THE ESSENTIAL MULTI-FACTORIAL SALMONELLA CONTROL IN SWINE:
THE ANIMAL WELFARE WAY

Murray J Hyden, M.I.Biol, C. Biol Technical Sales Director, Agil Ltd, UK
Dr Helen Stebbens, Consultant, HRS Animal Nutrition Consultancy

Consumer awareness concerning the problem of Salmonella in pigs is a major worry to the swine producing industry worldwide. In December 2000, The Veterinary Record reported the results of a survey, which indicated that to ignore this concern could result in damage to an industry just emerging from recession. Pig producers now have a chance to be "pro-active" in terms of animal welfare and in doing so should avoid some of the devastating problems suffered by the poultry industry in the 1980's.

John Thompson and Sons Ltd., the major Northern Irish Feed Compounder is leading the way with a novel approach to re-building consumer confidence thanks to a unique approach to welfare by co-operation between farmer, veterinarian, feed additive manufacturer and the feedmill.

Salmonella in Pigs - is it a real problem?

A British survey, conducted between March 1999 and February 2000, reported on the prevalence of salmonellae in pigs at slaughter (Davies, et al, 2000). The results indicate that using an indirect Elisa test and 40% optical density cut-off, 15.2% of pigs were seropositive. Salmonella were, however, identified in 23% of pig caeca, again by serological testing. Salmonella contamination levels of just 5.3% were found on carcasses when tested bacteriologically and the predominant serotypes were S. typhimurium and S. derby.

From this data a high frequency of Salmonella seropositives were found in slaughter pigs. However levels of viable salmonellae on processed pig carcasses were generally low. Similar results have emerged in the US and other major pig- producing European countries.

Since the Salmonella crisis in the poultry industry in the UK closer monitoring of feed ingredients and other bio-security procedures in animal feed manufacturing plants has been improved. Currently, less than 5% of all animal feed produced in the UK is contaminated with salmonella and the human food poisoning serotypes - S. enteritidis and S. typhimurium are rarely isolated.

Quality Assured Pig meat - The First Step

Quality Assurance Schemes have enabled closer monitoring of the way animals are bred, reared, and processed to produce quality food products. In 1997, Thompsons, the Belfast-based feed manufacturer, formed the Elite Pig Producers Group. In collaboration with meat processors, pig breeders, and specialist pig veterinarians Thompsons monitor the production of consistent pig carcasses for which premium prices are paid. This group now finishes 4000 bacon pigs per week. Pigs are produced to certain production criteria and conformity, under strict veterinary control. Thompson's also provide farmers and processors with on-going training in the form of workshops and seminars.

A salmonella-monitoring scheme has been introduced, following the identification of Salmonella as a potential problem on pig farms by the Group specialist pig vet, David Stewart. Interpretation of the Salmonella data is based on a combination of the schemes developed by the Danish Authorities and the Scottish Agricultural College in Edinburgh. Mr Stewart believes that the use of Salmonella antigen blood testing, and analyses makes the scheme more sensitive to fluctuations in Salmonella on-farm. Based on the data reported in previous trials, blood testing for Salmonella antigens will tend to eliminate false positives due to the persistence of Salmonella antibodies in the host animal.

In 1989, all 42 units within the Group were tested. Results revealed that only 3 of the 42 units were positive for Salmonella. Thirty-nine units were found to fall into Level 1. The three remaining units were classified as Level 3. Veterinary treatments combined with changes in management practices and improvements in pest control, resulted in 2 of the 3 units moving into Level 1, whilst the third unit was only able to achieve Level 2 status.

Even at the outset when this well respected herd was testing as Level 3, performance levels were acceptable. Weaners were achieving 591g/d with an FCR of 2.08, and finishers were growing at 885g/d with an FCR of 2.51. Blood samples taken in January 1998 revealed that 2 of the 12 samples from finishing pigs contained S. typhimurium; the most common serotype isolated from pigs on mainland Europe, Great Britain and the US.

Welfare improvements including a strict all-in-all-out policy in all accommodation was introduced. Thorough washing, disinfection and resting, between batches of pigs was introduced. Feeders, feed containers and barrows were made rodent proof to prevent contamination with bird or rodent faeces, which may be a source of Salmonellae.

Approximately 12 months later, following periods of medication to control the clinical problems of diarrhoea, further blood sampling revealed that the herd had still only moved into a Level 2 status. This may well have been the result of earlier use of prophylactic antibiotics for several years prior to the start of the investigation, during which time antibiotic resistance had developed. This is a problem of alarming importance in Europe and national Governments are attempting to solve this problem through legislation and bans on the use of prophylactic antibiotics.

This data published by the UK Government clearly shows that the main source of Salmonella on pig farms is likely to be horizontal and vertical transmission between pigs and not from the environment or the feed. Consequently the need to find a non-antibiotic remedy to work in the pig intestine and reduce Salmonella to achieve Level 1 status on this unit became a major challenge.

Even so it was necessary to identify and eliminate, all sources of S. typhimurium. Thompson's have always concentrated on quality and viewed themselves as pioneers within the UK feed industry. In 1992, they were the first feed mill in Europe to achieve the International Quality Assurance Standard, ISO 9002, for its entire manufacturing process. In 1996 Thompson's built a state-of-the-art feedmill designed to destroy Gram negative and non-sporing pathogenic bacteria by subjecting the feed to a pre-determined temperature for a defined period of time. Enclosing the entire manufacturing process within a complete hygiene barrier at positive air pressure prevents re-contamination of the heat-treated feed during cooling and packing or bulk loading operations. All of the air supplied is filtered through antibacterial filtration units. They did not believe that the feed constituted a possible risk and the fact that they were supplying all the other farms in the Elite Pig Producers group supported this belief.

The mill biosecurity programme for the detection and elimination of Salmonella focuses on all types of feed. Some Salmonella serotypes have been isolated in raw materials at the mill but these are predominantly S. montevideo and S. seftenberg rather than the S. typhimurium found in the pigs. The feed was therefore not considered to be the source of the Salmonella. The farm or breeding sows are believed to be the major sources of Salmonella on pig farms with water and housing being some of the most important areas of concern. Salmonellae from the environment can easily colonise the small and large intestines under certain stress conditions and especially the immature gut of the newly weaned pig.

However, two other feed factors about the feed may aggravate the problem of Salmonella colonisation:

1. The buffering capacity of the feed. The inability for the pig to acidify the feed completely restricts protein digestion and prevents destruction of invasive pathogens in the stomach. A problem in both newly weaned piglets and lactating sows, where high calcium levels dramatically increase the buffering capacity of the feed.

The compound feed industry has tried to utilise ingredients and balanced rations to alleviate the problem. However the buffering capacity of feed is such that it would require in excess of 20 kg/Te of Hydrochloric (stomach) acid per Tonne to lower the pH to a level where potential enteropathogens such as Salmonella might be killed. If the use of organic acids is considered then the inclusion rate rockets to a staggering 250 kg/Te. A quite unrealistic figure.

2. Inappropriate protein and carbohydrate nutrition in relation to the enzymatic development of the pig gut. Again a problem for newly weaned pigs as undigested materials may aggravate the gut lining, increasing the risk of pathogen colonisation and diarrhoea.

Correct selection of raw materials in relation to the digestive maturity of the pig can reduce but not eliminate this problem. This may be a simple approach to reducing antibiotic usage and the threat of antibiotic resistance.

Thompson's, working with specialist researchers in the Agricultural Institute at Hillsborough in Northern Ireland have selected raw materials, designed diets and identified efficacious products that will help to promote a healthy gut microflora.

However, once present in the gut invading pathogenic bacteria require only four things for growth.

1 Temperature - -The body temperature of a pig is ideal.
2 Nutrients - - - --The gut of the growing swine is full of nutrients.
3 Water - - - - - - Pigs drink.
4 pH - - - - - - - - The pH of the small intestine is perfect for E. coli and the pH of the large intestine and caecum is ideal for Salmonella.

Since the body temperature of the pig cannot be changed, and food and water cannot be restricted without compromising performance and welfare of the pig, an alternative means of altering the pH of the gut must be sought if the reliance on antibiotics is to cease. The addition of acid to the gut, either through the drinking water or the feed would appear to be an obvious solution but as mentioned earlier it is impossible to alter feed pH to a level where Salmonellae would be killed due to a pH effect.

Thompsons had already discovered that the use of dietary acids can be effective in preventing certain pathogenic bacteria, including Salmonella, from re-contaminating the feed after leaving the controlled environment of the feedmill. These acids work effectively up to the point of consumption. After consumption free organic acids or acid salts will immediately be metabolised as energy sources by the pig and will be unavailable for any microbiological control. For example organic acids such as lactic, formic and propionic which are widely accepted as feed additives are readily digested just as the lactic acid in yoghurt or the citric acid in fruit are digested in humans.

However, two products were claimed to continue to work in the gut and therefore help prevent non-feed derived Salmonellae from colonising the gut epithelium. The key component of these products is a carrier matrix that enables the acids to reach the site where they are most needed - the hindgut.

Different bacteria found in the gut colonise specific regions based on their ability to survive the local pH environment.

Acidification of the gut should be a possible means of preventing the growth of pathogens that tend to favour more neutral to slightly alkaline pH values for optimal growth if there is a reliable mechanism that lowers the gut pH.

Two products were identified that use a unique carrier system were identified and tested on the problem farm. The carrier system is claimed to protect the organic acids used allowing them to pass through the stomach to the small and large intestines. This protection is achieved through the use of a mineral carrier matrix that does not affect the acids, but absorbs them and transports them to the intestines with controlled release into the gut lumen.

This method of acid delivery has proved to be so effective that these products actually use buffered acids to prevent over acidification in areas of the small intestine where digestive enzymes may be affected by a major shift in pH. The buffering also has the benefit of reducing acid volatility and effectively eliminates the threat of reduced palatability of the treated feed.

The mineral carrier remains slightly more acidic than the surrounding digesta in the gut lumen and becomes colonised by acid tolerant and acid-producing organisms that ferment feed carbohydrates to produce organic acids such as lactic, propionic and acetic acids. These acids lower gut pH and create an environment that is antagonistic to the growth of enteropathogens such as Salmonella. The carrier therefore is used solely to provide a microenvironment that supports the growth of natural fermentative microflora in the gut. It is these natural gut microflora that produce the acids necessary to protect the hindgut from Salmonella colonisation. This is the same principle as achieved by using probiotics however probiotics are often not as effective as would be hoped for due to a long lag phase before colonisation if the gut lumen is too alkaline for effective colonisation. These acid products rely on the indigenous Lactobacillus population of the gut, and provide the essential environment for their multiplication.

Thompsons have examined two of these products. The first product is used as a protected formic/propionic blended biosecurity product to control pathogens such as Salmonella.

The second product is a more complex acid blend incorporating a fructo-oligosaccharide. Fructo-oligosaccharides are extracted from plants using hot water and are particularly valuable in creep and starter feeds.

At the time of weaning the main fermentable carbohydrate source (lactose) is lost from the diet. As a result of this loss of lactose the beneficial Bifidobacteria species rapidly die-out, as they are unable to ferment the complex carbohydrates such as cereal starches found in creep and starter diets. Fructo-oligosaccharides are not metabolised by pathogenic species such as Salmonella but are fermented by Bifidobacteria. So by providing a fructo-oligosaccharide we are able to supply an alternative and specific fermentable carbohydrate source for these Bifidobacteria, thereby extending the total period of protection afforded by these unique bacteria.

The main benefit of Bifidobacteria in the digestive tract of young pigs is that they convey a greatly improved immune response to the piglet through the production of pathogen specific bacteriocins. The Bifidobacteria are therefore vital in the development of a healthy gut microflora. These bacteria also produce lactic acid, which lowers gut pH and helps to prevent colonisation by neutral to alkaline pH loving pathogens such as Salmonella.

Thompsons introduced these products on the problematic unit. Diet acidification commenced in March 2000 using 3kg/Te of protected acid with fructo-oligosaccharide in the first stage diet (up to 12kg), with the biosecurity product at 3kg/Te in the finisher and lactating rations, and just 2kg/Te in the dry sow diet. Within four to five weeks of the introduction of these products the farmer noticed several improvements. The salmonella-type scour seen periodically was beginning to subside; the use of antibiotic injections to control this scour was stopped.

Over the next few months the use of prescription medicines was greatly reduced. The use of antibiotics in the grower diets ceased; potentiated sulphonamide was used at one stage to control an outbreak of pneumonia in the finishers. The pigs were then moved on to diets of lower specification, without any loss in performance.

David Stewart then repeated the tests for Salmonella - twenty finishing pigs were blood sampled in May 2000. The average optical density of 8.75%, illustrated a great improvement in the salmonella status of the herd. Only two pigs produced optical density values greater than 40 indicating, and the herd status had now reached Level 1

"The use of the acid/oligosaccharide combination, Prefect, in the youngest pig diets together with the acidifier, Bact-a-cid, in all other diets had proved very helpful in this situation. These products also have the advantage of being non-antibiotic, with a long life expectancy".

David Stewart,
Specialist Pig Veterinarian
Elite Pig Producers Group

Conclusive evidence

The elimination of Salmonella from a well-run pig herd in Northern Ireland was the result of a multi-factorial approach, instigated by the feedmill. With Veterinary input alone the problem could be reduced but a thorough programme of animal welfare, feed quality, biosecurity, veterinary care and the correct choice of feed additive combined to give a highly cost effective solution. The result is a top quality, Salmonella free herd with an excellent performance record that requires minimal veterinary intervention.

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