The persistence of swine vesicular disease virus infection in pigs

Two groups of pigs were infected with a recent Italian isolate of swine vesicular disease virus (SVDV). Blood, nasal swabs and faeces were collected for up to 6 months after exposure to infection and animals were killed at regular intervals to obtain tissues post-mortem. These samples were examined for virus by conventional means and for viral RNA (vRNA) by reverse transcription-nested polymerase chain reaction (RT-nPCR). Virus was identified intermittently from both clinically and subclinically infected animals in nasal swabs, faeces and tonsillar tissue by either virus isolation or RT-nPCR up to 63 days post infection (dpi). Between 63 and 119 dpi virus was not detected in the secretions, excretions or tissues of any pigs. Following mixing of the two groups of animals at 119 dpi, SVDV was again identified in faeces for up to 7 days suggesting that the stress of mixing reactivated the excretion of virus in pigs from which the agent could no longer be identified. Minor antigenic changes were identified between the parental virus and isolates recovered late in the course of infection. Altered antigenicity corresponded with deduced amino acid substitutions identified from differences in nucleotide sequence between early and late isolates. This investigation demonstrates that SVDV and vRNA can be present in pigs for considerably longer after exposure to infection than has previously been recognized and provides preliminary evidence for a carrier state in swine vesicular disease.