The Salmonella puzzle – what can we learn from Europe?

The fact that Salmonella can be found in poultry meat and eggs is well-known even by uninformed consumers. Programmes are implemented worldwide to control Salmonella infections in birds due to public health concerns. What can we learn from Europe?

By Anna Catharina Berge, Berge Veterinary Consulting and Zöe Kay, Alltech Inc.

The European Union (EU) is a role model for active Salmonella control in poultry and EU-wide regulations have set the targets to be obtained in all member countries. The control measures have generated decreasing incidence of Salmonella in poultry flocks and decreasing cases of human salmonellosis. The observed reduction of salmonellosis cases is mainly attributed to successful implementation of national control programmes in poultry populations. Salmonella control measures including strict biosecurity, sanitation measures, feed sanitation, vaccination, nutritional interventions, harvesting and post-harvest hygiene have contributed to the success of the poultry sector in decreasing Salmonella contamination.

Salmonella in live birds

Breeding poultry - It is essential that Salmonella control starts in breeding flocks, since they are the top of the poultry production pyramid and Salmonella can be transmitted through the egg to the offspring. EU regulation 1003/2005 ensures that all breeder flocks are tested regularly for Salmonella. If the breeder flock is positive, it is slaughtered and the eggs may not be used. A target was set for breeding flocks to ensure that no more than 1% of national breeding flocks (with more than 250 birds) remain positive for Salmonella serovars of human health significance by the end of 2009. Eighteen Member States (out of 27) reached the EU Salmonella reduction target for breeding flocks.

Broilers - EU study (2005-2006) indicated that 23% of broiler flocks were Salmonella positive and the levels varied from 0 to 68% between member countries. The most common serovars in broilers in decreasing order were S. Enteritidis, S. Infantis, S. Mbandaka, S. Typhimurium and S. Hadar. A reduction target in broiler flocks for S. Enteritidis and S. Typhimurium was set through Regulation 646/2007. The target is to reduce the maximum percentage of broiler flocks remaining positive for these serotypes, to 1% or less, by 31 December 2011. Eighteen EU countries had already met the Salmonella reduction target set for broiler flocks in 2009.

Layers - An earlier EU study (2004- 2005), conducted on commercial large-scale egg-laying hen farms, found the range of Salmonella levels in layer flocks was between 0% and 79% and 20% of all farms were positive for the serovars Enteritidis and/or Typhimurium.

The EU has two regulations for reducing and controlling the prevalence of Salmonella in poultry and eggs. Targets for the reduction of Salmonella in commercial laying flocks have been set in the EU through Regulations 1168/ 2006 and 1177/2006. Every country has to reduce the number of laying hens infected with Salmonella by a specific minimum percentage each year, with bigger reduction targets for countries with higher levels of Salmonella. The ultimate target is to reduce Salmonella levels to 2% or lower by 2011. All countries with Salmonella prevalence in commercial layers of above 10% are required to vaccinate their layers against Salmonella. Eggs from Salmonella-infected flocks have been banned as table eggs in the EU from 2010 and have to be sterilised if they are to be used for processing into egg products. Seventeen Member States met their 2009 reduction target for flocks of laying hens.

Turkeys - The targets set through EU Regulation 584/2008 outline that no more than 1% of breeding and fattening turkey flocks will be contaminated with S. Enteritidis and S. Typhimurium by 31 December 2012. Salmonella surveillance following EU regulations in turkey production was reported from nine EU countries in 2009 and levels ranged between 0.3 to 11.3 %.

Zero Salmonella in food

The EU considers that Salmonella is not an acceptable hazard to be found in food. The Salmonella criteria laid down by Regulation (EC) 2073/2005 modified by Regulation (EC) 1441/2007 prescribe rules for sampling and testing, and set limits for the presence of Salmonella in specific food categories. According to the criteria, Salmonella must be absent in food of animal origin. In 2009 less Salmonella was found in samples of egg products (0.2%) than in 2008 (2.8%), and this is attributed to the control programmes. The EU zoonosis report, published in 2011, indicated that in foodstuffs in 2009, Salmonella was detected in 5.4% fresh broiler and 8.7% turkey and 0.7% pig meat. Table eggs were found positive in only 0.2% of the samples, but it should be noted that recovering Salmonella in samples of eggs is difficult. Most of the foodborne outbreaks of Salmonella in 2009 were attributed to eggs and egg products.


Human salmonellosis

In 2009, 108,614 salmonellosis cases in humans were reported in the EU. However, cases of salmonellosis are not always diagnostically confirmed and the true number of Salmonella cases could range between one to over 10 million human salmonellosis cases per year. Nevertheless, the reported number of salmonellosis cases has decreased on average by 12% per year in the EU over the last five years (Figure 1). There is no exact information regarding how much human salmonellosis is related to poultry production. A recent analysis of the European data suggested that most human salmonellosis was related to layers, an estimated 44% of human cases, followed by 27% from pigs. Turkeys and broilers were estimated to be less important sources of Salmonella, contributing to 4% and 3% respectively. Another indication of the importance of eggs as a hazard for human salmonellosis is the experience of Denmark. The percentage of infected layer flocks was reduced from 13.4% in 1998 to 0.4% in 2006 in Denmark and during this period a parallel decrease was seen in human salmonellosis.


Control programmes

The basis for successful control of Salmonella infections in poultry farms are good farming and hygienic practices (including all the aspects covering feed, birds, management, cleaning and disinfection, control of rodents, biosecurity, etc.) as well as testing and removal of positive flocks from the production chain. The use of antimicrobials for Salmonella control in EU livestock, including poultry is forbidden because of the public health risks associated with development, selection and spread of antimicrobial resistance.

The control of Salmonella requires multiple measures and the whole production pyramid needs to be included, starting with Salmonella free breeding flocks. Prevention measures need to be coupled with routine monitoring to ensure that the measures are appropriate for each specific situation. Since Salmonella infections are a public health concern, the Salmonella preventive measures must include hygienic slaughter routines.

Vaccination has been shown to be an important tool in countries with high prevalence of Salmonella in poultry. Vaccination in layers is required in EU countries with Salmonella prevalence exceeding 10%. In Europe, turkeys are not vaccinated and broilers are seldom vaccinated due to their short lifespan and lack of cost-effectiveness. The vaccines are targeted for the most often reported serovars of human infections in Europe (Enteritidis and Typhimurium). After Salmonella vaccination in layers was initiated in 2004 in Belgium, laboratory confirmed cases of human salmonellosis dropped from 12,894 in 2003 (89% due to Enteritidis) to 3,831 in 2008.

Nutritional intervention methods

Since it is the bird that propagates Salmonella in poultry production, a key target is to find nutritional intervention methods that can modify the gut environment to prevent Salmonella colonisation, invasion, multiplication and shedding. Nutritional Salmonella intervention methods can be carried out via general diet formulation or feed additives.

A number of feed ingredients have gained commercial acceptance in helping to reduce Salmonella, by improving gut health and at the same time performance.

One strategy to minimise enteric pathogens is to optimise the gut microflora and enhance the immune system. Organic acids, short and medium chain fatty acids, probiotics, competitive exclusion cultures, prebiotics and certain specific carbohydrates, such as mannanoligosaccharides, egg proteins, essential oils and bacteriophages, and nutrient composition modulation of diet have been used or are under investigation to improve gut function as well as potentially reduce Salmonella levels.

An example of a nutritional tool to minimise Salmonella in poultry is the use of mannan-oligosaccharides. Mannose and its polymers have been shown to reduce Salmonella colonisation in chickens. The large majority of Salmonella contain mannose-specific lectins (Type 1 fimbriae) on the bacterial surface that binds to glycoproteins (rich in mannose) on the intestinal surface. Mannose sugars can thereby compete with the intestinal glycoproteins for attachment sites and prevent
colonisation. In a series of three trials in which 3-d-old chicks were orally challenged with 104 cfu S. Typhimurium, birds receiving 4,000 ppm of Bio-Mos had 1.4 log (more than a 10-fold) reduction in caecal Salmonella concentrations one week later (Figure 2). In another series of three trials with S. Dublin as challenge bacteria, the number of birds that tested Salmonella positive one week later was significantly reduced in birds fed Bio-Mos: 90% vs. 56%. This effect has been further demonstrated in commercial evaluations.

A series of trials in Hungary showed that the use of Bio-Mos throughout the cycle reduced Salmonella infection rates from 80% to 10% in broilers and by 88% in breeder flocks (Figure 3). In addition to decreasing Salmonella levels, Bio-Mos has been shown to improve feed conversion rate, growth rate and egg production. Therefore the producer may get a return of investment on Bio-Mos as a tool to minimise Salmonella.

How to prevent Salmonella problems

No single measure will eradicate or work efficiently to eliminate Salmonella from poultry production. In order to be successful a programme must include every stage and aspect of production.
Where does Salmonella come from?
  1. Breeders – ensure that the breeding flock is Salmonellafree.
  2. Hatcheries – thorough cleaning and disinfection, fumigate eggs, and Salmonella monitoring.
  3. Use day-old chickens that are from Salmonella-free flocks and hatcheries.
  4. Use stringent cleaning and disinfection between flocks.
  5. Use all-in/all-out system for raising birds and appropriate times between flocks.
  6. Use a pest control system to prevent access by wild rodents, birds or other animals to poultry houses.
  7. Use heat-pelleted feed and feed from a feed mill that performs Salmonella monitoring.
  8. Optimise nutrition and immune status in the birds.
  9. Eliminate stressors such as thinning the flock and moulting layers. Provide at least 6 hours of darkness per day.
  10. Implement monitoring programmes for Salmonella in the flocks, hatcheries and feed mills.
  11. Take appropriate action in cases of Salmonella outbreaks.
  12. Initiate effective biosecurity measures. Install dedicated equipment for poultry houses, do not share equipment, ensure that all visitors and workers have clean cloths and boots. Do not permit outside personnel into the poultry facility for loading of birds.
  13. Maximise the protective mechanisms of the individual birds; vaccination in high prevalence flocks; optimisation of the intestinal flora, competitive exclusion products, mannan-oligosaccharides, organic acids, etc.
  14. Thoroughly wash and sanitise transport crates.