Effectiveness of Salmonella control strategies in fattening pigs

The first aim of this study was to examine which control mechanism is the most effective and profitable to control Salmonella in fattening pigs. Three treatments at farm-level are examined: 1) hygiene and management measures; 2) acidified feed; 3) acidified drinking water. The second aim of this study was to examine the correlation between the Salmonella blood titre (immunology S/P value) and the Salmonella excretion in faeces of the slaughtered pig. Acidified drinking water and acidified feed showed significance reduction of the Salmonella prevalence in fattening pigs. Acidified drinking water showed improvement of the daily gain and feed conversion and is therefore the most profitable control strategy. Farms with high status or mean S/P value had significance more Salmonella excreted pigs than farms with low status or mean S/P value. Focusing on the height of the Salmonella titre on farm level reduces the chance of contamination of the meat processing plant. Introduction Salmonella spp. has often been mentioned as one of the main bacteria that can cause illness in humans and is therefore a health risk (Berends et al., 1998; Leclerc et al., 2002; Gomez et al., 1997). Contamination of meat and meat products with Salmonella spp. is of growing concern in the modern pig livestock sector. This concern is related to public health, animal health and welfare and to international trade of animals and products of animal origin. In Denmark, 22.2% of 1,363 investigated pig herds were Salmonella positive by microbiological testing (Baggesen et al, 1999). In Belgium, 92% of the herds were found positive (Nollet et al, 2004), in Canada 67% (Rajic et al, 2005). Berends et al (1998) estimated that 25-30% of primal cuts and retail-ready pork in butchers’ shops are contaminated with Salmonella while this figure is 50-55% for minced pork and pork sausages. Approximately 22% of the 50,000 Dutch human cases of salmonellosis in 2003 can be attributed to consumption of contaminated pork (Valkenburgh et al, 2004) These figures show that control strategies to reduce the risk of Salmonella infection in both pigs and humans are needed and several countries have started with its implementation. These control strategies are merely based on neutralization of specific risk factors like (a) prevention of introduction of the pathogen to the farm, (b) prevention of transmission between units within an infected herd, (c) prevention of colonization of the pathogen within the host, (d) prevention of cross-contamination during transport, slaughter and product handling in the meat processing industry (VanderWolf et al, 2001a; VanderGaag and Huirne, 2002; Lo Fong Wong et al, 2004). Prevention of introduction to a pig herd can (at least partly) be achieved by buying piglets that are not infected with Salmonella (e.g. SPF piglets) and maintaining a strict hygiene protocol for people and goods. Prevention of colonization can be induced by antimicrobial growth promoters (Baggesen et al, 1999), use of acidified drinking water (VanderWolf et al, 2001b) or by feeding fermented feed (VanWinsen et al, 2000, 2001). Acidification might have a direct toxic effect on the pathogens (VanWinsen et al, 2000) while its effect on the micro flora in the gut and thus on the colonization and growth of Salmonella still is unclear (VanWinsen et al, 2001). In the current study we examined the effect of acidified drinking water, acidified feed and an improved hygienic management on the level of antibodies in finishing pigs. Therefore, 36 herds were sampled 8 times within 2 years while the intervention measures were applied after the first year. Materials and Methods Participating herds In March 2002, 50 farmers running a pig fattening herd were approached to participate in the study. The farmers obtained the feed of a specific feed mill (Hendrix UTD Feed, Lochem) and their pigs were slaughtered in a specific factory (Hendrix Meat Group, Meppel). During the study, 14 herds were withdrawn for various reasons (end of farming, change of feed supplier, decreasing interest and motivation). Pig fattening herds (n = 36) were followed over a full period of 2 years (March 2002–March 2004). Blood samples were taken four times a year at regular intervals at the slaughter line from 15 randomly selected fattening pigs. Samples were collected immediately after bleeding and sent to the Animal Health Service to determine the level of Salmonella antibodies by an indirect ELISA using LPS antigen of S. Typhimurium and S. Livingstone (VanderWolf et al, 2001c) that represent over 90% of all S. enterica serovars found in Dutch finishing pigs (VanderHeijden et al, 1998). S/P