The experimental facilities were conventional weaning units with physical separation between rooms (solid walls, no air filtering). No other animals than those included in the study were housed for the duration of the experiment. After arrival to the experimental farm, piglets were left to acclimatize for 1 week. Animals were ear-tagged and randomly divided (random numbers) in two groups, designated as V (n = 40) and NV (n = 58) that were housed in separated rooms with no physical contact between them. V pigs were administered intramuscularly a 2 ml dose of a commercial modified live PRRSV vaccine (MLV) (PORCILIS PRRS® MSD Animal Health) according to manufacturer instructions. Group NV was left unvaccinated and remained as naïve controls. On the 35th day post-vaccination (dpv), 14 NV pigs were separated, housed in an isolated room and inoculated intranasally with 2 ml (1 ml/nostril) of a suspension containing 105.5 TCID50/ml of a LY317615 1 PRRSV strain designated as 3267 ( Darwich et al., 2011, Díaz et al., 2012 and Gimeno et al., 2011). The inoculated animals were designated as “seeder” pigs (SP). In parallel, the remaining V and NV animals were distributed in groups of 5 pigs each allocated in seven rooms, four of them allocating V pigs and three allocating NV animals. In each room two groups of animals were housed but direct physical contact between pens within the same treatment was avoided by using continuous pen separations and leaving an empty space between groups. Nevertheless, the air space was shared between pigs. Two days later, at 37 dpv, one SP was introduced into each pen to expose uninfected pigs to PRRSV (1 SP: 5 V and 1 SP: 5 NV pigs). Additionally, 2 sentinel groups of 5 NV pigs each were allocated in pens contiguous to NV + SP groups and a third sentinel group of 4 NV animals was placed adjacent to a V + SP pen. The reason for start the exposure phase after that long period of time (37 dpv), was to minimize the possibility of interference with the detection of vaccine virus. The experiment was terminated at 58 dpv, namely after 21 days of exposure of V and NV pigs to SP.
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