The Prrsv-Serumneutralization Test Detects Gaps In Herd Immunity

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) appears in two genotypes (EU and US), for both genotypes attenuated live-vaccines are available. A cross-sectional study in 38 Bavarian sow herds was performed to assess the level of neutralizing antibodies. Per herd 38 blood samples were collected (10 weaned piglets, 10 gilts and 6 sows of 1./2., 3./4. and 5./6. parity, respectively). Sera were tested by ELISA, serumneutralization test (SNT) against EU- and US-vaccine virus, and pooled sera were tested by real-time RT-PCR.Herds were classified by the last vaccination of sows as "Vacc EU" "Vacc US" and "nv" (non-vaccinated) and by detection of PRRSV-EU in the group of weaned piglets (EU+ and EU-). Detection of PRRSV-US and vaccination of piglets were not included as variables.Sows of group (2) Vacc EU/EU- showed the highest EU-SNT-titers irrespective of parity. Groups (5) Vacc US/EU+ and (1) Vacc EU/EU+ followed in descending order. Significantly lower SNT-titers in (1) Vacc EU/EU+ were especially observed in sows of 1./2. Parity (Kruskal-Wallis, p<0.05). Very low SNT-titers were observed in the three remaining groups (3) nv/EU+, (4) nv/EU- and (6) Vacc US/EU-.In US-vaccinated herds detection of PRRSV-EU coincided with strong ELISA-reactivity in all animal groups. In EU-vaccinated herds this was only observed for weaned piglets. Sows showed a strong ELISA-reactivity irrespective of detection of PRRSV-EU.The value of the ELISA is restricted to the certification of PRRSV-free herds. The EU-SNT reflects the level of herd immunity at least against vaccine virus; it indicates gaps in herd immunity.