The protective value of vaccine-induced neutralising antibody titres in swine fever

The relationship between vaccine-induced antibody titres against swine fever virus (SFV), as measured by the neutralisation peroxidase-linked assay (NPLA), and protection against virus multiplication, excretion and transmission, disease and death was studied in 46 pigs. The pigs were housed individually and challenged intranasally with 100 pig ID50 of the virulent Brescia strain of SFV. In order to detect virus transmission, a swine fever (SF)-susceptible sentinel pig was placed in contact with the vaccinated animal 2 days after challenge. All 11 pigs with pre-challenge NPLA titres less than 12.5 responded to the challenge with fever, 8 out of 10 showed leucopenia, 7 transmitted virus to their contact and 3 died. Of the 9 animals with titres greater than or equal to 12.5 and less than 25, 8 developed fever, 6 out of 7 had leucopenia, 2 excreted and/or transmitted virus and all survived. Of the 12 pigs with pre-challenge titres greater than or equal to 25 and less than 50, 5 responded with fever, 6 out of 10 had leucopenia, 4 excreted virus and none died. Although all pigs with prechallenge titres greater than or equal to 50 showed a booster response, virus transmission was not observed, indicating that in the case of exposure such animals would not contribute towards the spread of field virus. From an epidemiological point of view, titres less than 32 were found inadequate.