Experiences with eradication of Porcine Reproductive and Respiratory Syndrome in Danish swine herds

Abstract - In 1992, the PRRS virus (Danish field-strain) was detected for the first time in Denmark, and in the summer of 1996 an American PRRS-virus strain was introduced through vaccination. The objective of this study was to test and evaluate eradication programmes for both strains of the PRRS-virus at the herd level. All herds included in the study were single-sited commercial systems with a good external protection against reinfection. Eradication of PRRSV in three finishing herds and one farrow-to-finishing herd was attempted by a total depopulation programme followed by restocking from a PRRSV-free source. After cleaning and disinfection, the herds had a down period of a minimum of 14 days. In another 20 farrow-to-weaning or farrow-to-finishing herds (2-300 sow units), eradication of PRRSV was attempted through a partial depopulation procedure. The eradication programme for each herd was based on an initial serologic profile from the following animals: 20-30 breeding animals, 10 of the oldest nursery pigs, 5 growers, 5 finishers. The serologic profile was based on a combination of two serologic test systems: an immunoperoxidase monolayer assay (IPMA) and a blocking ELISA. As a minimum the following procedures were carried out: In 12 herds, the farrowing units were emptied for a period of at least 1 week during which the units were cleaned and disinfected. Four, 8 and 12 months after eradication 10 of the oldest nursery pigs, (minimum 5 growers and minimum 5 finishers) were bloodsampled. The eradication was considered successful when all sampled pigs had IPMA titres at the 4 and 8 month-samples. In five AIAO finishing herds eradication of PRRSV was sought by partial depopulation, putting PRRS-negative finishers into emptied, cleaned and disinfected sections, while other sections still contained PRRS-positive animals. The eradication was considered successful when samples from 5 pigs from each section were seronegative one month after the last PRRS-positive animals had left the herd. In all four herds that were totally depopulated, both strains of PRRSV were eradicated. The eradication was successful in 15 of the 20 sow herds undergoing a partial depopulation. There were two possible reasons for eradication failure: (1) the nurseries were not emptied as agreed upon; (2) the virus was still transmitted in the breeding herd posterior to nursery depopulation. The reasons for eradication failure could not, however, be established with certainty. The farrowing units of three herds with a failed eradication programme and 4 herds with a successful eradication of PRRSV were not emptied. In four of the five finishing herds, eradication by partial depopulation was successful. Both the Danish field strain of PRRS and the vaccine strain could be eradicated by the method of partial depopulation. The study showed that eradication of PRRS in herds is possible. The following criteria were found to be important: An important condition for the success of eradication by partial depopulation in finishing herds seemed to be the complete separation of PRRSV-positive animals and incoming PRRSV-negative animals (for example including an empty section between the two groups of animals).