Terrestrial Animal Health Code
Terrestrial Animal Health Code |
Prevention and control of Salmonella in commercial pig production systems
Introduction
Nontyphoidal salmonellosis is one of the most common foodborne bacterial diseases in the world with Salmonella Enteritidis and S. Typhimurium (including monophasic variants) being the predominant serotypes identified in humans in most countries. S. Enteritidis is primarily associated with poultry while S. Typhimurium may be present in many mammalian and avian hosts. These serotypes and several others occur at variable prevalence in pigs depending on the region. In some countries S. Infantis and S. Choleraesuis may cause salmonellosis in humans.
Salmonellainfection in pigs is mostly subclinical, although clinical disease such as enteritis and septicaemia in weaned pigs may occur. Subclinical infection, including a carrier state, can be of variable duration and can play an important role in the spread of Salmonella within and between herds and pose a public health risk.
Salmonella serotypes and their prevalence in pigs may vary considerably within and between farms, countries and regions. It is important for Veterinary Authorities and producers to consider serotypes of Salmonella, their occurrence and the disease burden in pig and human populations when they develop and implement strategies for the prevention and control of Salmonella in commercial pig production systems.
Definitions
For the purpose of this chapter:
’Commercial pig production systems’: means those systems in which the purposes of the operation include some or all of the following: breeding, rearing and management of pigs for the production and sale of pigs or pig meat.
Purpose and scope
This chapter provides recommendations for the prevention and control of Salmonella in commercial pig production systems, including outdoor pig production systems, where practicable, in order to reduce the burden of infection in pigs and the risk of human illness through foodborne contamination as well as human infections resulting from direct or indirect contact with infected pigs.
This chapter should be read in conjunction with the Codex Alimentarius Code of Hygienic Practice for Meat (CAC/RCP 58-2005), Code of Good Animal Feeding (CAC/RCP 54-2004), and the Guidelines for the Control of Nontyphoidal Salmonella spp. in Beef and Pork Meat (CAC/GL 87-2016), and the OIE/FAO Guide to Good Farming Practices for Animal Production Food Safety.
Objectives of prevention and control measures
Prevention and control measures may focus on those serotypes of Salmonella of greatest consequence to pigs and public health. These measures will also contribute to the reduction of other serotypes.
Prevention and control measures in commercial pig production systems may:
reduce the prevalence and amount of Salmonella entering the slaughterhouse/abattoir and therefore decrease the challenge to the slaughter and dressing procedures and the likelihood of pig meat contamination;
reduce Salmonella contamination of the environment via pig faecal waste, which in turn will limit infection of animals (including wildlife);
reduce the likelihood of infections in humans through contact with infected pigs or contaminated materials or water.
While control in the primary production phase can decrease the number of animals carrying or shedding Salmonella, controls after primary production are also important to minimise the contamination and cross-contamination of carcasses and meat products.
Good farming practices and, when appropriate, the principles of hazard analysis and critical control points (HACCP) should be taken into account when designing prevention and control measures.
Articles 6.14.5. to 6.14.15. provide recommendations for the prevention and control of Salmonella in commercial pig production systems. These recommendations may also contribute to the prevention and control of some other infections.
Biosecurity
Biosecurity is essential to prevent and control Salmonella. A biosecurity plan should be developed according to the commercial pig production system employed. The choice of specific measures will vary according to the type of commercial pig production system.
When including Salmonella as part of a biosecurity plan the following should be addressed:
location, design and management of the establishment;
veterinary supervision of pig health;
management of the introduction and mixing of pigs;
training of personnel in their responsibilities and their role in animal health, human health and food safety;
prevention of contamination of feed and water, including for irrigation;
maintenance of records including data on pig health, production, movements, feeding, water supply, medications, vaccination, mortality, and cleaning and disinfection of farm buildings and equipment;
availability of test results to the farm operator when Salmonellasurveillance is conducted;
removal of unwanted vegetation and debris that could attract or harbour pests around pig housing;
minimising the entry of domestic animals and wild birds into pig buildings and feed stores;
cleaning and disinfection procedures for buildings in which pigs are handled or housed in accordance with Chapter 4.13.;
control of pests such as rodents and arthropods, and regular assessment of effectiveness;
control and hygienic procedures for entry and movement of persons and vehicles;
biosecurity applied to all personnel and visitors entering the establishment. As a minimum, this should include hand washing and changing into clean clothes and footwear provided by the establishment. Similar precautions are recommended when they move between separate epidemiological units on large farms;
cleaning and disinfection of equipment and vehicles identified as posing a risk;
storage and disposal of dead animals, bedding, faeces and other potentially contaminated farm waste in a manner that minimises the likelihood of dissemination of Salmonella and prevents the direct or indirect exposure of humans, livestock and wildlife to Salmonella. Particular care should be taken when pig bedding and faeces are applied to land used for horticultural crops intended for human consumption;
procedures for prevention of dissemination of Salmonella when an animal is suspected or known to be infected.
Location and design of pig establishments
When making decisions on the location and design of pig establishments, reduction of the likelihood of transfer of pathogenic agents, including Salmonella, from major sources of contamination should be considered. Sources of Salmonella may include other livestock establishments or areas of application or disposal of contaminated waste or effluent. Other sources and vectors of Salmonella include vehicles, equipment, water-courses, personnel, domestic animals, birds, rodents, flies and wildlife.
The design of commercial pig production systems should consider the following:
proximity of other livestock establishments, and wild bird and rodent populations;
management of faecal waste to minimise contamination of the establishment;
adequate drainage for the site and control of run-off water and untreated waste water;
use of smooth impervious materials for construction of pig houses to enable effective cleaning and disinfection;
paving the area immediately surrounding pig houses or indoor establishments with concrete or other impervious material. This will facilitate rodent control and minimise recontamination after cleaning and disinfection;
a control of entry and movement of vehicles, equipment and persons, for example, locate delivery and collection points away from pig housing or feed storage;
preventing contamination of feed and water during storage and distribution;
pig handling and movements to minimise stress and spread of Salmonella;
restriction of entry of domestic animals, wild birds, rodents, flies and other relevant wildlife.
Management of pig introductions into the establishment
Introduction of pigs into a herd is a risk factor, especially in moderate and high prevalence regions. To minimise the likelihood of introducing Salmonella by replacement pigs:
good communication along the pig production chain should be encouraged to raise awareness of the risk of introducing Salmonella through pig introductions;
consideration should be given to minimising the number of sources for both replacement breeding stock and rearing pigs, and matching Salmonellaherd status in terms of Salmonella freedom or occurrence of priority serotypes such as S. Typhimurium;
new genetic material should be introduced through the use of semen whenever practicable;
if possible, pigs should be sourced directly from herds of origin because live animal markets or other places where pigs from multiple properties are mixed for resale may increase the likelihood of spread of Salmonella and other infectious agents among pigs;
newly introduced pigs should be kept separate from the rest of the herd for a suitable period before mixing with other pigs;
when appropriate, testing of pigs for Salmonella prior to introduction or mixing with other pigs should be considered to inform subsequent control measures, for example, when introducing pigs of unknown status.
Moving and mixing of pigs
The moving and mixing of pigs increases the likelihood of spread of Salmonella. To minimise the spread of Salmonella:
the number of pig movements and mixing of pigs should be minimised;
if possible, the 'all-in-all-out' system with a single age group of pigs should be used. In particular, the addition to younger groups of pigs held back from older groups should be avoided;
sick pigs should be segregated from healthy ones.
Feed and feed composition
Feed and feed ingredients
Feed and feed ingredients can be sources of Salmonella for pigs. This is especially important in herds, countries or regions of low prevalence. To minimise the spread of Salmonella through feed:
feed and feed ingredients should be produced, handled, stored, transported and distributed in accordance with Chapter 6.4.;
feed and feed ingredients should be transported, stored and fed in a manner that minimises contamination by faecal waste and, where practicable, minimises access by domestic animals, birds, rodents and wildlife;
when practicable, feed should be treated with heat, or with approved bactericidal or bacteriostatic treatments such as organic acids.
Feed composition
When Salmonella is present in a pig herd, the composition of feed may influence the occurrence of Salmonella in individual pigs.
For the control of Salmonella the following be considered:
liquid feed that is fermented or containing milk products has a protective effect due to the presence of beneficial bacteria and lowered pH;
coarsely ground feed may reduce the occurrence of Salmonella by slowing gastric transit (thereby increasing exposure to gastric acid) and reducing dysbacteriosis. Coarsely ground feed ingredients may be fed alongside pelleted feed;
fine grinding needed to produce heat treated pellets may result in dysbacteriosis which favours the colonisation and multiplication of Salmonella in the intestine. Therefore, heat treated pellets are more appropriate for situations in which Salmonella is uncommon;
when wheat is the predominant feed ingredient, reducing the proportion of this ingredient may reduce the occurrence of Salmonella because the rapid fermentation of wheat promotes dysbacteriosis.
Water
Water for drinking should be of an appropriate quality. To minimise the spread of Salmonella through water:
the water supply should be monitored and controlled to maintain it free from Salmonella contamination;
water holding tanks should be enclosed;
water supply and delivery systems should not be accessible to birds, rodents or wildlife;
the water delivery system should be regularly cleaned and disinfected. For example in an 'all-in-all-out' system this occurs before restocking.
Additional prevention and control measures
Vaccination may be considered as part of a Salmonella control programme. Vaccine production and use should be in accordance with Chapter 1.1.6. of the Terrestrial Manual. The protective effect of vaccines is generally serotype-specific and is influenced by factors such as timing of vaccination in relation to exposure.
Antimicrobial agents may modify normal flora in the gut and increase the likelihood of colonisation by Salmonella. In circumstances when antimicrobial agents are considered necessary for the treatment of clinical salmonellosis, they should be used in accordance with Chapter 6.10. Furthermore, antimicrobial agents should not be used to control subclinical infection with Salmonella in pigs because the effectiveness of the treatment is limited, they may increase the risk of Salmonella colonisation, and their use can contribute to the development of antimicrobial resistance.
Where approved by the Competent Authority, organic acids, probiotics and prebiotics may be added to feed or water to reduce shedding of Salmonella by pigs. However, efficacy is variable.
Transportation
Vehicles should be properly cleaned and disinfected after transportation of animals, in accordance with Chapter 4.13.
When transporting animals from multiple establishments, the Salmonella status of the establishments should be considered to avoid cross-contamination of pigs.
In addition, the relevant recommendations in Chapters 7.2., 7.3. and 7.4. apply.
Lairage
Lairage may be used at various stages in pig production, for example accumulation of weaned pigs before movement to nursery herds, holding finisher pigs before transport to slaughter and holding pigs at the slaughterhouse/abattoir before slaughter. Relevant aspects of lairage management include consideration of effective cleaning and disinfection between groups, minimising mixing of animals that have not continually been kept together and managing stress.
In addition, the relevant recommendations in Articles 7.5.1., 7.5.3. and 7.5.4. apply.
Surveillance for Salmonella in commercial pig production systems
Surveillance data provide information to assist the Competent Authorities in their decision making regarding the requirement for, and design of, control programmes and in setting and verifying performance objectives. Harmonised surveillance systems to determine the occurrence of Salmonella at herd level are in place in some countries. Communication between slaughterhouses/abattoirs, Veterinary Services and the herd manager or veterinarian of the results of Salmonellasurveillance systems is an important element of a Salmonella control programme.
Standards for diagnostic tests are described in the Terrestrial Manual. Serological testing, usually using 'meat juice' at slaughter, is one method for assessing exposure to Salmonella in pig herds. Benefits of serological testing include low cost per test, high throughput capability and the potential for automation of tests. Collection of samples at the slaughterhouse/abattoir enables centralised sampling of multiple herds. While serology is a useful tool for risk ranking of herds, serological testing does not detect exposure to all serotypes or differentiate between different serotypes within the serogroups included in the antigenic range of the test or the level of Salmonella in pigs at slaughter. If serology is used as the surveillance method, it may not be possible to distinguish between vaccinated and infected pigs.
Serological testing gives no indication of excretion of Salmonella in the herd and does not reflect how infectious is the tested group.
Microbiological testing, with additional phenotyping or genotyping, identifies serotypes of Salmonella present in pig herds and can provide epidemiological information on likely sources of Salmonella and on the presence of strains with enhanced virulence or resistance to antimicrobial agents. Bacteriological sampling of individual pigs has low sensitivity but this can be overcome by sampling at herd level or repeated sampling of individual animals. Pooling of samples (such as individual faecal samples or mesenteric lymph nodes) or sampling naturally pooled material (such as sampling of faeces from the floor of pig pens) will decrease the costs. Some serotypes of Salmonella such as S. Choleraesuis can be difficult to detect using microbiological methods.
Prevention and control in low prevalence
regions
In regions where Salmonellainfection of pigs is uncommon, it may be possible to maintain low prevalence status or eliminate infection from herds through a combination of good farming practices, herdsurveillance, individual testing, movement controls, and removal of persistent carriers.
In individual herds, for example valuable breeding herds, in higher prevalence regions, the success of this approach is dependent upon a low likelihood of reintroduction of infection.
Outdoor pig production
For outdoor pigs in commercial production systems, in addition to the prevention and control measures described in Articles 6.14.5. to 6.14.15., it is recommended that:
field rotation programmes be used to minimise Salmonella contamination and accumulation in soil and surface water and therefore ingestion by pigs;
systems used to provide feed, and where possible water, be designed to minimise attraction of, or access by, wildlife;
the location of other outdoor pig herds and the concentration and behaviour of wildlife in the area be considered.
nb: first adopted in 2017; most recent update adopted in 2018.
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