Identification and quantification of risk factors in animal management and transport regarding Salmonella spp. in pigs.

This article discusses the main elements of a descriptive epidemiological model for Salmonella spp. in the pre-harvest stages of pork production, and the subsequent quantification of risk factors. About two thirds of all Dutch pig farms are more or less permanently infected. At infected finishing farms, the current probability that Salmonella-free pigs will become infected is about 85%. In the case that a certain pen is infected, the current probability that further pen-to-pen transmission will occur is about 90% and that (human) vectors will also spread the infection 60%. Between 5-30% of the animals may still excrete Salmonella spp. at the end of the finishing period, and this percentage can double during transport and lairage. In infected pigs at slaughter weight, the Salmonella spp. are foremost located in the digestive tract, its contents and the closely associated lymph nodes. Under the current circumstances, the lack of farm hygiene (odds ratio (OR) 39.7), (re)contaminated feed (OR 1.6), the use of broad spectrum antibiotics (OR 5.6), a positive Salmonella-status of animals before transport (roughly estimated OR 4.0), the lack of transport hygiene (roughly estimated OR 1.1) and transport stress (OR 1.9) are the most important risk factors regarding infections with Salmonella spp. Currently the role of on-farm contamination cycles with endemic ('house flora') Salmonella spp. is so important, that the role of other factors is difficult to ascertain. It is also concluded that the farm-phase forms the core of all current problems, and that better prevention and control can be achieved by, (i) very strict and consistent farm hygiene in combination with promotion of the colonization resistance of animals kept together with a prudent use of broad spectrum antibiotics; (ii) simultaneous execution of control programmes at breeding farms, multiplying farms and finishing farms; (iii) separate transport, lairage and slaughter of the animals thus produced.