It is well known that Salmonella infection is a serious threat in poultry growing. Adequate measures are necessary to minimise transmission from one generation to another, eventually in broilers. This starts with stringent control of primary breeding stock, linked to intensive sampling and testing of both the birds and their environment. Also strict control in feed manufacturing and thorough cleaning and disinfection of premises and equipment are essential.
By Mogens Madsen , Dianova Ltd, Denmark
Enteric Salmonella infections are the second-most frequently reported zoonotic diseases of humans in the European Union, being responsible for significant human illness, loss of productivity and mortality. In foodstuffs, Salmonella is most often detected in fresh broiler, turkey and pig meat. Salmonella is widespread in domestic and wild animals, and commercial poultry flocks are a large reservoir. When flocks become infected on the farm, Salmonella is usually carried asymptomatically in the gastrointestinal tract of the birds and may consequently be transferred to processed carcasses via faecal contamination. Further spread may occur during processing due to cross-contamination.
EU control programmes
Salmonella prevalence within broiler flocks varies widely from one country to another. In a recent survey by the EU, flock prevalence was found to vary from 0 – 68.2% (Table 1
). Consequently, the EU has set targets for Salmonella reduction in individual Member States. Legislation has been introduced that makes testing compulsory and specifies deadlines for establishing the required targets in breeders, layers, broilers and turkeys.
For the industry, it is important to be assured that the investments in programs for controlling Salmonella in poultry have a genuine impact on public health. An examination in 2000 of the cost-benefit relationship of the Finnish national control program for Salmonella in broilers concluded that there was a large saving on public health costs as a result of the program, and that the value of only one death avoided exceeded the cost of the entire program. Similarly the Danish control programme for pigs and poultry saved the equivalent of 25.5 million US dollars on health costs in 2001, for an outlay of 14.1 million dollars, and it was clearly shown that introduction of specific pre-harvest control programs for broilers, pigs and egg layers had a direct and significant effect on the number of human cases of salmonellosis (Figure 1
Risk factors and intervention
The major risk factors for introducing Salmonella into poultry production are contaminated feed, infected breeder flocks, lack of effective biosecurity on farms, inadequate control of hygiene during harvest and transport of broiler chickens, and cross-contamination of carcasses during slaughter and processing.
Manufactured feed can be a source of Salmonella for animals. When present in dry feed, Salmonella can survive for more than a year and even low numbers may be significant since a level of just 1 cell/g is sufficient to colonise young chicks for some Salmonella strains. Feed ingredients can be classified according to risk and those presenting the lowest risk should be used wherever possible, especially in feeds for breeding stock. Animal-derived proteins and certain vegetable proteins, such as soybean meal, are in the highest-risk category. Because Salmonella tends to occur in very low numbers and is usually distributed unevenly in any contaminated batches of feed material, sampling and testing schemes have a low probability of detecting the organism. A better approach is to apply Hazard Analysis Critical Control Point (HACCP) principles to the manufacturing process.
Effective implementation of the HACCP system requires good storage conditions and appropriate control of transport vehicles, with regular cleaning and disinfection, and protection of the load up to and including the point of delivery.
· Breeding stock
Biosecurity at a high level is imperative for primary breeding and grandparent flocks. There should be frequent and comprehensive monitoring of the flock and its environment, and farm employees tested in order to detect any Salmonella carriers. Eggs must be collected regularly and treated to eliminate any Salmonella on the shell surface, and held in a pest-proof, temperature-controlled environment while awaiting transport to the hatchery. Various other preventive measures can be used to reduce the risk of Salmonella infection in breeding stock in addition to biosecurity requirements. One widely used control measure is vaccination that targets the invasive serovars S. Enteritidis and S. Typhimurium, and involves both live and inactivated vaccine strains. However, due to the risk of masking a low-level Salmonellainfection, vaccination in breeding stock is discouraged by the EU in the long term.
Even a single infected chick can transmit the organism to many other chicks during the hatching period, so good hatchery hygiene is an essential part of a Salmonella control program. The key elements for effective hygiene control are hatchery design, ventilation, isolation, cleaning and disinfection, waste handling, microbiological monitoring and good communication between management and staff. Even so, it is not unusual to find ‘resident’ Salmonella strains in incubators and washing machines. Egg handling practices are very important. Eggs should be treated with a suitable disinfectant on arrival at the hatchery.
Biosecurity requirements for broiler meat flocks are given in many national and international codes of practice, standards and guidelines, and aim to minimise the risk of Salmonella gaining access to the flocks. Other requirements include an all-in, all-out stocking policy and an adequate time allowance for thorough cleaning and disinfection of each house between crops. EU testing requirements for Salmonella at about four weeks of age enables any positive flocks to be processed separately or last in the day to prevent flock-to-flock transfer of Salmonella at slaughter and processing.
· Harvesting and transportation
Biosecurity may be breached during depopulation (thinning) when the house is opened and a proportion of the flock removed, so special care is needed to avoid infection of the remaining birds from contaminated catching equipment, transport crates and modules brought on to the site for thinning purposes. The containers used to transport birds from the farm to the processing plant may be contaminated with Salmonella on arrival at the farm. Equally, Salmonella has been isolated from previously negative birds, following transportation, and residual faecal material present in transport containers has been identified as a key factor in carcass contamination.