Pig farmers in Europe are being continually bombarded by new legislation affecting the way they can keep and produce their pigs and salmonella control schemes are just one example of the new standards that have to be met. At the same time over recent years several antimicrobial growth promoters (AMGPs) have been withdrawn, some with anti-salmonella and E.coli controlling activities, such as carbadox and olaquindox as well as therapeutics such as furazolidone, which may have contributed to an increase in salmonella challenge. The remaining AMGPs will be withdrawn in almost a year and a rise in enteric infections is generally anticipated. As a result a considerable amount of research work is presently being carried out to look at ways of minimising infections with salmonella. These include hygiene, diet and the use of new additives. It is becoming increasingly clear that the stomach plays a key role in preventing salmonella entering the gut where it can cause harm and trigger unwanted reactions.

Salmonella are taken in via the mouth, usually following faecal contamination of the pen by other infected pigs. Therefore, straw, solid floors, floor feeding, poor hygiene and rodent infestations all contribute to increased chances of microbial intake and challenge. Salmonella is widespread in pigs with on average 30% being infected, so the chances of infection are relatively high. There are a variety of species of salmonella but the most common in pigs (approximately 70%) is Salmonella enterica Typhimurium, which is the second most common salmonella infection in man, hence its public health significance. Typhimurium is also important for the pig, as well as colonising the gut, it is also invasive and can penetrate the lymphoid system and the body, stimulating an immune response and antibody production. This is an important and normal response to fight the infection but these antibodies are the means whereby infection is monitored using the meat juice ELISA tests on slaughter pigs. The test is a good indicator of infection during the life of the pig but does not necessarily represent the carrier status at slaughter (see graph 1).

Graph 1. Comparison of salmonella infection (faecal culture) and serologic antibody response (Kranker et al, 2002)

Another important factor is that if pigs are not continuously exposed to infection then the antibody levels fall with time and the meat juice ELISA test, being less sensitive than serology, can fall away below detection in approximately two months i.e. in the finishing period.

Methods of control

Many countries have been looking at different ways to control salmonella infection on the farm.

Effects of hygiene

In the UK, a pilot study on 11 farms (Cook, 2004) using a strict cleaning and hygiene program disappointingly showed very little difference with regard to salmonella isolation from pooled faeces samples compared to 11 standard farms (24% isolation on the standard and 21% on the hygiene program). The meat juice ELISA results did however improve with 58% positive on the standard system and 40% positive (-31%) on the hygiene program. Unfortunately they did not take into account the salmonella status of the grower pigs coming onto the farm and as a result carrier pigs were able to contaminate the pens and pen mates and keep the infection going. (See graph 2)

Graph 2. Effects of a hygiene program on the incidence of salmonella isolation and meat juice ELISA tests (Cook, 2004)

Effects of nutrition/feed processing

In Denmark they have made the most progress in developing different feeding programs to reduce the level of infection. In an excellent PhD thesis by Christian Fink Hansen (2004) of the Danish Royal Veterinary and Agricultural University, Copenhagen, he describes a number of trials. One study looked at the influence of a coarsely ground meal feed and  a finely ground pelleted feed on performance and meat juice ELISA results in pigs reared from 32-102 kg liveweight.

Table 1. Comparative performance and salmonella meat juice ELISA tests of pigs fed meal and pelleted feed (Hansen et al, 2004)

Coarsely ground meal feeding distinctly reduced the meat juice ELISA scores by 42% in comparison with a finely ground pelleted feed but there was a substantial reduction in performance, which economically would be difficult to justify.

Why should a differently processed feed have such an effect and how does it have an impact on salmonella? They showed that with the finely ground feed the stomach contents were more uniform and took upto five hours to reach a low pH (acidity) of about 2, (the level that kills salmonella). The coarse feed however caused stratification of the stomach contents with the coarser material in the lower (distal) part and the pH reached 2 at about two hours, thereby exposing the contents and the salmonella more quickly and longer to a killing level of acid before they passed into the small intestine. This was shown to have a marked effect on the numbers of enterobacteria in the stomach and their reduction broadly mirrored the pH level.

 

Graph 3. Effect of coarsely ground meal and finely ground pellets on acid pH in the distal stomach (Hansen et al, 2004)

Acid levels in the stomach have a major killing effect on the number of acid-susceptible bacteria such as salmonella and E. coli. This reduces their passage into the small intestine and beyond and appears to be the major factor in the reduction in salmonella colonisation of the gut, their invasion and hence reduction in meat juice ELISA positive pigs.

 

Acidification of feed - Formi

Acidification of the feed, with a recently EU approved non-antibiotic growth promoter based on formic acid, potassium diformate (Formi – BASF), appears to also enhance the stomach acidity, reducing pH and salmonella infection as well as improving pig performance.

 

In a second study by Hansen et al (2004) they showed that the addition of Formi at 6kg/tonne, combined with the pelleting of coarse ground meal, overcame much of the performance depression in comparison with finely ground pelleted feed, when given to pigs between 30-106kgs bodyweight.

Table 2. Effect of Formi and pelleting of coarse ground feed in comparison with fine ground pelleted feed (Hansen et al, 2004)

Also in a recent study in the UK (Dennis and Blanchard, 2004) showed that the inclusion of Formi at 6kg/tonne had a marked effect on reducing meat juice ELISA scores for salmonella. On a 2000 place finishing unit with a history of high levels of salmonella infection, 82% of pigs were shown to be serologically positive 7 weeks after arrival. All the grower and finisher diets had Formi added for 2 months. The batch results were compared with previous batches for performance and meat juice ELISA results.

Graph 4. Effects of Formi (potassium diformate) given for two months on meat juice ELISA scores

The meat juice ELISA scores were reduced from 59% positive to 27.3% (54% reduction) (see graph 4) taking it out of the penalty levels of the UK ZAP (zoonoses action plan) scheme. In addition the growth rate was improved by 7.7% and the mortality reduced from 4.25% to 2.53% (40% reduction). It was also reported that salmonella was eliminated from all mouse droppings tested.

 

With all the restrictions being imposed on antibiotic growth promoter use in the past and in the future, combined with the tightening up on salmonella control via the various national schemes, it is pleasing to see great progress in research is being made to help our understanding of the way nutritional factors and new additives can assist nature’s own defences, via the stomach, in salmonella control.

 

Copyright © Octagon Services Ltd   December 2004

 
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