Risk of Porcine reproductive and respiratory syndrome virus introduction into Danish pig farms: a register-based study

Porcine reproductive and respiratory syndrome (PRRS) remains one of the most important virus infections in the industrialised pig production sector. In 2022, a national PRRS control programme was launched in Denmark. Nevertheless, new cases of PRRS-positive farms continue to be reported. The objective of the present study was to identify routes of introduction and risk factors associated with the introduction of the PRRS virus onto Danish pig farms. Based on register data from 2023, we identified 175 cases (173 farms) that changed their serological status from PRRS-negative to PRRS-positive, and 1,909 controls (1,665 farms) that remained PRRS-negative. The overall incidence was estimated to be 9.5% (175/1,838), with an incidence of 5.7% (39/788) for farms with sows and 11.8% (136/1,151) for weaner and/or finisher farms. Inward movement of PRRS-positive pigs was identified as the most likely route of introduction for 20% (8/40) of the farms with sows and 82% (112/136) of the weaner and/or finisher farms. Cases resulting from the deliberate inward movement of known PRRS-positive pigs were excluded from the dataset before logistic regression models were developed with case/control as a response variable predicting the risk of PRRS introduction. Two logistic regression models were developed: one for farms with sows and one for weaner and/or finisher farms. Evaluated risk factors included farm size, number of supplying farms, the supplier becoming PRRS-positive within 90 days of movement, distance to PRRS-positive neighbours and season. The two final multivariable models included both two highly significant risk factors: the supplying farm becoming PRRS-positive within 90 days of movement of pigs (p < 0.001) and the distance to PRRS-positive neighbours (p < 0.001). Crude observations identified a 16 and 43 times higher risk of becoming PRRS-positive if the supplier became PRRS-positive within 90 days of movement of pigs, compared with farms where the supplier remained PRRS-negative (RRsow = 15.7 [9.1;27.3] CI95%, RRWF = 42.5 [28.3;64.0] CI95%). Likewise, there was a four and nine times higher risk of farms becoming PRRS-positive if PRRS-positive neighbours were located within a 5 km radius, compared with no positive neighbours within this distance (RRsow = 4.2 [1.8;10.6] CI95%; RRWF = 8.5 [3.4;20.9] CI95%). On-farm biosecurity measures that have been identified as major factors in other studies were not possible to include, as this study was based purely on register-data. The results from the present study confirm that the movement of pigs is the main driver of PRRS infections on Danish weaner and/or finisher farms. Local transmission from neighbouring farms appears to be of secondary importance.