The first PRRS outbreaks started in 1987, although we now know that there was serological evidence of the PRRS virus going back nearly a decade before outbreaks were seen. The flip side of course is that if the sow herd is not stable for these diseases then SEW technologies might NOT work. SEW technology was producing pigs free of many, but not all diseases. It doesn't take long for several things to start happening. Over time, PRRS and Mycoplasma pneumonia circulate to more and more rooms, and pigs from each room get exposed.
Pigs move out to the finisher barn.
Depending on the rate of circulation and spread among pigs, there is an eventual outbreak of respiratory disease in the finisher. One other observation should be added here. The good news in this story, if there is any good news, is that there are definite indications that SEW is doing exactly what it was supposed to do. Some agents, however, are slipping through in some pigs. The cracks in the system can be tracked down, identified and corrected.