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.

1. Risk assessment for Listeria monocytogenes in hotsmoked 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

2. BACKGROUND

3. • 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)
Pathogen
Campylobacter spp.
Illnesses
10,539
Deaths
99
Salmonella non-typhoidal
15,608
553
3.54
L. monocytogenes
2,298
499
21.71
Department of Nutrition and Food Science, Univ. of Ghana
% Deaths
0.95

4. 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

5. 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both unpublished)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

6. 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

7. 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 hotsmoked fish
•
•
•
Mackerel:
Scomber japonicus
Tuna: Katsuwonus pelamis
Herrings:
Sardinella eba
Department of Nutrition and Food Science, Univ. of Ghana

8. 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
Department of Nutrition and Food Science, Univ. of Ghana

9. METHODS

10. GENERAL STUDY DESIGN
Hazard Identification
Determination of the ability of an organism to
cause harm
Exposure
Assessment
Probability of ingesting hazard, and the likely
quantities thereof
Hazard Characterization
Dose-response
Assessment
Risk Characterization
Description of the (severity of the ) effect of
the hazard following consumption
Final quantitative or qualitative description of
the nature of the risk
Fig. 1: Codex Alimentarius Commission framework for risk assessment, 2003

11. HAZARD IDENTIFICATION
• Literature review for information on
–
–
–
–
–
Nature of organism
Occurrence and transmission
Pathogenicity
Outbreaks
Risk assessments

12. EXPOSURE ASSESSMENT
• 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)

13. EXPOSURE ASSESSMENT
Consumer surveys
• 150 consumers were interviewed with semistructured 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

14. EXPOSURE ASSESSMENT
Laboratory analysis
Primary Enrichment
LEB, 37oC, 24h
Secondary Enrichment
Fraser, 37oC, 24-48h
Plating on Oxford or
Chromagar, 37oC, 24-28h

15. EXPOSURE ASSESSMENT
• The likely number of L. monocytogenes ingested was
calculated as
N=CxQ
[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)

16. 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
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
[2]
N = C x Q [1]

17. 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

18. RESULTS & DISCUSSION

19. 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

20. PROCESSING AND HANDLING
Department of Nutrition and Food Science, Univ. of Ghana

21. Table 1: Average prevalence of Listeria monocytogenes in traditionally
processed fish purchased from informal market
Product
Number of
samples
purchased
Number of
samples positive Prevalence of L.
for L.
monocytogenes
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

22. 16.00
16.00
14.00
14.00
12.00
x102 CFU/g
10.00
8.00
6.00
5.00
4.00
4.00
4.00
2.00
1.00
2.00
0.00
KA
KO
MO
TU
SA
MA
HR
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
Department of Nutrition and Food Science, Univ. of Ghana
DR

23. Fish contaminated with Lm on
informal markets
No
Ingestion not likely
Yes
Fish eaten as is (without heating)
No
Lm survives in fish during cooking
No
Ingestion not likely
Yes
Risk of ingestion
N = CxQ
Yes
Risk of ingestion
N = CxQ
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

24. 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

25. RISK ESTIMATION FOR LOW-SUSCEPTIBILITY
CONSUMER
• Dose required for illness = ≥ 109 cells
(Buchanan et al., 1997; Schlech,1999)
• Mean Lm CFU
– Herrings
– Mackerel
– Tuna
= 8.33x102 CFU/g
= 2.90x103 CFU/g
= 2.10x103 CFU/g
Department of Nutrition and Food Science, Univ. of Ghana

26. 1
5.0%
185.6
90.0%
317.8
5.0%
0.8
0.4
95% = 0.000318
Mean = 0.000248
0.2
500
450
400
350
300
250
200
150
0
100
Confidence
0.6
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

27. 5.0%
1
5.56
83.5%
11.5%
10.00
0.8
0.4
0.2
95% = 1.088E-005
Mean = 8.012E-006
Probability of illness according
to Weibull-Gamma / NA
18
16
14
12
10
8
6
4
0
2
Confidence
0.6
Probability of illness
Values in Millionths
Probability of illness among 18-49yrs from consumption of traditionally
smoked herrings contaminated with L. monocytogenes
Department of Nutrition and Food Science, Univ. of Ghana

28. 5.0%
0.01
99.3 90.0%
169.8
5.0%
0.008
Confidence
0.006
0.002
95% = 0.000170
Mean = 0.000132
0.004
Tuna
Probability of illness
Values in Millionths
Risk of illness among 18-49yr tuna consumers
Department of Nutrition and Food Science, Univ. of Ghana
382.5
286.875
191.25
95.625
0
0

29. RISK ESTIMATION FOR LOW-SUSCEPTIBILITY CONSUMER
5.0%
100.0%
1
99.2
90.0%
0.0%
5.0%
0.0%
169.9
0.6
Tuna
Herrings / NA
95% = 0.000318
Mean = 0.000248
95% = 0.000170
Mean = 0.000132
0.2
Mean = 8.012E-006
Mackerel / Quantity Consumed
0.4
397.58
Probability of illness
Values in Millionths
298.185
198.79
99.395
0
0
Confidence
0.8
Comparison of probability of illness from consumption of tuna, mackerel and
herrings
Department of Nutrition and Food Science, Univ. of Ghana

30. RISK ESTIMATION FOR HIGH-SUSCEPTIBILITY
CONSUMER
• Dose required for illness = ≥ 104 cells
(Buchanan et al., 1997; Schlech,1999)
• Average Lm CFU
– Herrings
– Mackerel
– Tuna
= 8.33x102 CFU/g
= 2.90x103 CFU/g
= 2.10x103 CFU/g
Department of Nutrition and Food Science, Univ. of Ghana

31. 5.0%
100.0%
1
0.1993
90.0%
0.0%
5.0%
0.0%
0.2949
0.6
Tuna / Average
0.4
Herring / Average
Mean = 0.3250
0.2
Mean = 0.2478
Mean = 0.0285
Mackerel / Average
Probability of illness
Comparison of probability of illness
Department of Nutrition and Food Science, Univ. of Ghana
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
Confidence
0.8

32. HIGH-RISK vs. LOW-RISK GROUPS
Department of Nutrition and Food Science, Univ. of Ghana

33. 5.0% 90.0%
0.0% 0.0%
0.0000000
1
5.0%
100.0%
0.8
0.2
Mean = 0.0285
Mean = 8.012E-006
0.4
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
0.000064
0.0000492
0.0000344
0.0000196
0.0000048
0
-0.00001
Confidence
0.6

34. EFFECT OF MITIGATION
Department of Nutrition and Food Science, Univ. of Ghana

35. 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

36. 1
5.0%
90.0%
0.0%
0.0%
4.9
9.0
5.0%
100.0%
0.8
Red: mitigation
Blue: No mitigation
0.4
Probability of illness
Values in Millionths
Effect of mitigation on risk reduction among 18-49yrs, tuna
Department of Nutrition and Food Science, Univ. of Ghana
245.75
222.5
199.25
176
152.75
Mean = 0.000132
129.5
106.25
83
59.75
36.5
13.25
0
Mean = 6.829E-006
0.2
-10
Confidence
0.6

37. 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

38. 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

39. THANK YOU