Presentation by Kennedy Bomfeh, Kwaku Tano-Debrah, Firibu K. Saalia and Betty Bediako-Amoa at the first International Congress on Pathogens at the Human-Animal Interface, Addis Ababa, Ethiopia, 15-17 September 2011.

1. EXPOSURE ASSESSMENT FOR LISTERIA MONOCYTOGENES
IN HOT-SMOKED FISH IN GHANA
74
Kennedy Bomfeh1, Kwaku Tano-Debrah1*, Firibu K. Saalia1, Betty Bediako-Amoa1
1Department of Nutrition and Food Science, University of Ghana, Legon
*Corresponding author: ktanode@ug.edu.gh; ktanode@yahoo.com
The study sought to determine the occurrence and likelihood of ingestion of Listeria monocytogenes in hot-smoked fish (mackerel, tuna, and
herrings) in Ghana. A survey was conducted in two coastal fishing and traditional fish processing communities in Accra to determine the frequencies
of consumption of the products, and the quantities (Q) often consumed at an instance. The concentration, C, and prevalence of L. monocytogenes
was determined in three (3) samples of each product purchased from four central informal markets in Accra (total of 15 samples). Prevalence was
determined as the percentage of samples in which the pathogen was detected, and concentration as the colony forming units of the pathogen per
gram of fish (CFU/g). The likely numbers of L. monocytogenes, N, ingested through consumption of each type of smoked fish were determined as N=
C x Q. The respective prevalence and concentrations of L. monocytogenes were: hot-smoked mackerel - 100%, 1.60x103 CFU/g; hot-smoked tuna 92%, 1.40x103 CFU/g; and hot-smoked herrings 75%, 4.00x102 CFU/g. The quantity of hot-smoked tuna and mackerel often consumed in both
communities was >200g, and the respective values for hot-smoked herrings between the two communities were 151g and 101g. Therefore, the likely
numbers of the pathogen ingested were ≥3.20x105 cells in mackerel and ≥2.80x105 cells in tuna for both communities. In herrings, the likely numbers
were 6.04x104 cells and 4.04x104 cells between the towns. The results suggest that consumers of hot-smoked fish from informal markets are exposed
to possible ingestion of Listeria monocytogenes.
INTRODUCTION
Table 2: Forms in which in which traditionally hot-smoked
fish was consumed
Form in which consumed
As is
Cooked *
1 (%)
2 (%)
1 (%)
2 (%)
Product
Listeria monocytogenes is a food-borne pathogen
known to sporadically cause a fatal disease called
listeriosis. The organism is reported to cause the most
deaths among all food-borne bacterial pathogens. It is
able to grow at very low temperatures (-1.5oC), high
salinity (up to 10-12% NaCl), low pH (minimum 4.4),
and low water activity (0.83)1. L. monocytogenes has
been shown to grow in smoked fish2. Ghana is a heavy
consumer of fish3, yet no study had been conducted in
the country on the occurrence of the pathogen in the
products. Much of the fish consumed is traditionally
processed as hot-smoked, salted and/or sundried fish
(over 80% of fish landings are processed that way)4.
This study therefore sought to determine the
occurrence of L. monocytogenes in hot-smoked tuna,
mackerel and herrings, and to determine the likelihood
of ingestion of the pathogen by consumers.
Tuna
100
100
Mackerel
48
60
100
100
Herrings
0
0
100
100
1= Jamestown 2= Tema New Town %= percentage of
respondents *=cooked in soups or stews
Fish on informal markets contaminated with
L. monocytogenes
Yes
No
Fish eaten as is
(without heating)
METHODS
RESULTS
10
Fig. 2: Loading fish in basins for sales
Ingestion not
likely
With semi-structured questionnaires, 300 consumers
were interviewed on the frequency and consumption
of hot-smoked tuna, mackerel and herrings processed
traditionally, the quantities they often consumed at an
instance, and the form in which the products were
consumed. Three samples of each product were then
purchased from five informal markets (including one
market each in the two survey communities) where
consumers indicated they purchased fish. Samples
were transported to the laboratory under aseptic
conditions. 25g of each product was homogenized in
225ml of buffered peptone water and plated out on
Oxford agar by the spread plate technique and
incubated at 37oC for 24-72hrs. Colonies typical of L.
Monocytogenes were counted and tested for sugar
fermentation and β-haemolysis.
23
No
Yes
Risk of ingestion
N=QxS
Lm survives in fish during
cooking
No
Ingestion not
likely
Yes
Risk of ingestion
N=QxS
Fig. 2: Event tree for risk of ingesting Listeria monocytogenes
through consumption of traditionally hot-smoked fish
purchased from informal markets
Table 1: Likely numbers of L. monocytogenes ingested
through consumption of contaminated TPF fish on
informal markets in Ghana
Cty
CONCLUSION
Listeria monocytogenes occurs in traditionally
processed fish on informal markets in Ghana,
suggesting that the products could be vehicles for the
transmission of the pathogen to consumers.
Given the low counts of the pathogen in the products,
the risk of ingestion is low. However, individuals
who either consume traditionally processed fish
purchased from informal markets as is or do not heattreat the products sufficiently increase their risks of
ingestion, and vice versa.
Tuna on smoker
Fig. 1: Processing environment and some handling practices
during traditional fish smoking in Ghana
>200
≥3.20x105
Mackerel
1.60x103
>200
≥2.80x105
4.00x102
151
6.04x104
1.40x103
>200
≥3.20x105
1. Montville, T.J. and Matthews, K.R. (2005). Food Microbiology: An
Introduction. ASM Press, Washington D.C. pp 159 – 171.
Mackerel
1.60x103
>200
≥2.80x105
Herrings
Mackerel on smoker
C (CFU/g)
1.40x103
Tuna
2
Product
Herrings
1
Thawing mackerel
N
Although the hygienic conditions of traditional fish
smoking environments was generally unsatisfactory
(Fig.1&2), in another study, we did not detect Listeria
monocytogenes in hot-smoked fish sampled
immediately after processing. Therefore, the
occurrence of the pathogen in samples from informal
markets implicates improper post-processing handling
as a source of contamination.
Tuna
Sanitation of processing
environment
Q (g)
DISCUSSION
Consumers are exposed to ingesting Listeria
monocytogenes if they consume hot-smoked fish from
informal markets as is (which was they case among
some, Table 2), or if the pathogen survives in fish
during domestic cooking (Fig. 2). The former event is
more probable, and under such circumstances the
likely number of cells ingested would be as presented
in Table 1.
4.00x102
101
4.04x104
2. Lindqvist, R. and Westoo, A. (2000). Quantitative risk assessment f
or Listeria monocytogenes in smoked or gravad mackerel and
rainbow trout in Sweden. International Journal of Food
Microbiology 58 181 – 196.
Cty= Community 1= Jamestown 2= Tema New Town
C=average counts across five markets
Q = Most frequently consumed quantities N=likely number
of L. monocytogenes ingested
REFERENCES
3. Bank of Ghana. (2008). The Fishing Sub-sector and Ghana’s
Economy. Research Department, Bank of Ghana
4. Nketsia-Tabiri, J. and Sefa-Dedeh, S. (2000). Quality attributes and
utilization of cured fish products in Ghana. Journal of Applied
Science and Technology 5:1 148 – 155
PRESENTED AT THE 1ST INTERNATIONAL CONGRESS ON PATHOGENS AT THE HUMAN-ANIMAL INTERFACE (ICOPHAI) , ADDIS ABABA, SEPT. 15-17 2011