Key messages
Biosecurity measures should be of very high priority in order to reduce the risk arising from slaughter pigs regarding S. Typhimurium. Indeed, a very low level of biosecurity which allows the infection of the pigs from external sources in a very high rate will lead to heavily infected farms, where even vaccination could not lead to positive results. Biosecurity measures that reduce the rate of external infections to one in a thousand could lead to category 0 farms (almost clean of Salmonella), assuming that the same measures would have also effect in pseudo-external infection (within the farm, from room to room). This rate practically means that for a farm with size of a room of three hundred pigs, there will be only one pig infected in three consecutive production cycles. This is explained, taking into account that if there is only one infected individual in the beginning of the fattening period, there are 45% probabilities for the infection to go extinct, before propagating into the room (early extinctions). Measures such as the disinfection of the tracks, the use of disposable clothes and boot covers, the control of pests and other measures aiming the reduction of the mechanical transfer of the pathogen from external sources are towards the right direction. Similar measures should be taken in order to reduce the mechanical transfer of the pathogen from one room to another.
On the other hand, practices such as the movement of pigs from one room to another, due to delayed growth, aggressive behaviour or other possible reasons have limited effect on the propagation of the pathogen in the farm. Even cleaning and disinfection of the rooms between successive batches of pigs have limited effect. In any case, it is expected a farm with highbiosecurity measures to have the appropriate cleaning and disinfection process in place either way.
Finally, other issues such as parallel infections, feeding strategies and others that have an effect on the transmission rate of the pathogen should be handled with care. Given that for different levels of infection of a farm the results can be quite contradictory, a good understanding of the situation within the farm should be achieved before clear targets are set.
Regarding the length of the Weaning-Growing-Fattening cycle, again it has an effect that depends on the level of the infection of the farm. Given that each farm may have a different target regarding the weight of the slaughter pigs, the situation should be carefully evaluated before the appropriate measures are proposed. For example, a different vaccination strategy (double or triple scheme) may be opted for farms with different production direction, depending on the day of harvesting.
Finally, although vaccination is not yet available for S. Typhimurium, an optimum vaccine that could provide perfect immunity to all the vaccinated pigs, should be administered in two doses (4th and 14th week of age) in order to achieve the best results. Still for heavily infected farms, vaccination could not offer substantial results and a combination of biosecurity measures should be in place to achieve remarkable results. For less optimum vaccines, these could be tested in silico using the framework modelled that has been developed in order to find an optimum vaccination strategy, before they are applied in real farm conditions.
