How do herd closure and rollover contribute to eliminating PRRS?

Researchers from China demonstrated the application of herd closure and the rollover approach without deliberate live virus exposure in a farrow-to-finishing herd after a severe PRRS outbreak. Herd closure and rollover is a popular PRRSV elimination strategy with acceptable costs.

The herd closure and rollover strategy consists of 3 steps:

• PRRS-negative gilts are introduced in 1 batch. The number of PRRSV-negative gilts should be enough for the sow replacement during herd closure to minimise the costs.

• The next step is herd closure, which usually takes 210-250 days until the herd reaches a provisional negative status.

• In the last step, all pigs are exposed to the PRRS live virus at the same time. In this method, the sow population is infected at the same time via simultaneous vaccination of the PRRSV-modified live vaccine or inoculation with serum containing resident virus and recovers simultaneously, thus, the entire population will establish sterilizing immunity against field strains.

Data collection

The farrow-to-finish facility contained 1.252 sows and 630 gilts, with corresponding nursery houses, fattening houses, and a gilt development unit. A severe PRRS outbreak occured in the herd. The research team collected 5 blood samples from sows and 5 tissue samples from stillbirth fetuses within 3 days post-outbreak for qPCR and ELISA tests. Later, they designed a PRRS elimination programme, in which the deliberate exposure of the live virus was skipped. No gilt was introduced until after the PRRS outbreak, and the herd closure began 15 days post- outbreak.

The researchers delayed gilt introduction until the herd reached a provisional negative status. During the herd closure, all semen was purchased from outside suppliers to ensure it is negative for common pathogens via qPCR tests.

The PRRS-positive ratio of the pre-weaning pigs declined quickly and became 100% negative by 23 weeks post-outbreak. Depopulation was launched in the 27^th week after the pre-weaning piglets tested negative for PRRS for 4 consecutive weeks.