EN
In June 1998, 12 gilts were purchased at a new, small (annual production 250 piglets) farm. The animals were free from PRRSV antibodies. After one month of quarantine the gilts were placed in two separate pens - A and B. The first female was bred in mid-August and the last one at the beginning of October. In order to ensure the empty rooms at the nursery house were occupied, in mid-November the farmer purchased 120,8- -week old weaners from an epidemilogically unrecognised herd. Unfortunately, the weaners were immediately introduced onto the farm. One-week later antibodies to PRRSV were accidentally detected in this group of pigs. Because the pig holding was under the control of the Institute, there was an opportunity to monitor the eventual spreading of PRRSV from weaners to gilts. In order to do this, blood samples were taken from all gilts 2,4,6 and 10 weeks after the weaners had been introduced onto the farm. Four weeks after purchasing the weaners, antibodies were detected in 3 out of 6 gilts from pen A and 2 weeks later PRRSV antibodies were found in a further 2 females. The last gilt from this pen was sero-converted 10 weeks after the weaners had been introduced. In pen B, PRRSV sero-conversion was detected in the first 2 gilts 6 weeks after the weaners introduction, and 2 other gilts were sero-converted 4 weeks later. One gilt from this pen did not have antibodies at the end of the observation period. The consequences of PRRSV spreading among pregnant gilts were observed during the furrowing period. Ten out of twelve females delivered dead or mummified piglets. Three litters were normal and in these cases the gilts had been infected during the 8-9 weeks of pregnancy. The economical consequences of introducing weaners infected with PRRSV into the newly established herd were very serious: the average number of piglets born alive per gilt was 6.8. The average number of piglets weaned was 6.2 and fatteners sold per sow were 5.7.