Article pp.107-111 du Vol.23 n°1 (2003)

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FOCUS : JSMTV

Is there a way to act simultaneously against both Salmonella and Listeria monocytogenes

in pig herds?

P. Fravalo, Y. Hascoet, C. Fablet, P.A. Beloeil, F. Madec, G. Salvat¹

Survey and control programs in the industrialized countries focus on possible presence of pathogenic micro-organisms in meat or meat products. In swine production, the epidemiological data concerning Salmonella sp. and Listeria monocytognenes all along the production chain are not equivalently achieved.

Salmonella strains found in cutting plans and on carcasses have a common ori- gin: living pigs (1). For Listeria monocytogenes (L. mono), the existence of resi- dent strains in slaughter-houses as well as in cutting plans underlines the need for controlling the colonization conditions of open surfaces (2). Despite this, the contaminated animals are still main vectors of L. mono strains in slaughter-houses and in further steps of the production. As a consequence, research efforts focus at the herd level. This study aims on one hand, to describe for a batch of pig the circumstances associated with a Salmonella sp. shedding status and on the other hand to describe the first results of an investigation relating to pig herds contamination by L. mono. The precision state of these data is different, but their confrontation confirms the opposite physiology of these two bacteria and clearly raises the question: must one make the choice of one or the other of the zoonotic bacteria in the first stages of pig production ?

MATERIALS AND METHODS

Salmonella sp.

A prospective cohort study was carried out, until the end of 2001, in 119 far- row-to-finish pig herds in France. The follow-up of a pig batch, the epidemiological unit of this study, was carried out from the piglets post weaning to the pigs slaughtering. The answers of the stockbreeders to the questionnaire targeting the zootechnical practices in the herd were recorded. Additional data were collected from the monitoring of the followed batch, as well as from the evaluation of the

1. AFSSA LERAP Ploufragan, BP53 Zoopole Beaucemaine, 22440 Ploufragan

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residual contamination of the fattening room before the entry of the animals. More than 3000 variables potentially explanatory of the Salmonella sp. excretion status were totalized per batch. Excretion was monitored by bacteriological analyses of 12 swabs of the room floor at the end of the fattening period. A batch will be defined as Salmonella excretor if at least one of the 12 swabs proved to contain Salmonella. The Multiple Correspondence analysis followed by a Hierarchical Clustering carried out on the variables independently associated to the Salmo- nella status of the batch allowed to define an excretion risk profile. A logistic regression carried out on these variables makes it possible to quantify the relative risk of these variables with respect to the status of the batch (Systat).

Listeria monocytogenes

A study was carried out in 32 herds. Selected herds were distributed according to the mode of food distribution (15 with dry feeding and 17 with soup feeding systems). They came from the same manufacturer of cattle food. This study aimed to precise the role of microbiology of the feeding in L.mono contamination of pig herds. Microbiology of food (aerobic mesophile flora, Enterobacteriaceae, thermotolerant coliforms and lactic flora counts), physicochemistry (mode of distribution, pH, buffer value, content of volatile fatty acid) were regarded as potentially explanatory variables of the L. mono contamination status of the herd. This contamination was followed by presence of L. mono in different samples: 25g of food and swabing of the back of five pigs in each fattening room. All the data independently associated with the Listeria monocytogenes status of the breeding herd were retained for the logistic regression (Systat).

RESULTS

The population of breeding herds for Salmonella study was made up on the basis of the willingness of the participating stockbreeders. For this reason it cannot be regarded as a sample of the population of the French herds even if the average technico-economic criteria selected to describe these herds are not significantly different from the results of the French pig herds, except the average size of the herds significantly larger for our population (191 productive sows present against 148 on average in France). On the 95 exploited herds, 36,8% presented Salmonella in at least one of the twelve swabs realised at the end of the fattening. They were identified as Salmonella excretor batch produ- cers. The serotypes Salmonella Derby and Salmonella Typhimurium were mainly isolated, respectively for 50% and 27,5% of the isolates. A residual contamination of the fattening rooms before the input of the animals was found in 29,9%

of the pig farms. Following the bivariate analysis, 13 variables were independently associated with the salmonella excretion data. These variables descri- bing the Salmonella herd status were submitted to a factorial analysis multiple correspondences followed by an ascending hierarchical clustering (AFCM- CCAH). This allows to define a herd profile, associated or not with a shedder status. The logistic regression allowed us to quantify the relative weight of the circumstances associated with the Salmonella status of the herds (Table 1).

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Table 1

Obtained risk profiles regarding the salmonella excretion status for a pig batch in a herd

AFCM-CAH

VARIABLE

Logistic regression Profile

“excretor pig batch”

Profile

"not excretor pig batch"

Classes (%

of excretor batches in each class)

OR CI95%

Score 0/2

Hygiene* score in maternity

Score0 (50) 15.7 2.3-105.1 Score 1 (39) 3.5 0.6-20.9

Score 2/2 Score 2

(20.7) 1 –

Hygiene* score in Post weaning

Score 0

(58.1) 4.9 1.1-22.1 Score 1

(33.3) 1.4 0.4-5.3

Score 3 (9.5) 1 –

No Clinical digestive symptoms during the growing phase

None (30.4) Yes (66.3)

1 5.6

– 1.4-22.5 No Curative collective treatment during

the post weaning –growing period

Yes (30.3) No (66.4)

1 3.5

– 1.1-11.1 Yes

Food Distribution mode Dry (48.6) Soup (28.9)

2.5 1

0.8-7.7 Yes –

Weaning age

Less than 26 days (51.5)

3.3 1.1-10.5 More than

26 days (27.9)

1 –

Between 1 and 4

Less than 1

Number of herds within a radius of 2 km of the studied herd Less than

3 days

More than 3 days

Duration (empty and clean room) between two batches No Boots washing system at the entry

of the herd Only 1 Number of different food

during the growing phase No Seropositive status of the herd

regarding the SDRP virus Yes Residual contamination of the room

before the entry of the batch

*Hygiene scores are synthetic variables (higher scores for higher hygiene levels)

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In the part of the study relating to L.mono, the bacterial composition of the feeding differed following the distribution mode for the four bacterial groups studied (Table 2). Significant differences were found for the pH, the buffer capa- cities (p<0,05) and to a lesser extent for the lactic acid content (p<0,1) of the feed. On one hand, despite the large differences recorded between the bacterial counts for the different feeding distribution modes (more than 4 log for the lactic flora for example), no parameter alone explained the contamination of the herds by L.mono. On the other hand the variable "distribution mode" was associated with the risk of contamination of the batch (OR 7,7 IC95% 1.4-42 for a presentation of food in wet form). L.mono was found in 5 breeding herds out of the 32 follow-ups (15.6%). These 5 herds distributed soup in fattening rooms. In 4 of these 5 breeding herds the presence of L. mono is detected at the same time on the animals and in the food.

Table 2

Bacteriological counts in the feed depending on the distribution mode

DISCUSSION

According to these results the feed distribution mode played a role in the contamination of a herd for both zoonotic bacteria. A dry distribution mode (opposed to the soup mode) is on one hand favourable to a breeding herd not contaminated with Listeria monocytogenes. But, on the other hand, in the same conditions, this herd is more exposed to excretion of Salmonella (excretor profile). The alternative which consists in accepting the presence of one pathogen while fighting against the presence of the other could not be accepted. This will not be justified by the epidemiological data recorded downstream from the production. A herd origin for the bacterial strains found on carcasses is demonstra- ted for both bacteria. The importance of these two pathogenic agents in term of public health imposes a simultaneous and effective control of these bacteria all along the production chain.

The levels of details of the investigations concerning these two pathogens are not equivalent. The results concerning Salmonella made it possible to show a broad list of circumstances associated with the excretion. But some of these

Feed distribution mode

Total Plate Count

Enterobacteria ceae

Coliforms grown at

44˚C

Actic Flora count (on acid MRS)

Listeria monocytogenes WET in log CFU/g

mean (sd)

7.69

(0.56)* 4.34(0.74)* 3.29

(1.05)* 8.01 (0.56)* ** Presence in 5 out of 17 Dry in log CFU/g

mean (sd)

6.46 (0.63)

* 3.44(0.76) * 2.16

(0.49)* 3.25 (0.85)* **Absence in 15

*: Significative difference p<0,01;

**: Herds number, significative difference p<0,05.

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factors are not directly usable as a corrective measure (eg density of breeding herds). Nevertheless, others seem real means of action in the herds to direct the control of these pathogens. The key factors for the control are related to hygiene. High hygiene scores should be implemented as soon as the first stages of life of piglets and maintained throughout the growing. Additionally, the installation of element of biosecurity (effective foot bath) will participate in limi- ting recontaminations. Moreover, the implementation of protocols of effective cleaning and disinfection will take part in the improvement of the level of hygiene on the farm. The inadequation of the applied procedures is revealed by the 29.9% of residual contamination by Salmonellas sp. observed in the rooms of fattening before the input of the animals. Despite the feeding distribution mode constitutes an additional factor of risk for Salmonella sp., it presents such an importance in the explanation of the contamination of a herd by L. mono that it appears impossible to avoid it for Listeria control.

The main objective concerning both pathogenic agents is to define efficient measures to control contamination pressure as early as possible in the pig production chain. It is not realistic to project an eradication of such bacteria from the primary production. Nevertheless, the production of pigs very seldomly and slightly contaminated will take part in the effectiveness of the hazard control of these pathogens in the chains of production. The efforts carried out at each stage identified as at risk in order to avoid the multiplication of the bacteria to contaminant amounts for the consumer are not called into question. The work completed for Salmonella sp. must be developed for Listeria monocytogenes in order to obtain control tools adapted to the different physiology of the two pathogenic bacteria.

REFERENCES

1. GIOVANNACCI I., QUEGUINER S., RAGIMBEAU C., SALVAT G., VENDEU- VRE J. L., CARLIER V. and ERMEL G., 2001. Tracing of Salmonella spp. in two pork slaughter and cutting plants using serotyping and macrorestriction geno- typing, J Appl Microbiol 90, 131-47.

2. GIOVANNACCI I., RAGIMBEAU C., QUE- GUINER S., SALVAT G., VENDEUVRE J.L., CARLIER V. and ERMEL G., 1999.

Listeria monocytogenes in pork slaughtering and cutting plants. Use of RAPD, PFGE and PCR-REA for tracing and molecular epidemiology, Int J Food Microbiol 53, 127-40.