Based on these results, serum exposure can be an effective way of stabilizing a sow herd and weaning negative piglets on a consistent basis in large, commercial sow units faced with a variety of PRRS field strains. The sow herds in this study ranged from 1300 sows up to 2700 sows and were all PRRS negative prior to the PRRS outbreak that was evaluated. The same results may not occur in herds with multiple strains of PRRS circulating. After the outbreak, blood was collected and processed to obtain a source of the PRRS virus strain in each individual herd. The serum was then diluted out and injected back into the entire sow herd with the goal being to expose every sow. It is believed that when the entire population is exposed to PRRS, they will all become immune to that strain and eventually stop shedding virus, which then prevents exposure of the piglets in farrowing. When herds are naturally exposed to PRRS, negatives can still remain in the herd even after severe outbreaks. These negative animals are a potential source of prolonged PRRS activity. Nine of the ten herds enrolled in the study were able to produce negative pigs after serum exposure, the tenth herd was infected with a second strain of PRRS before there was enough time for the herd to produce negative pigs. The time from serum exposure until negative pigs were being weaned ranged from 12 weeks to 58 weeks, but of the nine herds that succeeded in producing negative pigs, 8 did so by 21 weeks after serum exposure. The amount of PRRS virus that is injected into the sow herd does not appear to be critical to the success of serum exposure; at least not within the range used in this trial, which varied from 1 ml of serum per 100 sows up to 1 ml per 1200 sows. It is recommended to test sows 2-3 weeks after serum exposure to make sure they have developed PRRS ELISA titers, indicating exposure. Pre-weaning mortality in these farms typically increased after PRRS infection but the level varied widely, most likely due to differences in PRRS strains, health status prior to the PRRS outbreak, and management differences. Pre-weaning mortality came back down to previous levels in most cases before the farm was consistently producing negative weaned pigs, so this is not a good indicator of the PRRS status of the weaned pigs.
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