Estimating the rate of pseudorabies virus introduction into pig-finishing herds at regional level

From February to June 1995, 5-12 blood samples were collected in each Dutch pig herd and tested for antibodies against pseudorabies virus (PRV). The percentage of PRV-seropositive pig-finishing herds in three regions with more than 1,000 pigs/km2 (regions 2, 4 and 7) was 6% (region 2), 12% (region 7), and 25% (region 4). The percentage of PRV-seropositive pig-finishing herds in five regions with fewer than 1,000 pigs/km2 (regions 1, 3, 5, 6, and 8) was 3% (region 1), 9% (region 3), 6% (region 5), 0% (region 6), and 4% (region 8). The small sample size allows only the detection of major outbreaks and the percentages of PRV-seropositive herds therefore under-estimate the actual virus circulation in the regions. The fraction of PRV introductions that will result in a major outbreak depends on the herd immunity and thus on the vaccination programme of the herds. By combining for each herd the occurrence or absence of a major outbreak with the herd immunity induced by the vaccination programme, we estimated the average rate at which PRV was introduced into finishing herds in the eight regions. The average number of PRV introductions per finishing herd per finishing period (16 weeks) in the pig-dense regions was estimated at 0.20 in region 2, 0.83 in region 4, and 0.48 in region 7. In the less densely populated regions this rate was estimated at 0.08 in region 1, 0.34 in region 3, 0.17 in region 5, 0.00 in region 6, and 0.09 in region 8. The eight regions could be classified into four areas with a statistically different (P < 0.05 in Mann Whithey U test) rate of PRV introduction: i) regions 1, 6, and 8; ii) regions 2 and 5; iii) regions 3 and 7; and iv) region 4.