As a result of new European regulations, all member states of the European Union have to implement monitoring and control programs for Salmonella in poultry (European Parliament and European Council, 2003a, b). In laying hens, vaccination can reduce shedding and egg contamination (Davies and Breslin, 2003a; Van Immerseel et al., 2005a). Vaccination is not recommended for broilers because the short lifetime of these chickens makes this option less economic (Van Immerseel et al., 2005a). For broilers, a combination of maternal vaccination, intensive hygienic measures, and the use of specific feed additives can aid in preventing and controlling the infection (Methner and Steinbach, 1997; Van Immerseel et al., 2002; Davies and Breslin, 2003b).
Short-chain fatty acids have been used extensively in the field to control Salmonella in broilers. These acid preparations have been empirically used for their antibacterial action against Salmonella. Organic acids of various forms and formulations have been used extensively to control Salmonella in broilers. These acidic preparations are used for their antibacterial action against Salmonella and other Gram negative bacteria. This function is based on the acidic nature of the products. In pH neutral conditions this does not work any more. For the acids to work properly, they need to be available in the feed and in the stomach. Beyond the acidic stomach, the animal effectively buffers the digesta, removing the antibacterial effect of organic acids.
There are two types of preparations on the market: uncoated and coated acid products. Uncoated products are powders or liquids that are used to supplement feed or drinking water, mainly to kill Salmonella in these matrices. After uptake, actions of these acid preparations are limited to the crop because of resorption (Thompson and Hinton, 1997). All acids that are antibacterial to Salmonella can potentially be used in these uncoated preparations (Al-Chalaby et al., 1985; Hume et al., 1993; Moore et al., 2004). In the coated or encapsulated products, mineral or lipid carriers are used.
The aim of coating or encapsulation is to carry the acids down to the intestinal tract of the chickens. In this way, also the gut epithelial cells could be exposed to the acids, and the composition of the intestinal microbiota can potentially be modified by the action of the acids. Moreover, the acids are thought to be in closer contact with Salmonella at the site where Salmonella mounts a crucial step in the pathogenesis, being invasion of the epithelial cells. In the case of coated products, care should be taken in choosing the ideal acid compounds, as more complex interactions than simply antibacterial activity can play a role.
Butyrate is indeed based on an organic acid; namely butyric acid. However, the anti-Salmonella mechanism of action is completely different. Butyrate works against Salmonella and other bacteria by making the host animal more robust. The butyrate molecule signals the tissues in the gut wall to tighten up and work harder. To summarize the anti-Salmonella action of butyrate: it makes the host animal more resilient against Salmonella. This could also be seen als an added benefit on top of the animal performance enhancement for which butyrate is normally used.
Contaminated pork has been the second most important source of human salmonellosis (Hauser et al. 2010). The pathogen is common and persistent in the environment of pigs and therefor not easy to control. In most cases, Salmonella Typhimurium will subclinically colonize the pigs, without causing obvious symptoms. These carrier pigs are a vast reservoir of Salmonella and pose a major threat to animal and human health (Boyen et al., 2008).
Inflammation is a contributing factor in Salmonella colonisation
Winter et al showed in 2010 that gut inflammation is in fact a crucial step in the pathogenesis of Salmonella typhimurium. As it turns out, Salmonella triggers and then utilises the inflammation response to compete with other bacteria. Drumo et al (2009) exposed piglets to S typhimurium and showed that is leads to inflammation and a reduction in beneficial (butyrate producing) bacteria. This begs the conclusion that lowering inflammation leads to a better natural resistance to Salmonella colonisation of the gut. Supplemental butyrate does just that.
Butyrate and salmonella research in broilers
It was shown that coated butyric acid was superior in controlling Salmonella colonization in broilers, compared with coated formic and especially acetic acid (Van Immerseel et al., 2004a). Butyric acid is known to decrease virulence gene expression and invasion of Salmonella in epithelial cells in vitro; acetic acid has opposite effects (Lawhon et al., 2002; Van Immerseel et al., 2004b).
Several research papers have been published about the effect of butyrate on Salmonella shedding in broilers. Van Immerseel et al. (2005b) has shown positive results giving sodium butyrate as a pure compound and in the form of a coated feed additive (both at an effective dose of 0,63 g of butyric acid/kg of feed), 10 Ross broiler chickens were infected at d 5 with 105 cfu of S. enteritidis and housed together with 40 noninfected broilers. A control group received nonsupplemented feed. The group of broilers receiving coated butyric acid had a significantly lower number of broilers shedding Salmonella bacteria, but cecal colonization at slaughter age was equal for both groups. They concluded, butyric acid decreases cecal colonization shortly after infection, decreases fecal shedding, and as a consequence, decreases environmental contamination by S. enteritidis-infected broilers. However, complete elimination can probably only be achieved with a combined approach using both hygienic measures and different protection measures, as the broilers still carried S. enteritidis bacteria in the ceca at slaughter age, although at very low level.
Also Fernandez-Rubio et al. (2009) showed a significant reduction (P < 0.05) of Salmonella Enteritidis infection in birds from d 27 onward in testing two different types of butyrate (one free and one protected, both dosed at an effective dose of 0,92g of sodium butyrate/kg). The partially protected butyrate additive was more effective at the late phase of infection. Partially protected butyrate treatment successfully decreased infection not only in the crop and cecum but also in the liver. There were no differences in the spleen. These results suggest that sodium butyrate partially protected with vegetable fats offers a unique balance of free and protected active substances effective all along the gastrointestinal tract because it is slowly released during digestion.
Butyrate and salmonella research in pigs
Barba-Vida et al. (2016) showed promissing results for dose of 3 g/kg of a mildly protected sodium butyrate in piglets. They found significant reduction in the colonisation and excretion of salmonella in piglets orally challenged with a pathogenic strain of the bacterium. Also they found a trend in crypt depth at day 4 after infection.
Boyen et al. (2008) tested multiple organic acids, coated and uncoated in pigs. When given as feed supplement to pigs experimentally infected with Salmonella Typhimurium, coated butyric acid decreased the levels of faecal shedding and intestinal colonization, but had no influence on the colonization of tonsils, spleen and liver. Uncoated fatty acids, however, did not influence fecal shedding, intestinal or tonsillar colonization in pigs. This work showed that supplementing feed with 2 g/kg of a coated butyric acid, can reduce Salmonella load in pigs. Boyen et al show that direct antimicrobial effects are not the cause of this effect, but rather the down regulation of pathogenicity genes in Salmonella spp.
A mixture of organic acids was also tested against coated calcium butyrate and vaccination by De Ridder et al. (2013). Again butyrate showed the most promising measure against excretion of Salmonella spp. in the feaces and gave the lowest transmission. The conclusion of this work was therefore “…that vaccination and supplementation of the feed with coated calcium-butyrate limited Salmonella transmission in pigs and might be useful control measures.”
The dosage of the mildly protected butyrate that was used in the research in pigs is higher compared to those in broilers; up to 3 g/kg. This is not surprising since transit times in digestive tracts of pigs is longer than in poultry, so it could take a larger dose to reach the more distal sections of the GIT.
Research based conclusion and recommendation for use of butyrate
Butyrate is beneficial for lowering Salmonella in both broilers and pigs, although in pig it may require a higher use level than in poultry. The mechanism is not direct antimicrobial effects from the butyrate. Rather, the two mechanisms at play are the reduction of pathogenicity of the Salmonella and the increased robustness of the host. The combination of hygienic measures and supplementation of butyrate in the feed is the recipe for Salmonella decrease in livestock farming.
Recommendation for butyrate supplementation
Supplementation levels of butyrate in literature show that even at a low dosage good results can be achieved in broilers. Van Immerseel et al. (2005b) showed that 0,63 g of sodium butyrate/kg of feed significantly lowered faecal shedding. This dosage is simuliar to the dosage used by Fernandez-Rubio et al. (2009), who used 0,92g sodium butyrate/kg of feed.
Besides the dosing the correct formulation of the butyrate is of high importance. Each target animal will require a specific release pattern. Sodium, calcium and a combination of both as well as degree of coating will give specific realease patterns to target different parts of the GIT.
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