PREVENTION OF THE SPREAD OF PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS IN ENDEMICALLY INFECTED-PIG HERDS BY NURSERY DEPOPULATION

An attempt was made to prevent the spread of the virus of porcine repr oductive and respiratory syndrome (PRRS) on three Minnesota pig farms that had been experiencing chronic nursing pig problems, including poo r growth rates and increased mortality of post weaning pigs. The PRRS virus and different bacterial pathogens were isolated from all three f arms during the initial investigation, and all the farms had a high pr evalence of PRRS virus-seronegative breeding animals. All the pigs tes ted within one week after meaning when they were 18 to 22 days old, we re seronegative, whereas 80 to 100 per cent of the pigs tested at eigh t to nine weeks had antibody titres ranging from 1:64 to 1:1024 by an indirect fluorescent antibody method. The seroprevalence among the fin ishing pigs on the three farms ranged from 25 to 50 per cent. An eradi cation protocol uas established on each farm, involving emptying the n urseries, followed by pumping out the slurry pits and cleaning, washin g and disinfecting three times in 14 days. After the nurseries H ere r epopulated there were improvements in nursery mortality and average da ily weight gain, and no seropositive animals were detected in the nurs eries on any of the farms; the seronegative status was maintained for the sis-month testing period.