Sub-clinical swine fever: a survey of neutralizing antibodies in ther sera of pigs from herds having reproductive failures.

Sera harvested from breeding farms where reproductive failures were observed but where swine fever vaccination was not carried out, where tested for the presence of neutralizing antibodies specific for swine fever virus. Neutralization tests were performed in tissue culture using two viruses strains: the american serological variant "331" strain isolated by MENGELING (1969) and the virulent "normal strain" Alfort. For comparison, sera harvested from healthy, vaccinated and unvaccinated breeding farms were also tested. Results show absence of antibodies against these two strains in healthy and unvaccinated animals. All sera harvested from healthy and vaccinated breeding farms have high levels of neutralizing activity against the two strains. From 1,251 harvested unvaccinated breeding farms have reproductive failures, 6.2 p. 100 had neutralizing antibodies against the "Alfort" strain and 17.1 p. 100 contained neutralizing antibodies against the "331" strain. In 87 out of these 189 breeding farms, positive serological reactions were observed. Considering the respective intensity of neutralizing activity against the two strains , the vaccination with live virus vaccines ("Chinese strain" or "Thiverval" strain) induces high levels of antibodies but a higher titer against "Alfort" strain. Inversely, low virulent strains responsible for sub-clinical swine fever induce low or moderate levels against the "331" strain and nil or low levels against the "Alfort" strain. These results suggest that low virulent strains having common antigenic properties with the serological variant "331" strain are circulating among pigs in unvaccinated breeding farms having reproductive failures. These features are important for future development and establishment of routine serological checking for diagnosis and herd testing.