Pathogenesis, diagnosis and epizootiology of swine vesicular disease

Abstract

The work described in this thesis was initiated after the outbreaks of swine vesicular disease (SVD) in the Netherlands in 1992. The thesis starts with a general introduction on SVD and the virus causing SVD. Infection with SVD virus had been absent from the Netherlands for 17 years, and before 1992 SVD infection was only reported in Italy. Even though it was logical to assume that the outbreaks in the Netherlands were caused by virus from Italy, detection of SVD virus in Dutch pigs in Italy initiated a discussion whether transport of Dutch pigs were responsible for the infection at an Italian slaughterhouse, where the pigs had been for 2 days. This question initiated the research described in chapter 1, where we show that already 1 day after exposure to an infected environment virus can be detected in pigs. The pathogenesis work in chapter 1 indicated that the virus is most likely enters the body through the skin, therefore we studied the route of infection through the skin using immunohistochemistry, in-situ hybridisation and in-situ RT-PCR (chapter 2). Because the Netherlands is an important exporter of live pigs, complaints from trading partners on possible infection with SVD virus was causing trade restriction for short or longer periods. The pig industry therefore wanted to perform a large scale serological screening programme. For this large scale screening simple tests were necessary, we therefore validated a liquid phase blocking ELISA for SVD antibodies using a polyclonal pig hyperimmune serum (chapter 3). In the mean time we produced monoclonal antibodies against SVD virus, and mapped the epitopes of these monoclonal antibodies using chimeric SVD viruses produced by fusion PCR (chapter 4). One of the monoclonal antibodies was very well suited for the development of a new blocking ELISA for SVD antibodies (chapter 5). Not only a blocking ELISA was developed also isotype specific ELISAs for the detection of SVD specific IgM, IgG, IgG1 and IgG2 antibodies were developed (chapter 6). These isotype specific ELISA were used to study the sera collected on outbreak farms, and confirm that SVD virus mainly spreads within the pen where infected pigs are housed. In the period 1994 - 1999 over 3.9 million pig sera were tested for antibodies against SVD virus (chapter 7). In the ELISA approximately 0.5 - 1% of the sera were found positive, after the virus neutralisation test only 0.04 to 0.05% of the sera were positive, but due to the large number of sera tested still 200 - 300 positive samples per year were detected. These samples were from almost the same number of holdings, which were visited by the official veterinarian to show that the positive results were only singleton reactors. Setting the cut-off of the neutralisation test at a slightly higher level reduced this number by 50%, without reducing the farm level sensitivity of the screening programme. The positive sera were tested for antibodies against related Coxsackie B5 virus, but there was no indication the antibodies were caused by Coxsackie B5 infection.