Effect of piglet vaccination against PRRS in Belgian farrow-to-finish herds

Porcine Reproductive and Respiratory Syndrome virus (PRRSv) may cause significant losses in pig herds due to pre-weaning mortality and reduced performance in growing pigs. In Belgium, the virus is widespread and endemic in most herds. An earlier trial showed that infection of PRRSv mainly occurs near the end of nursery (8-12 weeks of age (w)). PRRSv vaccines are effective under experimental conditions, but sometimes fail to cover the field expectations. Sow vaccination against PRRS is common practice in Belgian herds. Recently, also piglet vaccination strongly increased. The aim of this trial was to evaluate the serological response after vaccination and to investigate the effect of piglet vaccination on the timing of infection with PRRSv.

In 5 farrow-to-finish herds, with a PRRSv infection at the end of the nursery as shown by previous analyses, 1 batch of piglets was vaccinated with an attenuated EU PRRSv vaccine strain (Porcilis® PRRS, MSD, The Netherlands). Piglets were vaccinated by the farmer at 19-20 days of age, except for 1 herd in which piglets were 23-24 days old. In each herd, 60 piglets were randomly selected and individually earmarked (20 piglets from sows of parity 1, parity 2-4 and parity ≥5, resp). A horizontal serological screening was performed at 4, 7, 10, 13, 18 and 23w. Serum was examined for antibodies (ab) against PRRSv by IPMA and virus isolation was done on serum during seroconversion.

In 3 herds, 62, 57 and 57% of piglets with a maternal ab (mab) titre of ≥160 at 4w seroconverted at 10w. In contrast, in 1 herd interference with mab was observed: 83% of piglets with a mab titre ≥160 did not seroconvert at 10w. Seroconversion indicated a PRRSv infection between 18 and 23w in those 4 herds, which was confirmed by isolation of wild type virus in 2 herds at 18w. The percentage of serological non-responding piglets to vaccination in the 4 herds was 50, 43, 47 and 33%, resp. In the 5th herd, viraemic piglets were found. Extremely high ab titres (≥2560) were detected at 4w and wild type virus was already isolated in piglets of 7w, indicating an early infection. Biosecurity was poor in the latter herd.

The results demonstrate that a substantial number of piglets do not seroconvert after vaccination. Piglet vaccination could not eliminate the virus from the herd, but seems to delay the timing of PRRSv infection towards the second half of the finishing period. Results underline that biosecurity measures play an important role in an effective control program. Further research is needed to investigate the clinical protection induced by vaccination, especially in the serological non-responders.

Disclosure of Interest: None Declared.