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

Abstract

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 ID 50 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 < 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 ⩾12.5 and < 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 ⩾25 and <50, 5 responded with fever, 6 out of 10 had leucopenia, 4 excreted virus and none died. Although all pigs with pre-challenge titres ⩾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 <32 were found inadequate.