the human population and may

pose a short term, chronic, or fatal risk to a person.The

identification of microbial hazard aociated with a particular

food is generally based on information generated from

routine microbial analysis of the commodity or from an

epidemiological linkage of a particular pathogen with a

case of food borne infection.1.2.Exposure aement

Exposure aement is a quantitative estimation of the

presence of a contaminant in a serving of food at the time

of consumption, or as close to this stage as is scientifically

poible and practical.However, the final estimation of the numbers and prevalence of a pathogen in the food is of ten

based on an accumulation of data on the prevalence and

numbers of pathogen at key points in the food chain with

data included on how particular stages in the food chain

affect the numbers/prevalence of the pathogen.The final

step in the proce estimates the amount of contaminant

in a single serving, with information on the typical amount

of food consumed in a serving procured from nutritional

Databases.The exposure aement model can be ‘deterministic’,i.e.derived using single data points along the food chain.However, this approach may result in outlier values being

ignored and thus under or overestimating the risk.A more

common approach is to use a probablistic or stochastic

analysis, in which a distribution curve representing all data

is used as opposed to a single point estimate.Typically a

Monte Carlo analysis is used to include data from all the

distributions along the chain and is done using software

such as @Risk(Palisade, NY, USA).In these analyses, a

single data point is chosen at random from each distribution

curve and used to calculate an outcome.The proce

is repeated several thousand times(multiple iterations)with

a different data point in each distribution chosen each time

and with the final output being based on all the iterations.The error in the predicted risk may be due to variability or

uncertainty, and there is increasing emphasis being placed

on quantifying and separating the impact of both uncertainty

and variability in risk aements(Cohen, Lampson,& Bowers, 1996;Pouillot, Beaudeau, Denis, &

Derouin, 2004).1.3.Hazard characterisation

Hazard characterisation relates exposure to a hazard

with the probable public health outcome(illne/death).A

dose–response relationship can be used to estimate the

amount(number)of pathogens which causes illne.The

data used in generating dose–response models are derived

from a variety of sources including human clinical trials,epidemiological studies based on food poisoning outbreaks,animal clinical trials, in vitro studies using cell lines,biomarkers or expert opinion.In some cases, the dose–

responses will describe the susceptibility of different populations,i.e.general population and immunocompromised.1.4.Risk characterisation

The final stage in the proce estimates the adverse

public health effect, or risk as a consequence of exposure

to the hazard.This may be a prediction of illne per typical

serving or calculated as an annual risk of illne.Depending on the hazard characterisation data available,the risk estimates may be broken down into age categories,based on differences in immune status in order to

identify groups which may be at higher risk following

exposure to the contaminant.The risk characterisation

model is generally developed using commercial software such as @Risk or Crystal Ball(Decisioneering Inc., Denver,USA).These programs can separate the distribution

for the overall risk prediction into uncertainty and variability

to allow more complex risk estimation and analyses

of the data.The generated model can be used to ae

which parts of the chain significantly affect risk or to

ae the changes in predicted illne by incorporation

of a new hypothetical risk mitigation strategy at a particular

point in the chain.This paper reviews Escherichia coli O157:H7 in the farm

to fork beef chain and examines how quantitative risk

aement models have been applied to establish and manage

the risk posed.While other serovars of verocytotoxigenic

E.coli(including E.coli O26, O111, O103, O145)

are now emerging as a cause of similar illne to E.coli

O157:H7 they are not addreed in this paper as there is

still limited information on their transmiion thorough

the beef chain and they have not been included in any published

quantitative risk aement models.2.E.coli O157:H7: human clinical aspects

E.coli O157 is a member of the Enterhaemorrhagic

group of E.coli(EHEC)and was first implicated in infectious

disease in the early 1980s(Riley et al., 1983).The

symptoms of infection include bloody diarrhoea and severe

abdominal pain.Haemolytic uraemic syndrome(HUS), a

cause of acute renal failure, may be a complication of the

illne, and neurological problems in the form of thrombotic

thrombocytopaenic purpura(TTP)may also occur.Immuno-compromised patients, including young children

and the elderly, are at particular risk of developing HUS.The time from exposure to onset of symptoms ranges fromto 14 days(Coia, 1998).However, with complications the

illne may last many months and lead to permanent damage

or even death.Pathogenicity is related to the ability of

the organism to adhere to and colonise the human large

intestinal epithelial tiue, forming attachment and effacing

(AE)lesions and the production of verocytotoxins.The

E.coli verocytotoxins are closely related to the Shiga toxin

of Shigella dysenteriae and are typically bacteriophage

encoded.There are two main claes of verotoxin: VT1, a

homogeneous group of toxins, virtually identical to the

Shiga toxin of Shigella and VT2, a heterogeneous group

of toxins, more distantly related to the Shiga toxin.E.coli O157 with the eae gene and VT2 are most often

aociated with HUS in patients(Werber et al., 2003).Outbreaks of VTEC infections involving serovar O157

have now been reported from United States and Canada

Bell et al.(1994)(Lisbea), Asia(Michino et al., 1998), Australia

(Desmarchelier, 1996), Europe(Tozzi, Gorietti, &

Caprioli, 2001), and Africa(Germani, Soro, Vohito,Morel, & Morvan, 1997).However, the majority of cases

are sporadic and contribute significantly to overall cases

of infection.There is considerable variation in infection

rates between different geographical regions.In Europe, the

highest rates of infection are in Scotland with approximately 4 cases per 100,000(SCIEH, 2006).In the Republic of Ireland

the incidence per 100,000 has ranged from a peak of

2.2 in 2003 to 1.3 in 2004(HPSC, 2004).In Northern Europe

infection rates are very low ranging from 0.04 per

100,000 in Norway and Finland to 1.1 in Denmark in

2000 although Denmark has in 2006, reported its first general

outbreak of E.coli O157 attributed to contaminated

milk(Jensen et al., 2006).In 2004, the incidence rate for

E.coli O157:H7 in North America was 0.9, a drop from

1.1 cases in 2003.In Asia, Japan has experienced the most

problems related to E.coli O157:H7 with an average incidence

rate of 2.74 per 100,000 between 1999 and 2004

(Sakuma, Urashima, & Okabe, 2006).A number of sources

and reservoirs of E.coli O157 including beef and lamb,lettuce, sprouts, fruit juices, vegetables, raw milk, water

have been implicated as vehicles of transmiion(Bell

et al., 1994;Cowden, Ahmed, Donaghy, & Riley, 2001;

Hilborn et al., 2000;Michino et al., 1999).Person-to-person

is also an important mode of transmiion, particularly

in day care centers(O’Donnell et al., 2002)and direct contact

with animals carrying the organism or with faecally

contaminated mud(Anon, 1999;Crampin et al., 1999)

are also recognised sources of infection