Foodborne diseases are serious public health, economic and food safety issues, among them the role of Salmonella in pork is one of major interest. A report from European Food Safety Authority (EFSA) in 2013 indicated that there was 80.3 million cases of foodborne salmonellosis in humans per year in the world and nearly 56.8% were related to pigs, pork and pork products. Effective Salmonella control programmes have been proven in poultry in many countries, however, some challenges is still faced by the pig industry to minimize Salmonella contamination in the farm-to-fork approach as there is no unique strategy for eradication of Salmonella in pig herd. Therefore, implementation of biosecurity, sanitation, vaccination, medication and management are crucial measures.

Salmonella is one of the major foodborne diseases around the world. Because of its endemic and high morbidity nature, it makes this a zoonotic pathogen as well as public health issue.  Animal origin foods are the mainly accountable for this serious problem and among other meat products, pork is of remarkable interest. Salmonella enteritis and Salmonella typhimurium (S. typhimurium) are responsible for most of the human salmonellosis or enteric fever. Literature indicates that the enteric fever is largely prevalent in the low and middle income countries of South Asia.  S. typhimurium is the leading cause of the disease cases responsible for 12 -21 million infirmity episodes and over 140,000 causalities per year globally. Previous studies conducted between 1995 and 2006 in India estimated the incidence of salmonellosis was 178–801 per 100,000 people per years with the highest incidence in early age individuals.  Foods derived from animal origin are the main responsible for these serious problems and among other meat products, pork has a significant interest, and however, there may be some variation among countries.

Here it is worth to explain that the infection mainly occurs through the orofecal route. The organism colonizes in the distal portion of the small intestine followed by invasion of the epithelial tissue and the bacterium can remain in gut lymph nodes. The organisms colonized in the lymph node excreted intermittently during periods of stress, even though clinical signs are no longer present. This is the reason for the high potential of Salmonella contamination throughout the production chain of meat and products.

Sources of Salmonella contamination:

1. Swine contamination: The introduction of this pathogen may occur through the replacement stock or animals from different sources, feed sources, biological vectors (human, rat, infected pig, etc) during transportation and even at lairage at the pork processing plant. Among these main sources of infection on-farm are the purchase of replacement stock and feed because of high frequency of replacement stock and arrival of feed stuffs to the farm. Salmonella can be introduced into the feed by contaminated feed ingredients. Feed contamination may occur during transport, processing of ration, storage and distribution. Many biological vectors act as reservoir of Salmonella, for example, improper rodents control programs on-farm is of high risk of infection in pigs. It is important remember that the finisher pigs need especial care against Salmonella infection. Further, residual environmental contamination, after cleaning and disinfection, of finishing pens are also a common cause of Salmonella occurrence in the herd. 
2. Contamination of pork at slaughter and processing: Pigs are often healthy carrier for Salmonella spp. and the transportation stresses cause the pigs more susceptible for Salmonella spp. It was also evidence that longer transport time showed a higher level of Salmonella shedding. Therefore it is need to transport the pigs to be slaughter in a sterilized vehicle as a measure to reduce the potential for contamination of pigs. Further, continuous entrance of Salmonella carrier pigs in the abattoirs is considered the main risk factor for the contamination of pig carcasses and carcass contamination is directly related to the pig skin contamination prior to stunning.  After stunning and scalding the most important source of carcass contamination is the presence of faeces, feed and microorganisms in the water. Moreover, during evisceration, the main risk of carcass contamination is through a leakage of the intestinal content and the cross-contamination by equipment and butcher’s hands. Therefore special cares need to be taken in these steps of pork processing.

The above are the common mode of “Farm-to-fork” transmission of Salmonella infection. There are many ways to break the chain of “Farm-to-fork” transmission of Salmonella infection. However, this article emphasizes the use of alternative approach that is the use of non-nutritional dietary additives for prevention/control of “Farm-to-fork” transmission of Salmonella infection. Because, the demand for deletion  of antimicrobial use in animal production and the ban on their use as feed additives has contributed to need for alternative control strategies for bacterial pathogens of food-producing animals as well as S. typhimurium infection of pigs. Therefore, dietary strategies have focused on the prophylactic use of different non-nutritional dietary additives such as prebiotics, probiotics, phytogenics, organic acids and essential oils. Such approaches have been demonstrated to improve gut function of meat animals as well as for prevention of Salmonella infection for the consumers.

Role of non-nutritional feed additives against foodborne Salmonella

1. Prebiotics: The term prebiotic was defined as “A non-digestible food ingredient that beneficially affects the host by selectively stimulating the favourable growth and activity of one or a limited number of bacteria in the colon and therefore attempt to improve host health”. These are mainly oligosaccharides or soluble fibres, proteins, peptides and some types of lipids. A prebiotic must have the following property-

1. It should be resistant to acidic pH of stomach, cannot be hydrolyzed by gastric enzymes, and also should not be absorbed in the gastrointestinal tract
2. It can be fermented by intestinal microbiota.
3. Improve luminal or systemic aspects of the host defense system.
4. It should selectively stimulate the growth and/or activity of the gut microbiota to improve host’s health.  

Example of some commonly use prebiotic are Fructo-oligosaccharides (FOS), Mannan-oligosaccharides (MOS), Inulin, Lactulose, Xylo-oligosaccharides (XOS). Sometimes prebiotic and probiotic can be use together for better result. Combine use of both prebiotic and probiotic are known as symbiotic. Most of the prebiotic prevent colonisation of pathogens such as Salmonella by competing for the attachment site in the digestive tract. Study also revealed that pigs supplemented with prebiotics increased the total serum IgM and IgA levels against S. typhimurium.  

2. Probiotics: Probiotics are living microorganisms that are fed to animals to colonise the gut   environment to create a better microbial balance. It has been shown that probiotics stimulate intestinal epithelial integrity and systemic immunity through inhibiting growth and dissemination of pathogenic microorganisms. The intestinal epithelial integrity is a major defense mechanism against pathogens. Defenses of the intestinal barrier consist of the mucous layer, antimicrobial peptides, immunoglobin A (IgA) and the epithelial junction adhesion complex. These barriers disrupt adhesion of pathogen to intestinal wall and prevent entry of food antigens to submucosa for prevention of inflammatory responses. Disrupted intestinal epithelial integrity may result in intestinal disorders.  Most of the probiotic mechanisms of action include maintenance of intestinal epithelial integrity, competitive inhibition of pathogen adhesion to gut mucosa, production of anti-microbial substances and modulation of the immune system. A probiotic must have the following characteristics-

1.  It should be gram positive, acid and, bile resistant and contain a minimum 30×10⁹ CFU per gram to exert a positive effect on the host.
2. The cultured organism neither be pathogenic or nor toxic to the host and the culture should posses high survival rate and multiply faster in the digestive tract.
3. The adhesive capacity of a microorganism to gut mucosa must be firm and faster.

Commonly use probiotic are genus for pigs are Lactobacillus, Bifidobacterium, Saccharomyces, Enterococcus, Streptococcus, Pediococcus, Leuconostoc, Bacillus. Study on Bacillus subtilis and Bacillus licheniformis have been shown to reduce the aggression of Salmonella into swine intestinal epithelial cell and no Salmonella in faeces of experimental pigs. Further studies in pigs have shown that lactic acid bacteria can improve immune responses to Salmonella choleraesuis.

3. Organic acids: In order to prevent bacterial resistance against antibiotics, apart from other feed additives, fermented liquid feed has been suggested as alternative for antibiotic. Fermented liquid feeds are produced by incubating the feed together with either the water or ethanol or food by product. During the fermentative process there is production of different organic acids by the micro organisms. The organic acids produced during fermentation mainly lactic acid and acetic acid. These organic acids have both the properties of bacteriostatic and bacteriocidal. These acids can be effectively used along with other feed additives at certain quantities to get bacteriostatic and bacteriocidal action in the gut. The antimicrobial action of organic acids due to its ability to reduce stomach pH. At this lower pH the acids can penetrate the bacterial cell wall and disrupt the normal actions of different species of bacteria viz Salmonella spp, E. coli, Clostridia spp, Listeria spp. and coliforms. Therefore, reduction in numbers of some species of the normal gut flora as well as pathogenic bacteria can occur in animals fed organic acids. The first organic acid to be approved for use in swine diets, by the European Union was a formic acid that was reported to reduce the incidence of Salmonella in pigs. Studies revealed that use of short-chain fatty acids viz formic, acetic, propionic and butyric have been shown to inhibit Salmonella growth however, use of medium-chain fatty acids (i.e., caproic, caprylic and capric) in swine diet proved a better results in inhibition of Salmonella growth.   

4. Phytogenics and essential oils: Phytogenic feed additives or phytobiotics are commonly defined as plant-derived extracts used in feed to potentially improve performance of animals, whereas essential oils are volatile components of plants. Both additives can be incorporated into feed to improve the productivity and/or health status of livestock. For example Macleaya cordata is a natural plant-derived supplement having antimicrobial, immunomodulatory and anti-inflammatory properties. A range of essential oils have been shown to have bacteriostatic or bacteriocidal properties against Salmonella in vitro. Among the most studied essential oils/components known to have anti-Salmonella activity are rosemary, oregano, lemongrass, clove, sage, mustard, basil, thyme, α-terpineol, carvacrol, citral, eugenol, geraniol, perillaldehyde and thymol. However, studies are currently lacking regarding the applicability of different essential oils in feed against control of Salmonella in pigs.