risk assessment for listeria monocytogenes in hot-smoked fish in informal markets in madina, accra
DESCRIPTION
Presentation by K. Bomfeh, K. Tano-Debrah, F.K. Saalia and B. Bediako-Amoa at the 17th Faculty of Science Colloquium, University of Ghana, Accra, Ghana, 21 March 2012.TRANSCRIPT
Risk assessment for Listeria monocytogenes in hot-
smoked fish in informal markets in Madina, Accra
K. Bomfeh, K. Tano-Debrah, F.K. Saalia and B. Bediako-Amoa
Presented at the 17th Faculty of Science Colloquium, University of Ghana
Accra, Ghana, 21 March 2012
BACKGROUND
• Listeria monocytogenes is a non-spore forming
pathogenic bacterium that causes a highly fatal disease
called listeriosis
• L. monocytogenes is considered the leading cause of
death among food-borne bacterial pathogens, with a
fatality rate of 20-30%, and up to 75% in highly
immunocompromised individuals
Table 1: Fatality of L. monocytogenes infection (CDC, 2000)
Department of Nutrition and Food Science, Univ. of Ghana
Pathogen Illnesses Deaths % Deaths Campylobacter spp. 10,539 99 0.95
Salmonella non-typhoidal 15,608 553 3.54
L. monocytogenes 2,298 499 21.71
Foods frequently contaminated
• Milk and milk products
• Soft cheese
• Processed meats, red meat
• Vacuum packaged beef and poultry products
• Lettuce
• Coleslaw
• Fried rice
• Smoked fish
• Salted fish
Department of Nutrition and Food Science, Univ. of Ghana
Department of Nutrition and Food Science, Univ. of Ghana
RATIONALE
• Human listeriosis is not documented in Ghana.
However, the occurrence of the illness among herds of
sheep has been documented
• The pathogen has been isolated from fresh milk from
informal markets, and from coleslaw from restaurants
in some parts of Accra (Appiah, 2010; Dogbe 2010
both unpublished)
• Fish has been implicated in listeriosis outbreaks
elsewhere (Buchanan et al., 1994; Lindqvist and Westoo,
2000; FAO, 2000)
Department of Nutrition and Food Science, Univ. of Ghana
RATIONALE
• Ghana records high rates of fish consumption
– Per capita fish consumption of 20-25kg, about twice
the world average of 13kg
– Fish purchases account for 22.4% of household food
expenditures
– Fish contributes 60% of total animal protein consumed
in Ghana (BoG, 2008; Adu-Gyamfi, 2006; Oppey, 2003; Nketsia-Tabiri and
Sefa-Dedeh, 2000; Steiner-Asiedu, 1991; Plahar et al., 1991)
• Over 80% of fish landings are processed traditionally
Department of Nutrition and Food Science, Univ. of Ghana
OBJECTIVES
Main: Risk assessment of listeriosis associated
with the consumption of traditionally processed
fish in Ghana
Specific:
1. To determine the presence and concentration
of Listeria monocytogenes in traditionally hot-
smoked fish
• Mackerel: Scomber japonicus
• Tuna: Katsuwonus pelamis
• Herrings: Sardinella eba
Department of Nutrition and Food Science, Univ. of Ghana
OBJECTIVES Specific:
2. To determine the exposure of consumers to the
pathogen through consumption of the products
3. To determine the risk of illness following
ingestion of the pathogen through consumption
of traditionally processed fish
METHODS
Fig. 1: Codex Alimentarius Commission framework for risk assessment, 2003
GENERAL STUDY DESIGN
Exposure
Assessment Probability of ingesting hazard, and the likely
quantities thereof
Dose-response
Assessment
Hazard Characterization
Description of the (severity of the ) effect of
the hazard following consumption
Risk Characterization Final quantitative or qualitative description of
the nature of the risk
Determination of the ability of an organism to
cause harm Hazard Identification
• Literature review for information on
– Nature of organism
– Occurrence and transmission
– Pathogenicity
– Outbreaks
– Risk assessments
HAZARD IDENTIFICATION
• Involved consumer survey and laboratory analysis of
field samples
Consumer Survey
• Fish consumption patterns
– Frequency of consumption
– Quantities often consumed at an instance
– Form in which consumed
Laboratory analyses
• Prevalence of Listeria monocytogenes
(presence/absence)
• Concentration of the pathogen (CFU/g)
EXPOSURE ASSESSMENT
Consumer surveys
• 150 consumers were interviewed with semi-
structured questionnaires on
– frequency of consumption of the products
– quantities often consumed at an instance
– form in which products are often consumed
– proxy estimation of the consumption pattern among
the elderly, children, and pregnant women.
• Total respondents: 600
EXPOSURE ASSESSMENT
Secondary Enrichment
Fraser, 37oC, 24-48h
Plating on Oxford or
Chromagar, 37oC, 24-28h
Primary Enrichment
LEB, 37oC, 24h
Laboratory analysis
EXPOSURE ASSESSMENT
EXPOSURE ASSESSMENT
• The likely number of L. monocytogenes ingested was
calculated as
N = C x Q [1]
N = likely number of L. monocytogenes cells ingested
C = CFU/g of L. monocytogenes in the fish product
Q = serving size of fish product frequently consumed
(Lindqvist and Westoo, 2000)
HAZARD CHARACTERIZATION
• The Weibull-Gama model was used to determine the
probability of illness (Lindqvist and Westoo, 2000; Bemrah
et al., 1998; Farber et al.,1997)
• Weibull-Gama dose-response model
P = 1 – [ 1 + (Nb)/β ] – α [2]
P = probability of infection
N = dose of L. monocytogenes
α, β, b = model parameters
α=0.25, b=2.14
β=1010.98 for high-risk population
β=1015.26 for low risk population
N = C x Q [1]
RISK CHARACTERIZATION
• Quantitative estimate of the risk of illness following
consumption of contaminated fish
ASSUMPTIONS
• Fish were consumed as purchased OR were not
heat-treated to an extent that guaranteed
elimination of L. monocytogenes
• All strains of L. monocytogenes in the fish products
were virulent
RESULTS & DISCUSSION
Department of Nutrition and Food Science, Univ. of Ghana
Key Findings 1. Sanitary conditions of fish processing and handling were
unsatisfactory
2. L. monocytogenes was detected in at least one sample of
each product on informal markets
3. L. monocytogenes was detected in at least one sample
from each step in processing except
– Fresh, frozen mackerel and herrings
– Hot-smoked tuna, mackerel and herrings sampled immediately
after processing
4. Counts were low (102-103 CFU/g)
5. Low risk of ingestion and infection
Department of Nutrition and Food Science, Univ. of Ghana
PROCESSING AND HANDLING
Department of Nutrition and Food Science, Univ. of Ghana
Table 1: Average prevalence of Listeria monocytogenes in traditionally
processed fish purchased from informal market
Product
Number of
samples
purchased
Number of
samples positive
for L.
monocytogenes
Prevalence of L.
monocytogenes
(%)
Smoked tuna 15 12 80
Smoked mackerel 15 14 93
Smoked herrings 15 10 67
Department of Nutrition and Food Science, Univ. of Ghana
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
KA KO MO TU SA HR DR
4.00 4.00
2.00
14.00
16.00
5.00
1.00
x10
2 C
FU
/g
Type of fish
Fig. 1: Average counts of L. monocytogenes in fish samples
KA-kako (salted) KO-koobi (salted) MO-momoni (salted)
TU-tuna MA-Mackerel HR-herrings DR-dried fish
MA
Department of Nutrition and Food Science, Univ. of Ghana
Yes
Risk of ingestion
Yes No
Ingestion not likely Fish eaten as is (without heating)
Fish contaminated with Lm on
informal markets
No
Yes
Lm survives in fish during cooking N = CxQ
Risk of ingestion
N = CxQ
Ingestion not likely
No
Fig. 2: Event tree for risk of ingestion of Listeria monocytogenes through
consumption of traditionally smoked fish purchased from informal markets
Department of Nutrition and Food Science, Univ. of Ghana
Table 1: Likely numbers of L. monocytogenes ingested through
consumption of contaminated TPF fish on informal markets in Ghana
Product C (CFU/g) Q (g) N=CxQ
Tuna 1.40x103 >200 ≥3.20x105
Mackerel 1.60x103 >200 ≥2.80x105
Herrings 4.00x102 151 6.04x104
C : average counts across five markets
Q : Most frequently consumed quantities N=likely number of L.
monocytogenes ingested
P = 1 – [ 1 + (Nb)/β ] – α
Department of Nutrition and Food Science, Univ. of Ghana
RISK ESTIMATION FOR LOW-SUSCEPTIBILITY
CONSUMER
• Dose required for illness = ≥ 109 cells (Buchanan et al., 1997; Schlech,1999)
• Mean Lm CFU
– Herrings = 8.33x102 CFU/g
– Mackerel = 2.90x103 CFU/g
– Tuna = 2.10x103 CFU/g
Department of Nutrition and Food Science, Univ. of Ghana
185.6 317.8 5.0% 90.0% 5.0%
Mean =
0.0
00248
95%
= 0
.000318
0
0.2
0.4
0.6
0.8
1
100
150
200
250
300
350
400
450
500
Co
nfi
de
nce
Probability of illness Values in Millionths
Probability of illness among 18-49yrs from consuming smoked mackerel contaminated with L. monocytogenes
Department of Nutrition and Food Science, Univ. of Ghana
5.56 10.00 5.0% 83.5% 11.5%
Mean =
8.0
12E-0
06
95%
= 1
.088E-0
05
0
0.2
0.4
0.6
0.8
1
2 4 6 8
10
12
14
16
18
Co
nfi
de
nce
Probability of illness Values in Millionths
Probability of illness among 18-49yrs from consumption of traditionally smoked herrings contaminated with L. monocytogenes
Probability of illness accordingto Weibull-Gamma / NA
Department of Nutrition and Food Science, Univ. of Ghana
99.3 169.8 5.0% 90.0% 5.0%
Mean =
0.0
00132
95%
= 0
.000170
0
0.002
0.004
0.006
0.008
0.01
0
95.6
25
191.2
5
286.8
75
382.5
Co
nfi
de
nce
Probability of illness Values in Millionths
Risk of illness among 18-49yr tuna consumers
Tuna
Department of Nutrition and Food Science, Univ. of Ghana
RISK ESTIMATION FOR LOW-SUSCEPTIBILITY CONSUMER
99.2 169.9
5.0% 100.0%
90.0% 0.0%
5.0% 0.0%
Mean =
0.0
00132
Mean =
8.0
12E-0
06
Mean =
0.0
00248
95%
= 0
.000170
95%
= 0
.000318
0
0.2
0.4
0.6
0.8
1
0
99.3
95
198.7
9
298.1
85
397.5
8
Co
nfi
de
nce
Probability of illness Values in Millionths
Comparison of probability of illness from consumption of tuna, mackerel and herrings
Tuna
Herrings / NA
Mackerel / Quantity Consumed
Department of Nutrition and Food Science, Univ. of Ghana
RISK ESTIMATION FOR HIGH-SUSCEPTIBILITY
CONSUMER
• Dose required for illness = ≥ 104 cells (Buchanan et al., 1997; Schlech,1999)
• Average Lm CFU
– Herrings = 8.33x102 CFU/g
– Mackerel = 2.90x103 CFU/g
– Tuna = 2.10x103 CFU/g
Department of Nutrition and Food Science, Univ. of Ghana
0.1993 0.2949
5.0% 100.0%
90.0% 0.0%
5.0% 0.0%
Mean =
0.2
478
Mean =
0.0
285
Mean =
0.3
250
0
0.2
0.4
0.6
0.8
1
0
0.0
5
0.1
0.1
5
0.2
0.2
5
0.3
0.3
5
0.4
0.4
5
Co
nfi
de
nce
Probability of illness
Comparison of probability of illness
Tuna / Average
Herring / Average
Mackerel / Average
Department of Nutrition and Food Science, Univ. of Ghana
HIGH-RISK vs. LOW-RISK GROUPS
Department of Nutrition and Food Science, Univ. of Ghana
0.0000000 0.0000000
5.0% 0.0%
90.0% 0.0%
5.0% 100.0%
Mean =
8.0
12E-0
06
Mean =
0.0
285
0
0.2
0.4
0.6
0.8
1
-0.0
0001
0.0
000048
0.0
000196
0.0
000344
0.0
000492
0.0
00064
Co
nfi
de
nce
Probability of illness Values in Thousands
Comparison of risk of illness from herrings: Low risk vs. High Risk
Department of Nutrition and Food Science, Univ. of Ghana
EFFECT OF MITIGATION
Department of Nutrition and Food Science, Univ. of Ghana
Tuna Mode of
consumption AHI ADULT ELDERLY
As is 1.00E-04 2.07E-01
Mitigation 6.90E-06 1.93E-02
Department of Nutrition and Food Science, Univ. of Ghana
4.9 9.0
5.0% 0.0%
90.0% 0.0%
5.0% 100.0%
Mean =
6.8
29E-0
06
Mean =
0.0
00132
0
0.2
0.4
0.6
0.8
1
-10
13.2
5
36.5
59.7
5
83
106.2
5
129.5
152.7
5
176
199.2
5
222.5
245.7
5
Co
nfi
de
nce
Probability of illness Values in Millionths
Effect of mitigation on risk reduction among 18-49yrs, tuna
Red: mitigation
Blue: No mitigation
CONCLUSIONS
Exposure Assessment
• Traditionally smoked tuna, mackerel and herrings on
informal markets are potential vehicles for the
transmission of L. monocytogenes
• Depending on the kind of product and quantity
consumed at an instance, consumers are exposed to
ingesting 102 to 105 cells of L. monocytogenes
CONCLUSIONS
Hazard Characterization and Dose-Response
• Depending on dose ingested and susceptibility to
infection, consumers have a 1 in 10 to 1 in
100,000,000 risk of illness
Risk Characterization
• Risk of ingestion and infection are generally low.
However, individuals who either consume TPF
from informal markets as is or do not heat-treat
the products sufficiently increase their risks of
ingestion and infection, and vice versa
THANK YOU