Protection of piglets born from ruminant pestivirus experimentally infected sows, and their contacts, to the challenge with hog cholera virus.

Two groups, A and B, of two specific pathogen-free pregnant sows were experimentally infected between the 25th and 29th days post-breeding with two strains of ruminant pestivirus: NADL cytopathic bovine viral diarrhoea virus for group A and Aveyron non-cytopathic border disease virus French strain for group B. Two other pregnant sows (group C) were kept uninoculated as control. When 7 weeks old, 8 piglets of group C were put in contact with 4 piglets of group A (group D), and 8 other piglets of group C with 4 piglets of group B (group E) in two separate pens with the purpose of testing the horizontal transmission of the viruses. All animals were kept under observation and serologically controlled at weekly intervals; two pigs of each group were finally submitted to a challenge with hog cholera virus. The two pigs of group E which were put in contact with the offspring of the border disease virus infected sow were protected; all other animals developed typical hog cholera symptoms and died. The relation between neutralizing titres of the sera to ruminant pestiviruses and protection to the challenge with hog cholera virus is discussed. The two protected pigs had high neutralizing antibody titres to border disease virus but no antibody to hog cholera virus at the time of the challenge. Though the two viruses look serologically distant, we surprisingly observed that infection with border disease virus protects against a superinfection with hog cholera virus.