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 reproductive and respiratory syndrome (PRRS) on three Minnesota pig farms that had been experiencing chronic nursing pig problems, including poor growth rates and increased mortality of post weaning pigs. The PRRS virus and different bacterial pathogens were isolated from all three farms during the initial investigation, and all the farms had a high prevalence of PRRS virus-seronegative breeding animals. All the pigs tested within one week after weaning when they were 18 to 22 days old, were seronegative, whereas 80 to 100 per cent of the pigs tested at eight to nine weeks had antibody titres ranging from 1:64 to 1:1024 by an indirect fluorescent antibody method. The seroprevalence among the finishing pigs on the three farms ranged from 25 to 50 per cent. An eradication protocol was established on each farm, involving emptying the nurseries, followed by pumping out the slurry pits and cleaning, washing and disinfecting three times in 14 days. After the nurseries were repopulated there were improvements in nursery mortality and average daily weight gain, and no seropositive animals were detected in the nurseries on any of the farms; the seronegative status was maintained for the six-month testing period.