“A weak spot for PRRS control comes when you take boar semen or gilts from outside into a herd” Better surveillance for PRRS at boar stations in Denmark has followed a breakdown at a PRRS-negative boar station which led to sharply reduced sow productivity at herds in the area, reports Professor Lars Erik Larsen of the University of Copenhagen. Investigators discovered that the break involved a recombinant form of the virus, combining strains from two vaccines used at a sow herd about 5 kilometres from the boar station.