Gaag, M.A., van der

Huirne, R.B.M.

Saatkamp, H.W.

Wageningen Yield 

Elicitation of expert knowledge on dynamics of Salmonella 
infections and contamination in the pork chain 
Van der Gaag, M.A.U, H.W. Saatkamp2, R.B.M. Huirne2 
1 ) Research Institute for Animal Husbandry, Lelystad, The Netherlands 
2) Wageningen University (Hollandseweg 1, 6706 KN Wageningen, phone: (+31 )317 484065, fax: 
(+31)317 482745, e-mail: Monique.vanderGaag<5|a!g.abe.wau.nl) 
Summary There is little information available from experiments and literature on 
the course of Salmonella infections in pigs and on the course of contamination at 
slaughter. To obtain such information, an expert elicitation study was carried out in 
the Netherlands (23 respondents). The estimation of the experts for the most likely 
duration to be infectious after infection with Salmonella is for a pig 38 days. The 
most likely duration to be a carrier was estimated at 55 days and to become 
serological negative 109 days. Application of fermented feed decreases the 
infectious period by 11 days. At slaughter, the average number of carcasses at risk 
after one to four contaminated carcasses was 10 to 21. So the duration of infection 
can be shortened by e.g. feed measures. In a situation that infected pigs enter the 
slaughterhouse, the order of the pigs to be slaughtered is important. 
Keywords: survey, zoonosis, epidemiology, pigs, control 
Introduction There is little information available from experiments and 
literature on the course of Salmonella infections in pigs and on the course of 
contamination at slaughter. For efficient control of Salmonella in the pork chain, 
insight in the dynamics of the infection in live pigs and of contamination at the 
slaughterhouse is essential. A survey was designed for experts in the Netherlands 
and Denmark in order to bring together their knowledge on these items. 
Materials and Methods In total 75 experts were invited to participate in a suney, 
45 Dutch experts during February and March 2001 and 30 Danish experts in June 
2001. An expert was defined as someone who is related to the pork chain and who 
is known for his/her knowledge of Salmonella in pigs and/or pork. The survey 
focused on the best control strategy per stage and on eleven variables about 
introduction and spread of Salmonella in the chain. For each stage prevention and 
reduction measures were given and the experts divided 100 points to indicate the 
relative importance per measure. Seven variables regarding live pigs were the 
duration of the seroconversion period, of the infectious period (with or without 
fermented feed), of the carrier period, the period to return to the serologically 
negative status and the infection rate (for serological positive and negative pigs). 
259 
The four variables regarding contamination of carcasses at slaughter were: number 
of carcasses at risk when one or four preceding carcasse(s) are contaminated, 
probability for infected pigs to end up contaminated and probability for free pigs to 
end up contaminated when 50% of the pigs slaughtered that day were infected. For 
each variable the expected minimum, most likely and maximum value was asked. 
Results and discussion From the Dutch experts, 23 (51%) completed the survey 
and at the time of this paper the Danish survey was not finished yet, so the 
following results are based on Dutch respondents only. The individual variance 
within the estimations of the variables among respondents is large. To prevent 
inordinate impact of extreme values, the highest and lowest value per variable are 
excluded from the database. According to respondents, a pig that becomes infected 
is serologically positive in 12 days (sd=3, n=l 1). See Figure 1. The most likely 
duration of the infectious period after the pig is infected for the first time is 32 days 
(sd=23, n=7) and for pigs fed with fermented feed this period is 21 days (sd=17, 
n=7). Pigs that were recovered and are infected for the second time, the expected 
infectious period is 30 days (sd=27, n=7). After the infectious period the pig stops 
shedding but it still carries bacteria in the lymph nodes or intestines, the so-called 
carrier period. The most likely duration of this period is 58 days (sd=21, n=7). 
O iJHQimum duration g most likely duration with standard deviation ^maximum duration 
1?0 
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seroconversion infectious infectious infectious carrier from serological positive 
(1st infection) (fermented feed)(2nd infection) to serological negative 
Figure 1 Minimum, most likely and maximum duration in days of six epidemiological states 
(values are average values of 7 to 11 respondents) 
At slaughter the contamination of carcasses depends on the status of the animal 
before slaughter and of the status of the preceding carcasses. The most likely 
probability an infected pig ends up as a contaminated carcass when no other 
infected pigs are slaughtered is 37% (minimum probability is 11% and maximum is 
68%). The probability that a free pig ends up as a contaminated carcass when half 
of the slaughtered pigs are infected is 43% (minimum 21% and maximum 77%). 
260 
After one contaminated carcass at the slaughterline the 10 successive carcasses are 
at risk to get contaminated (sd=8, n=7). With four contaminated carcasses in a row 
the respondents estimated that 21 (sd=17, n=7) successive carcasses are at risk. 
The three most preferable control measures per stage are shown in Table 1 (17 to 
21 respondents per stage). Measures with respect to hygiene are preferable in all 
stages. For the primary stages application of fermented feed is also advisable. For 
the other stages logistic supply is useful according the respondents. This 
harmonises with the estimations of the variables of contamination in the 
slaughterhouse as mentioned above. 
Table 1 Three most preferable measures per stage to minimise the Salmonella prevalence 
1 
2 
3 
Multiply 
Certified 
Salmonella free 
feed 
Hygiene to 
prevent spread2 
Fermented feed 
for piglets 
Finish 
Certified Salmonella 
free piglets 
Hygiene to prevent 
introduction and 
reduce spread1,2 
Fermented feed for 
pigs 
Transport 
Hygiene to 
prevent 
introduction' 
Logistic transpoif 
1 compartment of 
finishing farm/ ride 
Lairage 
Logistic supply of 
Pigs 
Hygiene to prevent 
introduction' 
Hygiene to prevent 
spread2 
Slaughter 
Logistic 
supply of pigs3 
Hygiene to 
prevent spread2 
Careful 
evisceration 
1) Hygiene to prevent introduction: measures such as sanitary lock, rodent control. 2) Hygiene to 
reduce spread: measures such as closed compartments, cleansing and disinfecting, smooth surfaces. 3) 
Logistic supply: order of supply is based on the prevalence in herd or group (lowest prevalence first). 
Due to increasing food safety concerns most hygiene measures probably become 
regulation within several years. Although logistic transport and slaughter is 
expected to yield much improvement, more field experiments are necessary to 
justify such measures in practice. 
Conclusions The best Salmonella control measures are improved hygiene and 
logistic operating procedures and in the primary stages also the application of 
fermented feed. The duration of states in the course of infection shows a lot of 
variation and most experts also expect large variation between individual pigs. 
Nevertheless the ranges given by the respondents give more insight in the course of 
infection in live pigs and of contamination at slaughter. 
Acknowledgements Thanks to all respondents for their time and co-operation. 
261 


Elicitation of expert knowledge on dynamics of Salmonella infections and contamination in the pork chain

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