The production system is a decisive factor to a great extent in the control and prevention of the disease. In farrow-to-finish herds (including farrow-to-weaners and farrow-to-growers piggeries), the pathogen can persist in endemic infected sows, which have overcome the infection and developed protective immunity but still shed the pathogen in their faeces. The proximity of facilities and continuous flow of animals in this sort of production system will facilitate the transmission of infection to non-infected animals. On an endemically infected farrow-to-finish piggery transmission of infection to susceptible pigs occurs primarily by contact with faecal material that originates from clinically infected pigs or from asymptomatic carriers colonized by the spirochaete [45]. Depending on the herd immune status and the measures taken to control the disease (based on antimicrobial treatments and vaccination [46,47]), animals will be more or less severely affected and the disease will affect principally pigs at the growing or finishing period when the medication used to control respiratory infections is removed, favouring the expression of SD. In contrast, it is easier to halt the transmission of the pathogen in animals reared in integration systems in which each part of the production is physically independent from the other [48]. However, in integration systems at the growing and finishing stages pigs from different origins are mingled. In consequence these stages constitute a risk if pigs from farms with different status to SD (infected or non-infected) are mixed. Not only productive parameters but also the health status must therefore be considered in the acquisition of pigs. The replacement of breeders with others from the same source each year was shown to be protective against the appearance of the disease [49].